Miu Ling Lam

Discrete Element Model for Suppression of Coffee-Ring Effect

When a sessile droplet evaporates, coffee-ring effect drives the suspended particulate matters to the droplet edge, eventually forming a ring-shaped deposition. Because it causes a non-uniform distribution of solid contents, which is undesired in many applications, attempts have been made to eliminate the coffee-ring effect. Recent reports indicated that the coffee-ring effect can be suppressed by a mixture of spherical and non-spherical particles with enhanced particle-particle interaction at air-water interface. However, a model to comprehend the inter-particulate activities has been lacking. Here, we report a discrete element model (particle system) to investigate the phenomenon. The modeled dynamics included particle traveling following the capillary flow with Brownian motion, and its resultant 3D hexagonal close packing of particles along the contact line. For particles being adsorbed by air-water interface, we modeled cluster growth, cluster deformation, and cluster combination. We found that the suppression of coffee-ring effect does not require a circulatory flow driven by an inward Marangoni flow at air-water interface. Instead, the number of new cluster formation, which can be enhanced by increasing the ratio of non-spherical particles and the overall number of microspheres, is more dominant in the suppression process. Together, this model provides a useful platform elucidating insights for suppressing coffee-ring effect for practical applications in the future.

Nanowire Magnetoscope Reveals a Cellular Torque with Left–Right Bias

Cellular force regulates many types of cell mechanics and the associated physiological behaviors. Recent evidence suggested that cell motion with left-right (LR) bias may be the origin of LR asymmetry in tissue architecture. As actomyosin activity was found essential in the process, it predicts a type of cellular force that coordinates the development of LR asymmetry in tissue formation. However, due to the lack of appropriate platform, cellular force with LR bias has not yet been found. Here we report a nanowire magnetoscope that reveals a rotating force-torque-exerted by cells. Ferromagnetic nanowires were deposited and internalized by micropatterned cells. Within a uniform, horizontal magnetic field, the nanowires that initially aligned with the magnetic field were subsequently rotated due to the cellular torque. We found that the torque is LR-biased depending on cell types. While NIH 3T3 fibroblasts and human vascular endothelial cells exhibited counterclockwise torque, C2C12 myoblasts showed torque with slight clockwise bias. Moreover, an actin ring composed of transverse arcs and radial fibers was identified as a major factor determining the LR bias of cellular torque, since the disruption of actin ring by biochemical inhibitors or elongated cell shape abrogated the counterclockwise bias of NIH 3T3 fibroblasts. Our finding reveals a LR-biased torque of single cells and a fundamental origin of cytoskeletal chirality. More broadly, we anticipate that our method will provide a different perspective on mechanics-related cell physiology and force transmission necessary for LR propagation in tissue formation.

Substrate Stiffness Regulates the Development of Left-Right Asymmetry in Cell Orientation.

Left-right (LR) asymmetry of tissue/organ structure is a morphological feature essential for many tissue functions. The ability to incorporate the LR formation in constructing tissue/organ replacement is important for recapturing the inherent tissue structure and functions. However, how LR asymmetry is formed remains largely underdetermined, which creates significant hurdles to reproduce and regulate the formation of LR asymmetry in an engineering context. Here, we report substrate rigidity functioning as an effective switch that turns on the development of LR asymmetry. Using micropatterned cell-adherent stripes on rigid substrates, we found that cells collectively oriented at a LR-biased angle relative to the stripe boundary. This LR asymmetry was initiated by a LR-biased migration of cells at stripe boundary, which later generated a velocity gradient propagating from stripe boundary to the center. After a series of cell translocations and rotations, ultimately, an LR-biased cell orientation within the micropatterned stripe was formed. Importantly, this initiation and propagation of LR asymmetry was observed only on rigid but not on soft substrates, suggesting that the LR asymmetry was regulated by rigid substrate probably through the organization of actin cytoskeleton. Together, we demonstrated substrate rigidity as a determinant factor that mediates the self-organizing LR asymmetry being unfolded from single cells to multicellular organization. More broadly, we anticipate that our findings would pave the way for rebuilding artificial tissue constructs with inherent LR asymmetry in the future.

3D fog display using parallel linear motion platforms

Fog screens have been widely used in theme parks and exhibition halls for entertainment and advertising purposes as the semi-transparent images suspending in the mid-air can create impressive pseudo 3D effect. However, conventional fog screens are flat and the projected images are not true 3D in nature. This paper suggests a new 3D fog display that exploits spatial projection mapping technique on a non-planar, reconfigurable fog screen to display volumetric data in real 3D space. The fog screen is composed of columns of upward-flowing laminar fog that are traveling on a set of linear motion platforms. The light beams from the projector are scattered at different depth positions on the fog screen, thus allowing the viewers to perceive three-dimensionality intuitively. The 3D fog display does not require any headgears or eye-tracking equipment while allowing full color 3D image to be observed by multiple simultaneous viewers. The immaterial nature of fog screens can also facilitate interesting tangible interactions in three physical dimensions. We have constructed a prototype display unit and developed a software platform to render the projection video and control the stepper motors of the linear motion platforms in order to synchronize the fog screen with the projection. The performance of the proposed display is verified by a number of real display examples.

