NOTE: End date changed to 9/5/2020 per NSSC information (Ed., 11/12/19)

NOTE: End date changed to 9/5/2019 per NSSC information (Ed., 10/2/19)

Colloid science is entering a new era. Over the past 15 years, our NASA-sponsored research has mainly dealt with monodisperse suspensions of colloidal particles interacting via well-known forces. Using spherical particles and observations with light scattering and microscopy, we have gained a great deal of fundamental knowledge about different phases of matter and the dynamics and thermodynamics of their formation. In particular, our experimental results in microgravity have led to a basic understanding of why crystals and glasses form and their properties.

During the past decade, we have made great strides in synthesizing new classes of particles with different shapes and specific, reversible or irreversible, variable range interactions. We have also found new ways to manipulate the particles with flow, electric and magnetic fields, and light. We are therefore positioned at the threshold of a new technology, assembling equilibrium and non-equilibrium macroscopic structures with function and activity from well designed particles on the nano to micron scale.

Of course, there are still fundamental scientific questions which we can and will address including a host of new ordered phases, frozen configurations, frustration and glasses, and the process of self-organization itself. In particular, we plan to use the microscopy and light scattering instruments, in collaboration with our European colleagues, to study particles that we prepare through emulsion and dispersion polymerization. Physical lithographic techniques will also be employed, and the particles will be modified chemically for controllable interactions. We plan to use different phoretic techniques--electro-, dielectro-, and thermo-phoresis--to control the particles density and orientation. These will also serve as the driving forces to establish the rheological properties of these new systems.

[Ed. note: compiled from report sent to NASA Glenn Research Center]

The program’s principle goal is the understanding and explanation of the fundamental microscopic mechanisms of self-organization in model complex fluid systems. The experimental samples are composed of specially synthesized colloidal particles with well understood, well controlled, and sophisticated interactions. Our experiments feature recently developed colloidal systems with directional, specific, and externally controlled inter-particle interactions and motility.

During the past grant period we have been preparing ellipsoidal and spherical particles for temperature and temperature gradient experiments for the NASA Advanced Colloids Experiment (ACE) presently on the International Space Station. We have also been performing ground based experiments on these samples to compare with the upcoming microgravity experiments. In addition we have published several results on ground based colloidal experiments.

Two recent publications are directly connected with antibody tests related to combatting the Covid-19 pandemic. They use optical detection of proteins bound to colloids where in their medical application the colloids would be covered with antigens complementary to the antibodies found in patients blood. The holographic detection method is capable of observing size changes in colloids’ radii of much less than a monolayer of antibody.

We have also pursued experiments on the effects of non-equilibrium organization of colloids. In particular we have studied the formation of non-crystalline colloidal structures that exhibit a new form of order, hyperuniformity, the vanishing of long range density fluctuations. As we have demonstrated in previous work, hyperuniformity may be useful in making isotropic complete photonic bandgap materials. Isotropic hyperuniform structures cannot be made by any equilibrium self-organization. However, as we demonstrate, they can be made using a dynamic phase transition Random Organization at its critical point. We implement and study this transition by driving a system of controlled volume fraction with a specially constructed shear cell with optical access to a confocal microscope.

Colloid science is entering a new era. Over the past 15 years, our NASA-sponsored research has mainly dealt with monodisperse suspensions of colloidal particles interacting via well-known forces. Using spherical particles and observations with light scattering and microscopy, we have gained a great deal of fundamental knowledge about different phases of matter and the dynamics and thermodynamics of their formation. In particular, our experimental results in microgravity have led to a basic understanding of why crystals and glasses form and their properties.

During the past decade, we have made great strides in synthesizing new classes of particles with different shapes and specific, reversible or irreversible, variable range interactions. We have also found new ways to manipulate the particles with flow, electric and magnetic fields, and light. We are therefore positioned at the threshold of a new technology, assembling equilibrium and non-equilibrium macroscopic structures with function and activity from well designed particles on the nano to micron scale.

Of course, there are still fundamental scientific questions which we can and will address including a host of new ordered phases, frozen configurations, frustration and glasses, and the process of self-organization itself. In particular, we plan to use the microscopy and light scattering instruments, in collaboration with our European colleagues, to study particles that we prepare through emulsion and dispersion polymerization. Physical lithographic techniques will also be employed, and the particles will be modified chemically for controllable interactions. We plan to use different phoretic techniques--electro-, dielectro-, and thermo-phoresis--to control the particles density and orientation. These will also serve as the driving forces to establish the rheological properties of these new systems.

Freezing on a sphere

Using fluorescently-labeled PMMA colloid, we investigated the freezing process of a 2D crystal on a curved surface. The major result is that by employing a novel order parameter, we are able to demonstrate that the topology encodes the position of the disordered regions during the transition, which can be traced to a decreased mobility of the particles. The orientational and icosahedral order change simultaneously and both coincide with the hexatic transition observed in flat-space.

