NOTE: End date changed to 12/29/2022 per NSSC information (Ed., 1/26/22)

The basic idea of utilizing the color information in multi-dimensional space can have a broader significance in a wide range of photometric and image analysis areas. The prototype interferometer has been successfully applied to plane mirrors, concave mirrors, and patterned metal films. We believe this technique has a wider range of applications including, but not limited to, the characterization of liquid crystal cells and materials, as well as semiconductor chips.

During this reporting year, the team was also awarded a provisional patent on the technology concept for Multi-Dimensional Color-Space Interferometry: A Single-Shot, 2pi-Ambiguity-Free Interferometry [Ed. Note: See Bibliography.]

NOTE: End date changed to 12/29/2022 per NSSC information (Ed., 1/26/22)

The last phase of our effort has been on the use of the concept of color analysis to the interferometry. The white-light/monochromatic-light interferometry has been utilized for high precision surface profilometry. Common to all the different modes of operation is an issue associated with efficient and reliable counting of the interferometric fringes under the constraint of periodicity. The color-based thickness mapping method we developed can be directly applied to the interferometry, alleviating the complications of the fringe counting and phase correction. Using the narrow-band illuminator, it becomes possible to take full advantage of the color space. Using a narrow band LED illuminator, we successfully demonstrated the feasibility of this application. Another common problem of the conventional white light interferometry is the limited dynamic range in terms of the coverage of the optical phase difference. In the present scheme, we would also like to point out that a judicious manipulation of band widths allows a more efficient use of the color space and expand the dynamic range over multiple period of wavelengths.

NOTE: End date changed to 12/29/2022 per NSSC information (Ed., 1/26/22)

The research effort in 2021, as in 2020, has also suffered seriously from the spread of COVID-19 infections across the nation since March 2020. In particular, laboratory activities were almost entirely stopped through the rest of the year. We have therefore focused our research efforts on perfecting the thickness measurement method that was started in 2019. As is well known, the thickness of free-standing films is quantized by the number of molecular layers and plays a significant role in determining the structure and interaction of droplets and their structural evolution. In the Lehmann rotation, the sense and speed of rotation undergo drastic variation as the thickness changes. Rapid and precise determination of the thickness distribution over the free-standing film is critical for the analysis of the experimental observations. We have developed a novel approach for thickness measurement based on the quantitative analysis of color of the reflected light, not on the intensity of reflection, which has been used for decades as the standard technique. For 2D analysis of the film images, the color is a much more reliable and robust variable than the intensity. By using a state-of-the-art high-resolution camera, it became possible to determine the thickness of the film with an accuracy better than 1 nm over the entire range of interest from less than 10 nm up to several hundred nanometers. This year, we have drastically improved the acquisition speed of the thickness measurement system by employing a lookup table scheme in place of the previous direct point-wise calculation. It is now possible to obtain the 2D thickness map of an area of the image consisting over one million pixels in one second.

The research effort in 2020 has suffered seriously from the spread of COVID-19 infections across the nation since March. In particular, laboratory activities were almost entirely stopped through the rest of the year. We have therefore focused our research efforts on perfecting the thickness measurement method that was started last year. As is well known, the thickness of free-standing films is quantized by the number of molecular layers and plays a significant role in determining the structure and interaction of droplets and their structural evolution. In the Lehmann rotation, the sense and speed of rotation undergo drastic variation as the thickness changes. Rapid and precise determination of the thickness distribution over the free-standing film is critical for the analysis of the experimental observations. We have developed a novel approach for thickness measurement based on the quantitative analysis of color of the reflected light, not on the intensity of reflection, which has been used for decades as the standard technique. For 2D analysis of the film images, the color is a much more reliable and robust variable than the intensity. By using a state-of-the-art high-resolution camera, it became possible to determine the thickness of the film with an accuracy better than 1 nm over the entire range of interest from less than 10 nm up to several hundred nanometers.

Based on the feasibility study conducted in the past two years, we have developed a prototype hardware integrating the ultrafine inkjet device and a motorized film drawer. The core of the hardware is enclosed inside a metallic chamber to allow application of moderate level of differential gas pressure outside and inside the film compartment. For the purpose of Lehmann rotation experiments, the film must be subjected to a gradient of partial pressure for certain molecules to be transferred; for this reason, the core has two separate closed compartments on both sides of the film. The newly developed film drawer consists of two metallic plates through which 10 mm-diameter hole has been drilled. The holes are tapered toward the outside so that the inner surfaces in contact to each other have a sharp knife edge. The inner surfaces are polished to allow a good surface contact of the plates. The smectic liquid crystal is applied to the inner surfaces, and the two plates are relatively slid in such a way that the two holes begin to open where the smectic film is suspended. The performance of the film drawer is highly stable and reproducible; Once a small amount of smectic liquid crystal is applied continuously to the contact area, the success rate for preparation free standing film is practically 100% except in such a case an excessive vibration is induced during spreading.