Computational swept volume light painting via robotic non-linear motion

Light painting is a photography technique in which light sources are moved in specific patterns while being captured by long exposure. The movements of lights will result in bright strokes or selectively illuminated and colored areas in the scene being captured, thus decorating the real scene with special visual effects without the need for post-production. Light painting is not only a popular activity for hobbyists to express creativities, but also a practice for professional media artists and photographers to produce aesthetic visual arts and commercial photography. In conventional light paintings, the light sources are usually flashlights or other simple handheld lights made by attaching one or multiple LEDs to a stick or a ring. The patterns created are limited to abstract shapes or freehand strokes.

Interactive volumetric fog display

Traditional fog screens are 2D. We propose a new design of fog screen that can generate fog at different depth positions and at only where necessary. Our approach applies projection mapping onto a non-planar and reconfigurable fog screen, thus enabling interactive visual contents to be displayed at multiple depth levels. Viewers can perceive the three-dimensionality naturally, and interact with the unencumbered images by touching them directly in mid-air. The display can be used in mixed reality settings where physical objects co-exist and interact with the 3D imagery in physical space.

Real-time model slicing in arbitrary direction using octree

In applications such as computational swept volume light painting, real-time contour rendering is essential to ensure shape fidelity. However, there is, at yet, no real-time and scalable solution for slicing a model in arbitrary direction. We propose a new slicing method by organizing the triangular mesh into Octree data structure. The approach can significantly reduce the computational time and improve the performance of real-time rendering. The data structure is invariant to the slicing direction, thus constructing the Octree is a one-time, offline pre-process.

Optimization of Combinatory Nicking Endonucleases for Accurate Identification of Nucleic Acids in Low Abundance

Nucleic acid biomarkers embody inherent importance for differentiating disease-causing organisms or environmental pathogens. Identifying unknown nucleic acids in low abundance remains extremely challenging. Previously, we reported a method to identify complementary DNA (cDNA) molecules based on sequence-specific topographical labels measured by atomic force microscopy (AFM). However, the accuracy is limited because only one type of nicking endonuclease was used as the labeling agent. Here we investigate how accuracy is improved using multiple types of nicking endonucleases in combinations. The numerical experiments created cDNA molecules incorporating measurement error or labeling defects, which were later compared with the 29,563 human messenger RNA (mRNA) transcript database with ideal labels. After comparison, the unknown cDNA molecule was identified as the transcript with the highest matching score. Thus, the accuracy was determined by the rate of true positives. We found that the accuracy is positively proportional to the label number. Compared with cases using single nicking endonuclease, which has an average accuracy of 51.2% ± 34.4%, the average accuracy was improved to 97.1% ± 5.6% using an optimized combination of NtBsmAI + NtBstNBI + NtAlwI. This improved accuracy is applicable to more than 85% of human mRNA transcripts. Together, our study suggests an optimization strategy for identifying nucleic acids in low abundance using the AFM-based method, with implications for diseases diagnosis, pathogen identification, and forensics at the single molecule level.

Computational light painting and kinetic photography

We present a computational framework for creating swept volume light painting and kinetic photography. Unlike conventional light painting technique using hand-held point light source or LED arrays, we move a flat-panel display with robot in a curved path. The display shows real-time rendered contours of a 3D object being sliced by the display plane along the path. All light contours are captured in a long exposure and constitute the virtual 3D object augmented in the real space. To ensure geometric accuracy, we use hand-eye calibration method to precisely obtain the transformation between the display and the robot. A path generation algorithm is developed to automatically yield the robot path that can best accommodate the 3D shape of the target model. To further avoid shape distortion due to asynchronization between the displays pose and the image content, we propose a real-time slicing method for arbitrary slicing direction. By organizing the triangular mesh into Octree data structure, the approach can significantly reduce the computational time and improve the performance of real-time rendering. We study the optimal tree level for different ranges of triangle numbers so as to attain competitive computational time. Texture mapping is also implemented to produce colored light painting. We extend our methodologies to computational kinetic photography, which is dual to light painting. Instead of keeping the camera stationary, we move the camera with robot and capture long exposures of a stationary display showing light contours. We transform the display path for light painting to the camera path for kinetic photography. A variety of 3D models are used to verify that the proposed techniques can produce stunning long exposures with high-fidelity volumetric imagery. The techniques have great potential for innovative applications including animation, visible light communication, invisible information visualization and creative art.

Outline-etching image segmentation reveals enhanced cell chirality through intercellular alignment: HUANG et al.

Cells cultured on micropatterns exhibit a chiral orientation, which may underlie the development of left–right asymmetry in tissue microarchitectures. To investigate this phenomenon, fluorescence staining of nuclei has been used to reveal such orientation. However, for images with high cell density, analysis is difficult because of the overlapping nuclei. Here, we report an image processing method that can acquire cell orientations within dense cell populations. After initial separation based on Boolean addition of binarized images using global and adaptive thresholds, the overlapping nucleus contours in the binarized images were segmented by iteratively etching the outlines of nuclei, which allowed the orientations of each cell to be extracted from densely packed cell clusters. In applying this technique to cultured C2C12 myoblasts in micropatterned stripes on different substrates, we found an enhanced chiral orientation on glass substrate. More important, this enhanced chirality was consistently observed with increased intercellular alignment and independent of cell–cell distance or cell density, suggesting that intercellular alignment plays a role in determining the chiral orientation. By segmenting single cells with intact orientation, this technique offers an automated method for quantitative analysis with improved accuracy, providing an essential tool for studying left–right asymmetry and other morphogenic dynamics in tissue formation.