Ho-Young Kim

Engineering microscale cellular niches for three-dimensional multicellular co-cultures

Modeling the in vivo microenvironment typically involves placing cells in a three-dimensional (3D) extracellular matrix (ECM) in physiologically relevant context with respect to other cells. The mechanical and chemical features of 3D microenvironments play important roles in tissue engineering, tumor growth and metastasis, and in defining stem cell niches, and it is increasingly recognized that cells behave much differently when surrounded by a 3D ECM than when anchored to a 2D substrate. To create microenvironments that more closely mimic in vivo settings, here we describe a novel microfluidic device that allows multiple discrete constructs of 3D cell-laden hydrogels to be patterned in a sequence of simple steps. The microfluidic platform allows for real-time imaging of the interactions between multiple cell types exposed to both autocrine and paracrine signaling molecules, all within a 3D ECM environment. Detailed modeling determined that surface tension, hydrophobic interactions, and spatial geometry were important factors in containing the gels within distinct separate channels during the filling process. This allowed us to pattern multiple gel types side-by-side and pattern 3D gels spatially with tight dimensional control. Cells embedded in gels could be patterned by culturing MDA-MB-231 metastatic breast cancer cells and RAW 264.1 macrophage cells within distinct collagen type I and Matrigel ECM environments, respectively. Over a 7 day culture experiment, RAW cells invaded into neighboring gels containing MDA-MB-231 cells, but not into gels lacking cells. These studies demonstrate the versatility and potential of this new microfluidic platform to engineer 3D microscale architectures to investigate cell-cell and cell-matrix interactions.

Flutter-driven triboelectrification for harvesting wind energy

Technologies to harvest electrical energy from wind have vast potentials because wind is one of the cleanest and most sustainable energy sources that nature provides. Here we propose a flutter-driven triboelectric generator that uses contact electrification caused by the self-sustained oscillation of flags. We study the coupled interaction between a fluttering flexible flag and a rigid plate. In doing so, we find three distinct contact modes: single, double and chaotic. The flutter-driven triboelectric generator having small dimensions of 7.5 × 5 cm at wind speed of 15 ms(-1) exhibits high-electrical performances: an instantaneous output voltage of 200 V and a current of 60 μA with a high frequency of 158 Hz, giving an average power density of approximately 0.86 mW. The flutter-driven triboelectric generation is a promising technology to drive electric devices in the outdoor environments in a sustainable manner.

The recoiling of liquid droplets upon collision with solid surfaces

Although the spreading behavior of liquid droplets impacting on solid surfaces has been extensively studied, the mechanism of recoiling which takes place after the droplet reaches its maximum spread diameter has not yet been fully understood. This paper reports the study of the recoiling behavior of different liquid droplets (water, ink, and silicone oil) on different solid surfaces (polycarbonate and silicon oxide). The droplet dynamics are experimentally studied using a high speed video system. Analytical methods using the variational principle, which were originated by Kendall and Rohsenow (MIT Technical Report 85694-100, 1978) and Bechtel et al. [IBM J. Res. Dev. 25, 963 (1981)], are modified to account for wetting and viscous effects. In our model, an empirically determined dissipation factor is used to estimate the viscous friction. It is shown that the model closely predicts the experimental results obtained for the varying dynamic impact conditions and wetting characteristics. This study shows tha...

Water harvest via dewing.

Harvesting water from humid air via dewing can provide a viable solution to a water shortage problem where liquid-phase water is not available. Here we experimentally quantify the effects of wettability and geometry of the condensation substrate on the water harvest efficiency. Uniformly hydrophilic surfaces are found to exhibit higher rates of water condensation and collection than surfaces with lower wettability. This is in contrast to a fog basking method where the most efficient surface consists of hydrophilic islands surrounded by hydrophobic background. A thin drainage path in the lower portion of the condensation substrate is revealed to greatly enhance the water collection efficiency. The optimal surface conditions found in this work can be used to design a practical device that harvests water as its biological counterpart, a green tree frog, Litoria caerulea , does during the dry season in tropical northern Australia.

Nanoscale patterning of microtextured surfaces to control superhydrophobic robustness.

Most naturally existing superhydrophobic surfaces have a dual roughness structure where the entire microtextured area is covered with nanoscale roughness. Despite numerous studies aiming to mimic the biological surfaces, there is a lack of understanding of the role of the nanostructure covering the entire surface. Here we measure and compare the nonwetting behavior of microscopically rough surfaces by changing the coverage of nanoroughness imposed on them. We test the surfaces covered with micropillars, with nanopillars, with partially dual roughness (where micropillar tops are decorated with nanopillars), and with entirely dual roughness and a real lotus leaf surface. It is found that the superhydrophobic robustness of the surface with entirely dual roughness, with respect to the increased liquid pressure caused by the drop evaporation and with respect to the sagging of the liquid meniscus due to increased micropillar spacing, is greatly enhanced compared to that of other surfaces. This is attributed to the nanoroughness on the pillar bases that keeps the bottom surface highly water-repellent. In particular, when a drop sits on the entirely dual surface with a very low micropillar density, the dramatic loss of hydrophobicity is prevented because a novel wetting state is achieved where the drop wets the micropillars while supported by the tips of the basal nanopillars.