The lower compartment of the core hardware houses the ultrafine inkjet device. In the previous term of the project, we have used a cutoff edge of 10 µm - 50 µm thick tungsten wire as the inkjet tip. Due to the flexibility of wire, it was not possible to precisely position the Wire. To alleviate this difficulty, we employed in the new design an electrochemically etched tungsten wire. Starting with a 500 µm-thick tungsten wire, the edge of the wire can be etched in KOH solution in 1 hr under the application of 3V DC voltage with the tungsten wire being the anode. To obtain the sharp edge, it is necessary to slowly extract the wire through the surface of the KOH solution. The rate of extraction determines the taper angle. The radius of curvature at the apex is well below 1 µm. The sharp needle is then inserted in a glass capillary. This procedure is straightforward compared to the previous preparations for the rigidity of the main wire. The bottom end of the glass capillary is partly glued to the tungsten wire. The ink materials can be easily applied by capillary action. The inkjet nozzle thus prepared is then set inside the core hardware, which has 6 degrees of freedom x,y,z positioning system.

The thickness of the smectic film is a crucial factor in determining the behavior of smectic films. Hence, measuring the thickness distribution over the film is the basis for quantitative analysis and interpretation of experimentally observed results. A standard technique is to measure the reflectivity of monochromatic light from the film. This method works for a point wise observation, but accurate mapping of the film thickness is difficult for poor areal definition of illumination and the inevitable distortion of the film caused by an accidental trapping of the menisci at the rim.

Since the brilliant coloration is one of the unique features of the free standing smectic film as well known for soup films, we decided to make use of this color information rather than the reflected light intensity. Under white light illumination, indeed, the molecular layer steps are clearly visible, and the discrete change of color results from the layer-by-layer thickness variation, indicating the possibility to estimate the thickness from quantitative analysis of color. Instead of a monochromatic light as in the intensity method, a stable broadband light source needs to be used for illumination of the film. For this purpose we use a 3-LED data projector as the light source for its outstanding stability and capability to computationally control the color and the intensity. There are three spectral peaks corresponding to the center color of the three LEDs, namely Red, Green, and Blue. This light from the projector is launched on the optical fiber bundle and is conveyed to one of the optical paths of a stereo microscope which has a zoom mechanism automatically optimizing the illumination beam size.

Based on the characteristics of the light source and the color sensitivity of the CMOS (complementary metal-oxide semiconductor) image sensor, we developed a theoretical color model as a function of the film thickness. The color information is extracted from the CMOS camera and is numerically compared with the theoretical model, yielding the thickness estimates. The relationship between color coordinates and the thickness is unique except for a few points and are confirmed to be used for precise determination of the thickness to the accuracy of molecular layer thickness around 3 nm or even better.

We have continued the development of sub-femtolitter inkjet device for use with viscous liquid crystals with a view to establishing the level of stability, reliability, and controllability in terms of the size of droplet, the position and timing of deposition. The common technical challenge in the inkjet technology is to avoid the clogging of nozzles. Since clogged nozzles are fatal in the space experiment, our design employs an open cell made of a 1mm-diamater capillary tube with a fine tungsten electrode (~10 microm-diameter) at the center. The ink liquid wets the tungsten electrode, forming a meniscus. Application of high bias voltages (<1kV) induce the electrowetting behavior, which attracts more ink liquid to toward the apex of the electrode. Above a certain threshold voltage, which depends on the height and the shape of the tungsten electrode, there occurs a sufficient supply of ink liquid to the tip of the electrode, starting the electrostatic deposition of the ink. Using a pulsed voltage, we confirmed that a fine droplet can be deposited at a rate up to 100 droplets/sec. Under application of a relatively low DC voltage compared to the pulsed operation, a continuous deposition with a controllable rate is also possible.

The experimental setup for quantitative real-time observation of inkjet deposition of islands on free standing smectic films consists of a pair of stereo microscopes, installed in such a way that the free standing film and the inkject nozzle can be observed simultaneously from both top and side directions. The top view microscope is equipped with a top down illumination to image the smectic film in the reflected light microscopy (RLM) mode. The RLM allows determination of the film thickness from the color and contrast of the image.