Two dimensional freezing proceeds by the rapid eradication of lattice defects as the temperature is lowered below a critical threshold. But crystals that assemble on closed surfaces are required by topology to have a minimum number of lattice defects, called disclinations that act as conserved topological charges. Consider the 12 pentagons on a classic soccer ball or the 12 pentamers on a viral capsid. Moreover, crystals assembled on curved surfaces can spontaneously develop additional lattice defects to alleviate the stress imposed by the curvature. It is therefore unclear how crystallization can proceed on a sphere, the simplest curved surface on which it is impossible to eliminate such defects.

In our study, we show that freezing on the surface of a sphere proceeds by the formation of a single, encompassing crystalline ‘continent,’ which forces defects into 12 isolated ‘seas’ with the same icosahedral symmetry as footballs and viruses. We use this broken symmetry –– aligning the vertices of an icosahedron with the defect seas and unfolding the faces onto a plane –– to construct a new order parameter that reveals the underlying long-range orientational order of the lattice. The effects of geometry on crystallization could be taken into account in the design of nanometer- and micrometer-scale structures in which mobile defects are sequestered into self-ordered arrays. Our results may also be relevant in understanding the properties and occurrence of natural icosahedral structures such as viruses.

National Science Foundation (NSF) / Center for the Advancement of Science in Space (CASIS) collaboration

During the period, our project entitled “Nonequilibrium processing of particle suspensions with thermal and electrical field gradients” was recommended for an award. The project team consists of investigators from New Jersey Institute of Technology (NJIT), including Prof. Boris Khusid serving as PI, and the New York University (NYU) team. The main objective of the proposed ISS experiments is to elucidate mechanisms underlying the nonequilibrium assembly of colloidal particles assisted by temperature and electric field gradients and suggest novel routes for processing functional materials. Experiments in the ISS will be conducted to investigate the evolution of phase transitions, instabilities, and the nucleation and growth of crystalline structures in model colloids subjected to temperature and electric field gradients on a fundamental level without the interference of gravity. We hypothesize that the contrast in the structure formation in model colloids under microgravity in the ISS and normal gravity in Earth-based experiments should reveal the salient features of the influence of a temperature gradient, an electric field and gravity on nonequilibrium structure formation in a suspension of interacting particles. These particles are essential for the development and operation of a wide range of terrestrial and space applications. This NSF grant is entirely for complementary support of our NASA supported microgravity studies.

Advanced Colloids Experiment (ACE)-T7: Sample preparation, characterization and operations

An outstanding problem in condensed matter science concerns the relation between particle shape, crystal symmetry, and structure. The simplest and most symmetric crystal is cubic and is naturally comprised of cube-shaped particles. In atomic systems, these are cubic lattices of spherical atoms, made anisotropic by their atomic orbitals; in our crystalline structures, the constituent particles are, in fact, colloidal cubes. Our research goal is to produce such colloidal structures, and study the dynamics of crystal nucleation and growth. ACE-T7 will vary the size and concentration of the depletant in several samples with the goal of seeing the effect on 3D crystallization in microgravity. Prof. Sacanna’s group (NYU Chemistry) synthesized the silica cubes and prepared the samples, carefully formulating stable particle suspensions and observing 2D crystallization in the lab.

During ISS Increment 55–56, samples 4–6 were investigated. Several experimental runs were conducted over the seven week period (weeks 12–18). These included Module 2: ACE Module S/N 2009 (702); capillary nos. 1 (sample 4), 2 (sample 5), 3 (sample 6). The three samples were homogenized and temperature gradient experiments were performed. Both surface and bulk crystallization were observed in capillaries 1–3. During six weeks of inactivity, the samples continued crystallizing in microgravity. We proposed an additional run to be performed during the August 12–21 period to resume imaging the colloid along the length of the capillaries in module 2. Our expectation is to observe 3D crystals that may have grown over time. We will focus on previously imaged positions to evaluate growth rate and shape evolution.

Initial observation of cubic colloidal crystals: Summary of June 13, 2018 operations

“Fluorescence is observed throughout the entire length of the capillary and particles can be found with the 100X oil objective, indicating that the capillary is well mixed. We set the left side of capillary #2 to 45°C and the right side to 25°C and took Z-stacks at five ROIs along the length of the capillary over time. We are currently seeing if there is a difference along the length of the capillary, i.e., crystals on the cooler side of the capillary and low particle concentration on the hotter side of the capillary. We have also done some image optimization to make visualizing and processing data easier, as some particles contain a lower amount of fluorophore.”

