How micropatterns and air pressure affect splashing on surfaces

We experimentally investigate the splashing mechanism of a millimeter-sized ethanol drop impinging on a structured solid surface, comprised of micro-pillars, through side-view and top-view high speed imaging. By increasing the impact velocity we can tune the impact outcome from a gentle deposition to a violent splash, at which tiny droplets are emitted as the liquid sheet spreads laterally. We measure the splashing threshold for different micropatterns and find that the arrangement of the pillars significantly affects the splashing outcome. In particular, directional splashing in direction in which air flow through pattern is possible. Our top-view observations of impact dynamics reveal that an trapped air is responsible for the splashing. Indeed by lowering the pressure of the surrounding air we show that we can suppress the splashing in the explored parameter regime.Comment: 7 pages, 9 figure

The Zipping-wetting Dynamics at the Breakdown of Superhydrophobicity

Under some conditions water droplets can completely wet micro-structured superhydrophobic surfaces. The dynamics of this rapid process is investigated with ultra-high-speed imaging. Depending on the scales of the micro-structure, the wetting fronts propagate smoothly and circularly or – more interestingly – in a stepwise manner for a smaller periodicity of the microstructure. The latter phenomenon leads to a growing square-shaped wetted area: liquid laterally enters a new row on a slow timescale of milliseconds, once it happens the row then fills itself towards the sides in microseconds (“zipping”)

Focus on fluids: Slippery interfaces for drag reduction

Inspired by natural interfaces with surprising transport properties, innovative modifications of surfaces have been engineered to reduce drag. The common theme across these new developments is the presence of lubricant patches or layers that decrease the direct contact of viscous liquid with non-slippery solid walls. For laminar flow, the traditional assumption regarding the lubricant layer is a constant shear rate or a steady pressure gradient, implying a net flow rate of the lubricant film. By challenging this assumption, Busse et al. (J. Fluid Mech., vol. 727, 2013, pp. 488–508) rigorously found that the hydrodynamic slip is reduced by the presence of a reversal of lubricant flow close to the wall. The analytical results for velocity field and change in drag provide insight into the optimal design of slippery surfaces with lubricant layers for drag reductio

Renewable Power Generation by Reverse Electrodialysis Using an Ion Exchange Membrane

Reverse electrodialysis (RED) is a promising technology to extract sustainable salinity gradient energy. However, the RED technology has not reached its full potential due to membrane efficiency and fouling and the complex interplay between ionic flows and fluidic configurations. We investigate renewable power generation by harnessing salinity gradient energy during reverse electrodialysis using a lab-scaled fluidic cell, consisting of two reservoirs separated by a nanoporous ion exchange membrane, under various flow rates (qf) and salt-concentration difference (Δc). The current-voltage (I-V) characteristics of the single RED unit reveals a linear dependence, similar to an electrochemical cell. The experimental results show that the change of inflow velocity has an insignificant impact on the I-V data for a wide range of flow rates explored (0.01–1 mL/min), corresponding to a low-Peclet number regime. Both the maximum RED power density (Pc,m) and open-circuit voltage (ϕ0) increase with increasing Δc. On the one hand, the RED cell’s internal resistance (Rc) empirically reveals a power-law dependence of Rc∝Δc−α. On the other hand, the open-circuit voltage shows a logarithmic relationship of ϕ0=BlnΔc+β. These experimental results are consistent with those by a nonlinear numerical simulation considering a single charged nanochannel, suggesting that parallelization of charged nano-capillaries might be a good upscaling model for a nanoporous membrane for RED applications

Numerical Investigation of Diffusioosmotic Flow in a Tapered Nanochannel

Diffusioosmosis concerns ionic flow driven by a concentration difference in a charged nano-confinement and has significant applications in micro/nano-fluidics because of its nonlinear current-voltage response, thereby acting as an active electric gating. We carry out a comprehensive computation fluid dynamics simulation to investigate diffusioosmotic flow in a charged nanochannel of linearly varying height under an electrolyte concentration gradient. We analyze the effects of cone angle (α), nanochannel length (l) and tip diameter (dt), concentration difference (Δc = 0–1 mM), and external flow on the diffusioosmotic velocity in a tapered nanochannel with a constant surface charge density (σ). External flow velocity (varied over five orders of magnitude) shows a negligible influence on the diffusioosmotic flow inside the tapered nanochannel. We observed that a cone angle causes diffusioosmotic flow to move towards the direction of increasing gap thickness because of stronger local electric field caused by the overlapping of electric double layers near the smaller orifice. Moreover, the magnitude of average nanoflow velocity increases with increasing |α|. Flow velocity at the nanochannel tip increases when dt is smaller or when l is greater. In addition, the magnitude of diffusioosmotic velocity increases with increasing Δc. Our numerical results demonstrate the nonlinear dependence of tapered, diffusioosmotic flow on various crucial control parameters, e.g., concentration difference, cone angle, tip diameter, and nanochannel length, whereas an insignificant relationship on flow rate in the low Peclet number regime is observed

Upscaling production of droplets and magnetic particles with additive manufacturing

International audiencePurpose Monodisperse microfluidic emulsions - droplets in another immiscible liquid - are beneficial to various technological applications in analytical chemistry, material and chemical engineering, biology and medicine. Upscaling the mass production of micron-sized monodisperse emulsions, however, has been a challenge because of the complexity and technical difficulty of fabricating or upscaling three-dimensional (3 D) microfluidic structures on a chip. Therefore, the authors develop a fluid dynamical design that uses a standard and straightforward 3 D printer for the mass production of monodisperse droplets. Design/methodology/approach The authors combine additive manufacturing, fluid dynamical design and suitable surface treatment to create an easy-to-fabricate device for the upscaling production of monodisperse emulsions. Considering hydrodynamic networks and associated flow resistance, the authors adapt microfluidic flow-focusing junctions to produce (water-in-oil) emulsions in parallel in one integrated fluidic device, under suitable flow rates and channel sizes. Findings The device consists of 32 droplet-makers in parallel and is capable of mass-producing 14 L/day of monodisperse emulsions. This convenient method can produce 50,000 millimetric droplets per hour. Finally, the authors extend the current 3 D printed fluidics with the generated emulsions to synthesize magnetic microspheres. Originality/value Combining additive manufacturing and hydrodynamical concepts and designs, the authors experimentally demonstrate a facile method of upscaling the production of useful monodisperse emulsions. The design and approach will be beneficial for mass productions of smart and functional microfluidic materials useful in a myriad of applications