We observed the deposited droplets of ink cumulatively obtained by 100 times pulsed-deposition of liquid crystal 8CB on an ITO glass substrate. From the total volume of the liquid crystal deposited, the volume of individual inkjet droplets is estimated to be at most 20 femtoliter. We observed a clear tendency that the lower the bias voltage, the smaller the minimum droplet size becomes, reaching even sub-femtoliter. However, the deposition condition becomes progressively unstable as the bias voltage is decreased. Moreover, at high pulse voltages over 1.5 kV, deposition of multiple drops often happens. We suspect that at such high voltages, the droplet is so much charged that the surface tension of the ink can no longer be large enough to keep the integrity of the droplet. The over charged droplet splits into pieces creating the multiple droplet deposition.

We successfully carried out the deposition of liquid crystal islands deposited on a free standing semectic film using the electrostatic inkjet. The initial size of the island is 20 microm~30 microm in diameter, and the islands rapidly coalesce with the neighboring islands, forming larger domains. Since the droplet before reaching the film is too small to observe even at the highest magnification, the process of island deposition appears as if the island is suddenly born and grows until it reaches the equilibrium thickness and size. An apparent drawback of the electrostatic deposition is that the electric force due to the voltage pulse vibrates the film and disturbs the configuration of the deposited islands. In order to prevent this behavior, it is necessary to install an extraction electrode held at the same potential as the free standing film.

In order to assess the effect of extraction electrode upon deposition of liquid crystal islands on a free standing smectic film, we have set up a simple extraction electrode. The electrode has a 200 microm-wide gap, through which the deposition can be made, and is placed about 0.5 mm from the film. We could confirm that the electrostatic vibration of the film was completely suppressed, and a fairly gentle deposition of islands was made possible. However, the operation of the extraction electrode is not as yet as reliable as it should due to the required delicate control of the trajectory of the ejected droplet. In the open cell design, the meniscus around the central electrode is not perfectly stable, thereby causing unpredictable fluctuation of the deposition conditions. To eliminate the influence of meniscus, we are currently developing a new design that is to electrically shield the meniscus.

Design and Implementation of Film Drawers

Automated preparation of free standing smectic films is a key hardware requirement for our experiments in space. The conventional method is to drill a hole in a glass slide and manually spread the film over the hole with a razor blade from a small amount of liquid crystal applied to the edge of the hole. Although this method works well in the ordinary lab environment, it is difficult to establish a level of repeatability and accuracy of the film thickness as required in the present space experiment. Furthermore, it is not straightforward to prepare a free standing film in such a way as to separate two isolated compartments for Lehman rotation experiments.

The earliest versions of the film drawer were designed such that instead of the razor blade, a 20 mirometer or so thick plastic or metallic sheet with a large hole is traversed between the metal holders across the hole. With the liquid crystal applied at the edge of the hole, the free standing film grows smoothly as the exposed area through the sliding sheet expands. The operation is quite stable and reproducible, yet the drawback is the difficulty to avoid the self-thinning of the free standing film as the liquid crystal is sucked into the narrow gap between the metal plate and the sliding sheet. After one day or two, the film becomes almost two molecular layers thin everywhere, making it rather fragile. A more recent version of the film drawer with the improvement in the thinning action utilizes a thin tungsten wire instead of the sliding sheet to draw the film from the knife edge. Since the contact area between the wire and the cover plates is much smaller than that in the sliding sheet-based drawer, the free standing film is much less likely to sucked back into the gap as before. The drawback of this design, however, is the relative difficulty in driving the wire. Further improvement is currently underway.

Additional Activities

On April 24 and 25, 2018, the Science Concept Review was held at NASA Glenn Research Center inviting external reviewers. From our team, Yokoyama and Tabe attended the meeting and discussed the basic idea, the current status, and the remaining challenges of the project. Due to a visa problem, Emelyanenko could not attend the meeting in person, but participated in discussions using an online conference. The review committee was of the opinion that the scientific goal and approach of the project is scientifically quite meaningful, yet the hardware, especially the femtoliter inkjet must be more sophisticated to realize the true on-demand deposition of femtoliter droplets without disturbing the free standing film.

Graduate Students : Following graduate students at the LCI & Chemical Physics Interdisciplinary Program contributed to the work described: Mengfei Wang, Joseph Angelo, and Wei Chen.

Future Plan: The ground based scientific activity will continue to establish the sub-femtoliter inkjet technology up to the level that electrically neutral droplets of variable sizes from submicron to tens of microns can be deposited with negligible disruption on the part of the smectic films. We have made a concept design of the ultimate setup with symmetrically positioned ejection and guard electrodes. Precise microfabrication of the extraction gate electrode will be the next step along with the development of neutralizing mechanism. Precise quantification of the droplet volume and charges must be made in order to identify the optimal deposition condition. In addition, it is necessary to further develop a reliable technique to reproducibly prepare a free standing smectic film and control the film thickness by external voltages to allow Lehmann rotation studies. A compact reflected light microscopy will be developed together with a real time image analysis system to monitor the two dimension distribution of the film thickness.