Synthesis of particle samples for ACE-T4 and ACE-T11

Colloidal particles of controlled size are promising building blocks for the self-assembly of functional materials. To support the ACE experimental program, the NYU Colloid Synthesis Facility is developing various materials and colloids including fluorescently-labeled poly(N-isopropylacrylamide-co-acrylic acid) microgel particles (ACE-T4), as well as fluorescently-labeled, poly(12-hydroxystearic acid)-stabilized poly(methyl methacrylate) colloid and trimethoxysilyl-terminated PHS-g-PMMA surfactant (ACE-T11). We have synthesized a new reactive dye (HEMA-modified julolidine rhodol), and are also working on synthesizing a reactive Cy3 cyanine fluorophore for copolymerization with methyl methacrylate and methacrylic acid.

Colloid science is entering a new era. Over the past 15 years, our NASA-sponsored research has mainly dealt with monodisperse suspensions of colloidal particles interacting via well-known forces. Using spherical particles and observations with light scattering and microscopy, we have gained a great deal of fundamental knowledge about different phases of matter and the dynamics and thermodynamics of their formation. In particular, our experimental results in microgravity have led to a basic understanding of why crystals and glasses form and their properties.

During the past decade, we have made great strides in synthesizing new classes of particles with different shapes and specific, reversible or irreversible, variable range interactions. We have also found new ways to manipulate the particles with flow, electric and magnetic fields, and light. We are therefore positioned at the threshold of a new technology, assembling equilibrium and non-equilibrium macroscopic structures with function and activity from well designed particles on the nano to micron scale.

Of course, there are still fundamental scientific questions which we can and will address including a host of new ordered phases, frozen configurations, frustration and glasses, and the process of self-organization itself. In particular, we plan to use the microscopy and light scattering instruments, in collaboration with our European colleagues, to study particles that we prepare through emulsion and dispersion polymerization. Physical lithographic techniques will also be employed, and the particles will be modified chemically for controllable interactions. We plan to use different phoretic techniques--electro-, dielectro-, and thermo-phoresis--to control the particles density and orientation. These will also serve as the driving forces to establish the rheological properties of these new systems.

We have continued our studies of charged colloidal particles held by image charge forces to fluid interfaces. In flat space we looked at the two dimensional glass transition. It had been theoretically (computationally) predicted that 2D glasses had a different diffusive behavior than 3D glasses. It is well known that freezing in 3D is a single first order transition while in 2D it is two second order transitions.

Phase transitions significantly differ between 2D and 3D systems, but the influence of dimensionality on the glass transition is unresolved. We used microscopy to study colloidal systems as they approached their glass transitions at high concentrations and found differences between two dimensions and three dimensions. We found that, in two dimensions, particles can undergo large displacements without changing their position relative to their neighbors, in contrast with three dimensions. This is related to Mermin–Wagner long-wavelength fluctuations that influence phase transitions in two dimensions. However, when measuring particle motion only relative to their neighbors, two dimensions and three dimensions have similar behavior as the glass transition is approached, showing that the long-wavelength fluctuations do not cause a fundamental distinction between 2D and 3D glass transitions. This work resolved the controversy created by the simulations.

Microrollers: Magnetically-driven Magnetic Colloids

Our previously reported work on colloidal swimmers involved particles specially prepared by Prof. Stefano Sacanna. These are 3-(trimethoxysilyl)propyl methacrylate (TPM) polymer particles with an imbedded magnetic hematite cube. We found that driving these particles with a rotating magnetic field produced an entirely new set of phenomena. When rotated near a wall the particles “rolled” and produced very large fluid flows which advected nearby particles and led to strong hydrodynamic interactions and new collective effects. These include the formation of a shock front, the instability of the front, a new type of fingering instability and finally the creation of self-sustaining “critters” –– clusters that broke off from the fingers and self propelled. One of the remarkable features of these critters is that they form from hydrodynamic interactions alone with no potential interactions. These microrollers and critters may lead to new ways to control microfluidic flows.

Freezing in Two Dimensions

We are also interested in how the 2D melting transition is frustrated by the effects of Gaussian curvature and the topological defects that it requires. As a warm-up, we studied the flat space crystallization. We found that even in a small system of charged dipoles residing at a fluid-fluid interface we could reproduce the effects previous seen in magnet dipole systems.

We studied the phase behavior of a system of charged colloidal particles that are electrostatically bound to an almost flat interface between two fluids. We showed that, despite the fact that our experimental system consists of only 103–104 particles, the phase behavior is consistent with the theory of melting due to Kosterlitz, Thouless, Halperin, Nelson, and Young. Using spatial and temporal correlations of the bond-orientational order parameter, we classified our samples into solid, isotropic fluid, and hexatic phases. We demonstrated that the topological defect structure we observe in each phase corresponds to the predictions of KTHNY theory. By measuring the dynamic Lindemann parameter, and the non-Gaussian parameter of the particle displacements relative to their neighbors, we showed that each of the phases displays distinctive dynamical behavior.