Instability of a liquid jet emerging from a droplet upon collision with a solid surface

A linear perturbation theory is developed to investigate the interface instabilities of a radially-expanding, liquid jet in cylindrical geometries. The theory is applied to rapidly spreading droplets upon collision with solid surfaces as the fundamental mechanism behind splashing. The analysis is based on the observation that the instability of the liquid sheet, i.e., the formation of the fingers at the spreading front, develops in the extremely early stages of droplet impact. The shape evolution of the interface in the very early stages of spreading is numerically simulated based on the axisymmetric solutions obtained by a theoretical model. The effects that factors such as the transient profile of an interface radius, the perturbation onset time, and the Weber number have on the analysis results are examined. This study shows that a large impact inertia, associated with a high Weber number, promotes interface instability, and prefers high wave number for maximum instability. The numbers of fingers at the spreading front of droplets predicted by the model agree well with those experimentally observed.

Mechanism of particle removal by megasonic waves

We elucidate the major mechanism of microparticle removal in the megasonic cleaning process through the direct visualization experiments. It is revealed that particles sitting on solids are removed by adjacent microbubbles that oscillate near the substrates and exert interfacial and pressure gradient forces on the particles. Other pressure and streaming effects are shown to be too weak to detach the particles.

Jumping on water: Surface tension–dominated jumping of water striders and robotic insects

How to walk and jump on water Jumping on land requires the coordinated motion of a number of muscles and joints in order to overcome gravity. Walking on water requires specialized legs that are designed to avoid breaking the surface tension during motion. But how do insects, such as water striders and fishing spiders, manage to jump on water, where extra force is needed to generate lift? Koh et al. studied water striders to determine the structure of the legs needed to make jumping possible, as well as the limits on the range of motion that avoids breaking the surface tension (see the Perspective by Vella). They then built water-jumping robots to verify the key parameters of leg design and motion. Science, this issue p. 517; see also p. 472 Specialized leg design and motions allow both insects and robots to jump on water. [Also see Perspective by Vella] Jumping on water is a unique locomotion mode found in semi-aquatic arthropods, such as water striders. To reproduce this feat in a surface tension–dominant jumping robot, we elucidated the hydrodynamics involved and applied them to develop a bio-inspired impulsive mechanism that maximizes momentum transfer to water. We found that water striders rotate the curved tips of their legs inward at a relatively low descending velocity with a force just below that required to break the water surface (144 millinewtons/meter). We built a 68-milligram at-scale jumping robotic insect and verified that it jumps on water with maximum momentum transfer. The results suggest an understanding of the hydrodynamic phenomena used by semi-aquatic arthropods during water jumping and prescribe a method for reproducing these capabilities in artificial systems.

Kinematic Condition for Maximizing the Thrust of a Robotic Fish Using a Compliant Caudal Fin

The compliance of a fin affects the thrust of underwater vehicles mimicking the undulatory motion of fish. Determining the optimal compliance of a fin to maximize thrust is an important issue in designing robotic fish using a compliant fin. We present a simple method to identify the condition for maximizing the thrust generated by a compliant fin propulsion system. When a fin oscillates in a sinusoidal manner, it also bends in a sinusoidal manner. We focus on a particular kinematic parameter of this motion: the phase difference between the sinusoidal motion of the driving angle and the fin-bending angle. By observing the relationship between the thrust and phase difference, we conclude that while satisfying the zero velocity condition, the maximum thrust is obtained when a compliance creates a phase difference of approximately π/2 at a certain undulation frequency. This half-pi phase delay condition is supported by thrust measurements from different compliant fins (four caudal-shaped fins with different aspect ratios) and a beam bending model of the compliant fin. This condition can be used as a guideline to select the proper compliance of a fin when designing a robotic fish.

Extreme water repellency of nanostructured low-surface-energy non-woven fabrics

We report the extreme water repellent nature of non-woven fabrics of PET (polyethyleneterephthalate) whose fiber surfaces are nanotextured with oxygen plasma and coated with a low-surface-energy nanofilm. The surface effectively suppresses vapor condensation and repels condensed water droplets in addition to exhibiting a high contact angle and a low contact angle hysteresis with a millimetre-sized water drop. We also show that the surface maintains its superhydrophobicity after water-vapor condensation and after oil-wetting due to high-aspect-ratio nanohairs on the fibers. The superior water-repellent ability of the plasma treated non-woven fabric can be exploited in a variety of industrial applications including water harvesting and fuel cell water management even under oily contaminations.