Of particular fundamental significance in the liquid crystal molecular science is the coupling between molecular rotation and flow vortex. This is a multiscale phenomenon, covering the length scale from a single molecule to macroscopic flow. The microgravity environment, combined with a highly accurate theoretical modelling, is expected to address this subtle, yet fundamental issue in liquid crystal science that has evaded full understanding for decades due to the experimental difficulty in the ground-based studies.

Although liquid crystal is a macroscopic state of matter, the local interaction inside and between the molecules is decisive in determining the critical material parameters such as the rotational viscosity. The outcomes of the proposed microgravity study will shed new light on the rational design of high performance liquid crystals with regard to the underexploited yet attractive features of liquid crystals.

Research Activities and Outcomes

As the first year of the project, we focused on the development of sub-femtolitter inkjet device for use with viscous liquid crystals. The rest of the research hinges on the availability of this device, and this will be the integral component of the flight experiment hardware. Currently commercially available are two types of technologies for dispensing small volume of liquid below 1nl: Inkjet printers of various types and the electrospinning or electrospray technology. The inkjet printers for office use and for 3D printing is based on a fine nozzle connected to a source of impulsive pressure generated by piezo actuators or bubbling. This type of device is sensitive to the viscosity of ink and is easily vulnerable to clogging. There also exists a finer inkjet technology based on electrostatic driving; this still uses even finer nozzles to deposit femtoliter droplets, which shares the common problem of clogged nozzle. Our approach here is to combine the electrospray, which is free from clogging issue, with the electrostatic inkjet to facilitate a clog-free sub-femtoliter inkjet.

The key feature of the present design is the open construction without a fine aperture nozzle. In the electrospray, which uses the same open construction, the application of high voltage to the ink results in the formation of the Taylor cone that generates a continuous flow of liquid. In order to achieve the drop-on-demand action with the capability of dispensing sub-femtoliter drops, a fine metal wire is inserted in such a way that the edge of the wire is slightly out of the surface of the liquid. By applying a high enough pulse voltage between the wire and the target, a small droplet is forced to detach from the edge of the wire. The most significant factors determining the droplet size are the thickness of the wetting liquid layer at the wire edge and the magnitude and width of the voltage pulse. Unlike the electrospray, the supply of the liquid is severely limited by the presence of the wetting liquid layer covering the metal wire extended out of the meniscus. We have carried out a preliminary yet substantial characterization of the inkjet performances under various operation conditions. We used a room temperature smectic liquid crystal, octyl-cyanobiphenyl (8CB). Its viscosity is in the range of 30-40 mPa.s, while the viscosity of commercial inkjet inks is formulated to be around 2 mPa.s, which is roughly twice as large as that of water. Despite the high viscosity, our device could deposit 8CB micrometer sized droplets in the drop-on-demand mode, provided the proper operation conditions are set. The driving scheme has been proven to be decisive to facilitate the drop-on-demand operation in the desired range of droplet size. The bias DC voltage (400V-1.5KV) is necessary to charge up the liquid crystal to a certain level and to attract the liquid crystal near the edge of the wire based on the focused electric field at the edge. The pulse voltage (400V-1KV) is to propel a charged droplet to be detached from the wetting layer. The width of the pulse, which turned out to be in the range of 2-13 ms, must be adjusted to make an isolated droplet rather than a continuous flow of liquid as in the electrospray. The target must be within the range 30-100 micrometer from the wire edge. The size of the droplet can be controlled from below 1 mm diameter to several tens of micrometer by adjusting the applied voltages. For a constant deposition condition, the size remains highly uniform.

The fundamental design of the sub-femtoliter inkjet device has been made and the performance characterization proved its potential utility in the projected space experiments. A crucial remaining task is to integrate an extraction gate electrode that replaces the target substrate used in the present experiment. It is also necessary to add a charge neutralizing chamber before the deposition of the droplets on the smectic liquid crystal film.

Of particular fundamental significance in the liquid crystal molecular science is the coupling between molecular rotation and flow vortex. This is a multiscale phenomenon, covering the length scale from a single molecule to macroscopic flow. The microgravity environment, combined with a highly accurate theoretical modelling, is expected to address this subtle, yet fundamental issue in liquid crystal science that has evaded full understanding for decades due to the experimental difficulty in the ground-based studies.

Although liquid crystal is a macroscopic state of matter, the local interaction inside and between the molecules is decisive in determining the critical material parameters such as the rotational viscosity. The outcomes of the proposed microgravity study will shed new light on the rational design of high performance liquid crystals with regard to the underexploited yet attractive features of liquid crystals.