Freezing on a sphere: In studies recently submitted to the journal Nature, we have seen crystallization on a spherical interface despite the topological frustration. The mechanism involves the sequestration of the defects to the vertices of an icosahedron. We can see this by locating the defects on the confocal image of the colloids bound to the surface of a water droplet in oil, fitting an icosahedron to the defects positions, projecting the particle positions onto the triangular faces and then unfolding the icosahedron. This approach leads to some intuition on why viruses often form in an icosahedral shape.

Synthesis of Monodisperse, Micron-sized Organosilica Spheres

Colloidal particles of controlled size are promising building blocks for the self-assembly of functional materials. We systematically studied a method to synthesize monodisperse, micrometer-sized spheres from 3-(trimethoxysilyl)propyl methacrylate (TPM) in a benchtop experiment. Their ease of preparation, smoothness, and physical properties provide distinct advantages over other widely employed materials such as silica, polystyrene, and poly(methyl methacrylate).

We determined that the spontaneous emulsification of TPM droplets in water is caused by base-catalyzed hydrolysis, self-condensation, and the deprotonation of TPM. By studying the time-dependent size evolution, we found that the droplet size increases without any detectable secondary nucleation. Resulting TPM droplets are polymerized to form solid particles. The particle diameter can be controlled in the range of 0.4 to 2.8 µm by adjusting the volume fraction of added monomer and the pH of the solution. Droplets can be grown to diameters of up to 4 µm by adding TPM monomer after the initial emulsification. Additionally, we characterized various physical parameters of the TPM particles, and we described methods to incorporate several fluorescent dyes.

Colloid science is entering a new era. Over the past 15 years, our NASA-sponsored research has mainly dealt with monodisperse suspensions of colloidal particles interacting via well-known forces. Using spherical particles and observations with light scattering and microscopy, we have gained a great deal of fundamental knowledge about different phases of matter and the dynamics and thermodynamics of their formation. In particular, our experimental results in microgravity have lead to a basic understanding of why crystals and glasses form and their properties.

During the past decade, we have made great strides in synthesizing new classes of particles with different shapes and specific, reversible or irreversible, variable range interactions. We have also found new ways to manipulate the particles with flow, electric and magnetic fields, and light. We are therefore positioned at the threshold of a new technology, assembling equilibrium and non-equilibrium macroscopic structures with function and activity from well designed particles on the nano to micron scale.

Of course, there are still fundamental scientific questions which we can and will address including a host of new ordered phases, frozen configurations, frustration and glasses, and the process of self-organization itself. In particular, we plan to use the microscopy and light scattering instruments, in collaboration with our European colleagues, to study particles that we prepare through emulsion and dispersion polymerization. Physical lithographic techniques will also be employed, and the particles will be modified chemically for controllable interactions. We plan to use different phoretic techniques--electro-, dielectro-, and thermo-phoresis--to control the particles density and orientation. These will also serve as the driving forces to establish the rheological properties of these new systems.

We have previously reported how DNA-functionalized colloids can be used to perform self-assembly protocols with specific particle-particle recognition and association. In our experiments, micron and sub-micron scale particles are designed to recognize and selectively interact with each other by DNA-sequence encoded coatings. Using cinnamate functionalization in the DNA fabrication, even more sophisticated colloidal assembly has been realized. This is achieved by a UV-light initiated crosslinking step. After particles or structures are assembled by the specific binding of complementary strands of DNA, certain bonds are made permanent by shining on UV, allowing flexibility in design and construction.

During this research period, two US patents were assigned to New York University (Chaikin, et al.; Lang and Chaikin) regarding “Self-replicating materials” and “DNA photolithography with cinnamate crosslinkers.” Success in creating a self-replicating system with polystyrene colloid can be translated to a wide range of materials such as metals and ceramics, semiconductors and plastics. Such composite, microscopically-designed materials should find wide application as sensors, solar cells, battery and fuel cell components, as well as new materials for personal products and pharmaceuticals.

The upcoming Advanced Colloid Experiment (ACE)-T and ACE-E experiments will involve DNA-coated colloidal spheres, patchy particles, and emulsion droplets, which will be manipulated by applied electric fields and thermal gradients to produce 3D micron scale structures. Confocal microscopy is key to the observation and tracking of the constituent particles in real time and real space.

Colloidal Swimmers

During this period, we continued our study of light-activated colloidal “swimmers” propelled by a combination of osmotic and phoretic effects. Motility is a basic feature of living microorganisms, and how it works is often determined by environmental cues. Recent efforts have focused on developing artificial systems that can mimic microorganisms, in particular their self-propulsion. We have reported on the design and characterization of synthetic self-propelled particles that migrate upstream, known as positive rheotaxis. This phenomenon results from a purely physical mechanism involving the interplay between the polarity of the particles and their alignment by a viscous torque. We show quantitative agreement between experimental data and a simple model of an overdamped Brownian pendulum. The model notably predicts the existence of a stagnation point in a diverging flow. Taking advantage of this property, we demonstrated that our active particles can sense and predictably organize in an imposed flow. Our colloidal system represents an important step toward the realization of biomimetic microsystems with the ability to sense and respond to environmental changes.

