Byung Mook Weon

Patterning droplets with durotaxis

Numerous cell types have shown a remarkable ability to detect and move along gradients in stiffness of an underlying substrate—a process known as durotaxis. The mechanisms underlying durotaxis are still unresolved, but generally believed to involve active sensing and locomotion. Here, we show that simple liquid droplets also undergo durotaxis. By modulating substrate stiffness, we obtain fine control of droplet position on soft, flat substrates. Unlike other control mechanisms, droplet durotaxis works without imposing chemical, thermal, electrical, or topographical gradients. We show that droplet durotaxis can be used to create large-scale droplet patterns and is potentially useful for many applications, such as microfluidics, thermal control, and microfabrication.

X-ray phase-contrast imaging of dynamics of complex fluids

Complex fluids often exhibit unusual and/or unexpected behaviours in response to external stresses because of their complicated structures and compositions. It is not easy to understand dynamic behaviours of complex fluids based on using conventional imaging methods such as optical or electron microscopy. Recently, x-ray phase-contrast imaging, as one of the most powerful methods, has been introduced for elucidating the dynamic nature of complex fluids, enabling directly looking into the insides of complex fluids thanks to the strong penetration capability and small refractivity of hard x-rays. In this paper, we review representative x-ray imaging studies on dynamics of various complex fluid systems from droplets, bubbles, granular materials and foams to colloids. It is demonstrated that x-ray phase-contrast imaging would help us better identify and utilize the properties of complex fluids.

Making porous conductive carbon films with unbalanced magnetron sputtering

Porous conductive carbon films are useful for application in fuel cells and biomedical sensors. Controllability of the porosity in conductive carbon films was investigated by using unbalanced magnetron sputtering (UBMS). Here, we show through porosity analysis and plasma diagnostics that carbon films can be tuned to have porosity ranging from amorphous to porous by varying the working pressures from 3 to 140 mTorr in UBMS. The porosity control is attributed to the carbon adatom energy change by control of the working pressures. This approach enabled us to obtain porous carbon films of 44–68% with a high conductivity of 20–0.001 S/cm, implying the feasibility of porous conductive carbon films for advanced applications.

Droplet evaporation with complexity of evaporation modes

Evaporation of a sessile droplet often exhibits a mixed evaporation mode, where the contact radius and the contact angle simultaneously vary with time. For sessile water droplets containing polymers with different initial polymer concentrations, we experimentally study their evaporation dynamics by measuring mass and volume changes. We show how diffusion-limited evaporation governs droplet evaporation, regardless of the complexity of evaporation behavior, and how the evaporation rate depends on the polymer concentration. Finally, we suggest a unified expression for a diffusion-limited evaporation rate for a sessile droplet with complexity in evaporation dynamics.

Polymer composite microtube array produced by meniscus-guided approach

Single freestanding microtubes of poly(methyl methacrylate)/polypyrrole (PMMA/PPy) are produced based on a meniscus-guided approach. A ring-deposit of nanoparticles is first formed in a meniscus solution of PMMA/PPy nanoparticles by outward liquid flow in fast solvent evaporation. Continuous accumulation of nanoparticles on the ring-deposit is then made by guiding the meniscus upward under the outward flow, thereby forming single composite microtube with controlled outer diameter and wall thickness. The meniscus-guiding enables us to produce an array of freestanding microtubes that are individually controlled in size at the desired positions. We demonstrate individually addressable gas sensors by integrating PMMA/PPy microtubes on electrodes.

Drying-mediated patterns in colloid-polymer suspensions

Drying-mediated patterning of colloidal particles is a physical phenomenon that must be understood in inkjet printing technology to obtain crack-free uniform colloidal films. Here we experimentally study the drying-mediated patterns of a model colloid-polymer suspension and specifically observe how the deposit pattern appears after droplet evaporation by varying particle size and polymer concentration. We find that at a high polymer concentration, the ring-like pattern appears in suspensions with large colloids, contrary to suppression of ring formation in suspensions with small colloids thanks to colloid-polymer interactions. We attribute this unexpected reversal behavior to hydrodynamics and size dependence of colloid-polymer interactions. This finding would be very useful in developing control of drying-mediated self-assembly to produce crack-free uniform patterns from colloidal fluids.

A solution to debates over the behavior of mortality at old ages

As humans live longer, the precise modeling of mortality curves in very old age is becoming more important in aging research and public health. Here, we address a methodology that utilizes a modified stretched exponential survival function where a stretched exponent is relevant to heterogeneity in human populations. This function allows better estimation of the maximum human lifespan by providing a good description of the mortality curves in very old age. Demographic analysis of Swedish females over three recent decades revealed an important trend: the maximum human lifespan (existing around 125xa0years) gradually decreased at a constant rate of ~1.6xa0years per decade, while the characteristic life gradually increased at a constant rate of ~1.2xa0years per decade. This trend indicates that the number of aging people is increasingly concentrated at very old age, which is consistent with the definition of population aging. Importantly analyzing the stretched exponents would help in evaluating the heterogeneity trends in human populations.

Short time dynamics of water coalescence on a flat water pool

Coalescence is an important hydrodynamic event that frequently takes place in nature as well as in industry. Here we provide an experimental study on short time dynamics of water coalescence, particularly when a water droplet comes in contact with a flat water surface, by utilizing high-resolution high-penetration ultrafast X-ray microscopy. Our results demonstrate a possibility that an extreme curvature difference between a drop and a flat surface can significantly modify the hydrodynamics of water coalescence, which is unexpected in the existing theory. We suggest a plausible explanation for why coalescence can be modified by an extreme curvature difference.

Creation of additional electrical pathways for the robust stretchable electrode by using UV irradiated CNT-elastomer composite

In this study, we fabricate an electrode structure, in which a carbon nanotube (CNT) film is coated on the composite of carbon nanotubes (CNTs) and polydimethylsiloxane (PDMS). Electromechanical tests disclose that incorporation of a high density of well-dispersed CNTs (with the aid of UV/O3 treatment) in the PDMS substrate offers an alternative current path when the CNT film cracked, helping demonstrate much improved electromechanical characteristics. Further analysis using scanning electron microscopy (SEM) combined with the data from the electromechanical tests illuminates some interesting electromechanical features and the failure mechanism of the material.

Trends in female lifespan in South Korea, 1987-2016

South Korea shows a remarkable rapid increase in lifespan in recent decades. Employing a mathematical model that is appropriate for human survival curves, we evaluate current trends in female lifespan for South Korea over three recent decades, 1987–2016, and predict coming trends in female lifespan until 2030. From comparative analyses with industrialized countries such as Japan, France, Australia, Switzerland, UK, Sweden, and USA, we confirm that South Korea has the highest increase rate of female lifespan in recent decades, and estimate that maximum lifespan would reach 125 years and characteristic life would surpass 95 years for South Korean female by 2030. South Korea would deserve much attention in study on human health and longevity as the longest-lived country in coming decades.