Colloidal interactions in low polar fluids

We performed high-resolution measurements of the pair interactions between dielectric spheres (PMMA colloid) dispersed in a fluid medium with a low dielectric constant. Despite the absence of charge control agents or added organic salts, these measurements revealed strong and long-ranged repulsions consistent with substantial charges on the particles whose interactions are screened by trace concentrations of mobile ions in solution. The dependence of the estimated charge on the particles’ radii is consistent with charge renormalization theory and, thus, offers insights into the charging mechanism in this interesting class of model systems. The measurement technique, based on optical-tweezer manipulation and artifact-free particle tracking, makes use of optimal statistical methods to reduce measurement errors to the femtonewton frontier while covering an extremely wide range of interaction energies.

Charged colloids at an oil-water interface

Our poly(methyl methacrylate) (PMMA) colloidal particles have nonionogenic surfaces that are sterically stabilized with poly(12-hydroxystearic acid) (PHS). These spheres can be rendered nearly neutrally buoyant through the addition of density-matching cosolvents such as tetrachloroethylene (TCE) without affecting their long-ranged interactions, although TCE is a good solvent for PMMA. When dispersed in cyclohexyl bromide (CHB), however, these particles can display extremely long-ranged repulsions that are strong enough to stabilize colloidal crystals at volume fractions below 0.001. These repulsions arise from positive surface charges that are believed to be built up by association of positively charged species in solution with the particles’ surfaces, or with the PHS layer covering the surfaces, or both. These positive species, in turn, are believed to arise from hydrolysis of CHB leading to dehydrobromination. The resulting hydrogen bromide then dissociates slightly in the moderately polar oil to produce protons that contribute to the spheres’ charges and Br- ions that remain in solution.

In the presence of an interface with a conducting aqueous phase, image-charge effects lead to strong binding of the hydrophobic colloidal particles to the interface, even though the particles are wetted very little by the aqueous phase. We studied both the behavior of individual colloidal particles as they approach the interface and the interactions between particles that are already interfacially bound. We demonstrated that using particles which are minimally wetted by the aqueous phase allows us to isolate and study those interactions which are due solely to charging of the particle surface in oil. Finally, we showed that these interactions can be understood by a simple image-charge model in which the particle charge q is the sole fitting parameter.

ACE-E Science Concept Review (SCR) October 19–20, 2015 NASA Glenn Research Center (GRC). PM Chaikin attended and presented the New York University (NYU) SCR.

ACE particle development

Throughout and into the next reporting period, AD Hollingsworth (NYU Colloid Synthesis Facility) is preparing colloid samples for LMM (Light Microscopy Module) confocal microscope performance testing. We synthesized a cyanine-based reactive dye (Cy3-MMA), which will be incorporated into PMMA spheres, in addition to the traditional rhodamine fluorophore.

Colloid science is entering a new era. Over the past 15 years, our NASA-sponsored research has mainly dealt with monodisperse suspensions of colloidal particles interacting via well-known forces. Using spherical particles and observations with light scattering and microscopy, we have gained a great deal of fundamental knowledge about different phases of matter and the dynamics and thermodynamics of their formation. In particular, our experimental results in microgravity have lead to a basic understanding of why crystals and glasses form and their properties.

During the past decade, we have made great strides in synthesizing new classes of particles with different shapes and specific, reversible or irreversible, variable range interactions. We have also found new ways to manipulate the particles with flow, electric and magnetic fields, and light. We are therefore positioned at the threshold of a new technology, assembling equilibrium and non-equilibrium macroscopic structures with function and activity from well designed particles on the nano to micron scale.

Of course, there are still fundamental scientific questions which we can and will address including a host of new ordered phases, frozen configurations, frustration and glasses, and the process of self-organization itself. In particular, we plan to use the microscopy and light scattering instruments, in collaboration with our European colleagues, to study particles that we prepare through emulsion and dispersion polymerization. Physical lithographic techniques will also be employed, and the particles will be modified chemically for controllable interactions. We plan to use different phoretic techniques--electro-, dielectro-, and thermo-phoresis--to control the particles density and orientation. These will also serve as the driving forces to establish the rheological properties of these new systems.

Microgravity experiments: Superballs and cubic crystals

During the previous grant period, we delivered to NASA, for launch to the ISS on SpaceX CRS-4 (21 September 2014), a series of samples of synthetic colloidal cubes, actually superballs, with different corner roundings. Spheres, with complete rounding, pack most densely in a face centered cubic lattice, whereas cubes pack in a simple cubic lattice. We have shown in previous publications under this grant and in more recent publications that using depletion interaction forces, we can observe these different phases. A 'depletion force' is an effective attractive interaction that arises between colloidal particles suspended in a dilute solution of smaller solutes that are preferentially excluded from the vicinity of the larger particles.

Classical depletants are non-adsorbing polymers, but in our flight samples, they were nanoparticles suspended along with the micron-size cubes, which were fluorescently dyed. Ground-based fluorescence microscopy experiments at NYU (New York University) on the flight samples before delivery and subsequently on the same samples at NASA Glenn showed 2D crystallization in the cubic phase as expected. The samples were delivered to ISS and a series of experiments were carried out during several days over a period of months during 4Q 2014 and 1Q–2Q 2015.

After astronaut manual homogenization of the samples using an external magnet and an internal stirbar, the samples were inspected with ground-based control using several different magnification objectives and brightfield and fluorescent illumination. Most images of the hollow cubic colloids in microgravity were obtained using a 63x air objective. The resolution was sufficiently clear to observe the hollow cubic particles. Depth scans were taken of several samples, with good quality images for depths of up to 100 microns from the surface. “Movies” were taken by sequentially capturing 30, 40, or 100 images of the same region and depth at fixed intervals. They indicated that particles in the bulk were mobile and diffusive while particles near the surfaces were immobile. Overall the microscope and ground control performed at or above expectations.

Unfortunately, the microgravity samples did not show the expected three dimensional cubic crystals that were expected. We are presently studying the microgravity data and performing more ground-based tests to determine why samples from the same batch performed differently in micro-g than in 1g. One immediate difference to be investigated is the presence of dense clumps of the colloidal particles around the stirbar and the fill ports of the space samples. Such aggregation did not occur on similarly prepared samples which remained Earthbound and were studied at the same time as the observations were made on orbit. It is possible that the clustering reduced the concentration of colloids in suspension below that required for crystallization. We will try to mimic the clustering phenomena in future ground-based work.

Shape-sensitive crystallization in colloidal superball fluids

During this period, we continued our study of superballs to understand the effect of particle shape and the range of interactions in forming different crystal structures. Guiding the self-assembly of materials by controlling the shape of the individual particle constituents is a powerful approach to material design. We have shown that colloidal silica superballs crystallize into canted phases in the presence of depletants. Some of these phases are consistent with the so-called “lambda-1” lattice that was recently predicted as the densest packing of superdisks. As the size of the depletant is reduced, however, we observe a transition to a square phase. The differences in these entropically stabilized phases result from an interplay between the size of the depletants and the fine structure of the superball shape. We find qualitative agreement of our experimental results both with a phase diagram computed on the basis of the volume accessible to the depletants and with simulations. By using a mixture of depletants, one of which is thermosensitive, we could induce solid-to-solid phase transitions between square and canted structures. The use of depletant size to leverage fine features of the shape of particles in driving their self-assembly demonstrates a general and powerful mechanism for engineering novel materials.

Light activated, self-propelled colloids

Light-activated, self-propelled colloids were synthesized and their active motion was studied using optical microscopy. We proposed a versatile route using different photoactive materials, and demonstrate a multi-wavelength activation and propulsion. Thanks to the photoelectrochemical properties of two semiconductor materials (alpha-Fe2O3 and TiO2), a light with an energy higher than the bandgap triggers the reaction of decomposition of hydrogen peroxide and produces a chemical ‘cloud’ around the particle. The effect induces a phoretic attraction with neighboring colloids as well as an osmotic self-propulsion of the particle on the substrate. We use these mechanisms to form colloidal cargoes, as well as self-propelled particles where the light-activated component is embedded into a dielectric sphere. The particles are self-propelled along a direction otherwise randomized by thermal fluctuations, and exhibit a persistent random walk. For sufficient surface density, the particles spontaneously form ‘living crystals,’ which are mobile, break apart, and reform. Steering the particle with an external magnetic field, we show that the formation of the dense phase results from the collisions, "heads-on," of the particles. This effect is intrinsically non-equilibrium and a novel principle of organization for systems without detailed balance. Engineering families of particles self-propelled by different wavelengths demonstrate a good understanding of both the physics and the chemistry behind the system and points to a general route for designing new families of self-propelled particles.

Colloid science is entering a new era. Over the past 15 years, our NASA-sponsored research has mainly dealt with monodisperse suspensions of colloidal particles interacting via well-known forces. Using spherical particles and observations with light scattering and microscopy, we have gained a great deal of fundamental knowledge about different phases of matter and the dynamics and thermodynamics of their formation. In particular, our experimental results in microgravity have lead to a basic understanding of why crystals and glasses form and their properties.

During the past decade, we have made great strides in synthesizing new classes of particles with different shapes and specific, reversible or irreversible, variable range interactions. We have also found new ways to manipulate the particles with flow, electric and magnetic fields, and light. We are therefore positioned at the threshold of a new technology, assembling equilibrium and non-equilibrium macroscopic structures with function and activity from well designed particles on the nano to micron scale.

Of course, there are still fundamental scientific questions which we can and will address including a host of new ordered phases, frozen configurations, frustration and glasses, and the process of self-organization itself. In particular, we plan to use the microscopy and light scattering instruments, in collaboration with our European colleagues, to study particles that we prepare through emulsion and dispersion polymerization. Physical lithographic techniques will also be employed, and the particles will be modified chemically for controllable interactions. We plan to use different phoretic techniques– electro-, dielectro-, and thermo-phoresis– to control the particles density and orientation. These will also serve as the driving forces to establish the rheological properties of these new systems.

The program’s principle goal is to explain fundamental, microscopic mechanisms of self-organization. Self-organization can be described as a process leading to some form of overall order, which results from local interactions between the components of an initially disordered system.

A classic example is the so-called hard sphere colloidal crystal that we have produced and studied in both 1-g and microgravity. Here, microscopic particles – similar in shape and size – spontaneously arrange themselves into structurally well-defined arrays. Thermal fluctuations trigger the (entropically-) favorable structures whose physical size and number are amplified by positive feedback. The process is called crystal nucleation and growth. This system is large enough and slow enough to be observed directly under an optical microscope, and is used extensively as a model for atomic and molecular scale phenomena.

Nature produces these structures, too: the opal is composed of silica spheres, 150 to 300 nanometers in diameter, arranged in a hexagonal or face centered cubic (fcc) lattice. Opals shows a range of visible colors due to their internal structure, which causes the interference and diffraction of light passing through the microstructure.

Our latest model complex fluids are composed of specially synthesized colloidal particles with well understood, well controlled and sophisticated interactions as described below. The experiments we propose feature recently introduced colloidal systems with directional, specific, and externally controlled inter-particle interactions and motility.

• Colloidal particle synthesis, ‘Superballs’

We have delivered to NASA, for launch to the space station in Fall 2014, a series of samples of polymer cubes, actually superballs, with different corner roundings described by the simple equation: 1 = (x/a)^m + (y/a)^m + (z/a)^m, where m, the shape parameter, varies from 2 to 4; a is the edge length; x, y, z are spatial coordinates. The usual spheres, m = 2, pack most densely in a fcc lattice. Cubes, m = 8, pack in a simple cubic lattice. The cubes with rounded edges pack most densely in a tilted lattice taking advantage of the space at their corners (1). Most interestingly, using depletants of different sizes we can fill the edges and corners of the cubes and change the packing/crystal structure (2,3). The depletants in the flight samples are nanoparticles and the cubes are fluorescently dyed. Our more recent experiments quantitatively show the role of different sized depletants and different shaped particles in the phase diagram of these particles. To date, our experiments have been limited to two dimensions due to the disruptive action of gravity. In other words, the particles tend to sediment because of their density mismatch with the suspending media, precluding three dimensional structures. In microgravity, we hope to observe the formation of 3D crystallites.

• Colloidal Swimmers

In the previous report, we discussed our fabrication and early experiments with light activated colloidal swimmers propelled by a combination of osmotic and phoretic effects (4). The micron-size particles are driven by the catalytic decomposition of hydrogen peroxide into water and oxygen only when blue light is applied to the system which consists of a polymer sphere with a slightly protruding, photosensitive / catalytic hematite cube inside. We therefore have a dynamical, non-equilibrium, system which is externally controllable. This property was used to demonstrate that we can capture and move other colloidal particles, ‘colloidal dockers’, to desired positions. This special feature should allow for the directed assembly of micro- and nano-scale structures (5). We have also demonstrated that the swimmers can sense and respond to external system variations in a way usually associated only with living creatures. In particular, they can flow upstream mimicking the behavior of, e.g., salmon (6). In the presence of an external flow, when the light is off and the particles are not active, the flow advects the swimmers downstream. In a 4 micron/sec flow, when the light is turned on, the swimmers reverse direction and move upstream. In the faster flow, they direct themselves upstream but at a swimming speed of 8 microns/sec they cannot overcome the downward flow. The upstream motion results from the active hematite element being attracted to the surface and acting as a pivot while the flow forces the polymer sphere into a relative downstream position.

• DNA Coated Colloids

In previous reports we have shown how DNA functionalized colloids can be used to perform self-assembly protocols with specific recognition and association of a particle to many other different particles. We had worked out the thermodynamics of these interactions in detail but the kinetics were largely unknown. In reference (7), we have performed a detailed set of experiments and developed a model which quantitatively accounts for the rate of aggregation of these particles as a function of the DNA sequences, length, areal coverage and salt concentration. These results allow us and others to further design synthetic routes for making complex structures taking into account the different rates at which separate parts will assemble.

Even more sophisticated colloidal assembly is enabled by our fabrication, along with the Weck and Seeman groups in NYU chemistry of DNA with cinnamate substituted for a set of complementary base pairs (8). Cinnamate is photocrosslinkable by exposure to UV light. Thus after particles or structures are assembled by the specific binding of complementary strands of DNA, the bonds can be made permanent by shining on UV. The usual assembly of structures with DNA hybridization is reversible upon heating, but now we can choose which links are reversible and which are permanent.

• Control of defect structure using optical tweezers

Our early NASA flight experiments using poly(methyl methacrylate) (PMMA) hard spheres yielded some extraordinary science from microgravity. Several researchers attempted to modify the system to get it density matched so that experiments could be done on the ground. While the hard sphere density matching did not work, the new systems became a beautiful example of charged spheres with remarkably long range interactions. We have used these systems to study Coulomb crystals and most recently to use them as ideal ways to study and manipulate topological defects such as dislocations. We performed confocal microscope imaging of the bulk system (PMMA colloids in oil) and a layer of PMMA particles which was bound by electrostatic forces to a thin water layer on a cover slip. This 2D system can be readily perturbed by introducing isolated defects using laser tweezers. In reference (9), we published our results on the formation and manipulation of grain boundaries and dislocation pairs, and dislocation reactions in this system. Our results point to many of the fundamental studies that could be done in microgravity with the addition of a laser tweezer setup to the flight confocal microscope.

References

1. Y. Jiao, F. H. Stillinger and S. Torquato, Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys., 2009, 79, 041309. R. D. Batten, F. H. Stillinger and S. Torquato, Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys., 2010, 81, 061105.

2. Laura Rossi, Stefano Sacanna, William T. M. Irvine, Paul M. Chaikin, David J. Pine and Albert P. Philipse, Cubic crystals from cubic colloids, Soft Matter, 7, 4139 ( 2011).

3. Laura Rossi, Vishal Soni, Stefano Sacanna, Paul M. Chaikin, David J. Pine, Albert P. Philipse and William T. M. Irvine, Shape-sensitive crystallization in colloidal superball fluids, submitted.

4. Jeremie Palacci, Stefano Sacanna, Asher Preska Steinberg, David J. Pine, Paul M. Chaikin, Living Crystals of Light-Activated Colloidal Surfers, Science 339, 936–940 (2013).

5. Jeremie Palacci, Stefano Sacanna, Adrian Vatchinsky, Paul M. Chaikin, and David J. Pine, “Photoactivated Colloidal Dockers for Cargo Transportation”, J. Am. Chem. Soc. 135, 15978-15981 (2013).

6. Jeremie Palacci, Anais Abramian, Stefano Sacanna, Jeremie Barral , Kasey Hanson, Alexander Grosberg, David J. Pine, Paul M. Chaikin, Artificial Rheotaxis, to be published.

7. Kun-Ta Wu, Lang Feng, Ruojie Sha, Remi Dreyfus, Alexander Y. Grosberg, Nadrian C. Seeman, and Paul M. Chaikin, “Kinetics of DNA-coated sticky particles”, Physical Review E88, 022304-8 (2013).

8. Lang Feng, Joy Romulus, Ruojie Sha, Marcus Weck, Nadrian Seeman, and Paul Chaikin, “Cinnamate-based DNA photolithography”, Nature Materials, 12 ,747-753, (2013).

9. William T.M. Irvine, Andrew D. Hollingsworth, David G. Grier and Paul M. Chaikin, “Dislocation Reactions, Grain Boundaries and Irreversibility in Two Dimensional Lattices using Topological Tweezers”, PNAS 110,15544-15548 (2013).

Colloid science is entering a new era. Over the past 15 years, our NASA-sponsored research has mainly dealt with monodisperse suspensions of colloidal particles interacting via well-known forces. Using spherical particles and observations with light scattering and microscopy, we have gained a great deal of fundamental knowledge about different phases of matter and the dynamics and thermodynamics of their formation. In particular, our experimental results in microgravity have led to a basic understanding of why crystals and glasses form and their properties.

During the past decade, we have made great strides in synthesizing new classes of particles with different shapes and specific, reversible or irreversible, variable range interactions. We have also found new ways to manipulate the particles with flow, electric and magnetic fields, and light. We are therefore positioned at the threshold of a new technology, assembling equilibrium and non-equilibrium macroscopic structures with function and activity from well designed particles on the nano to micron scale.

Of course, there are still fundamental scientific questions which we can and will address including a host of new ordered phases, frozen configurations, frustration and glasses and the process of self-organization itself. In particular, we plan to use the microscopy and light scattering instruments, in collaboration with our European colleagues, to study particles that we prepare through emulsion and dispersion polymerization. Physical lithographic techniques will also be employed, and the particles will be modified chemically for controllable interactions. We plan to use different phoretic techniques– electro-, dielectro-, and thermo-phoresis– to control the particles density and orientation. These will also serve as the driving forces to establish the rheological properties of these new systems.

NOTE (Ed., March 2014): Continuation of "The Control and Dynamics of Hard Sphere Colloidal Dispersions--NNX08AK04G", grant # NNX08AK04G with the same Principal Investigator.