Session A25: Microscale Flows: Flow in Microchannels

Experimental and numerical study of a complex cross-junction microchannel

Microfluidic devices occur in various fields such as inkjet printing, DNA chips, lab-on-a-chip technology, micro-propulsion and droplet-based microfluidics. Here, we examine drop and plug formation of immiscible liquids in a cross-shaped microchannel via high-speed imaging, shadowgraphy and PIV that allows interface topology and flow field tracking. We also present comparisons with direct numerical simulations using the new solver, BLUE, for massively parallel simulations of fully three-dimensional multiphase flows in complex solid geometries.   

Flow control mechanism of capillary driven flow in microchannel using non-mechanical forces.

The capillary driven flow in microchannel is a self-driven flow by the natural phenomenon called surface tension of the fluid. The gradients in surface tension force which influence the flow field in microchannel is generated by the modulation of contact angle through a defined hydrophilization of the PDMS (Polydimethylsiloxane) microchannel surface. PDMS which is hydrophobic in nature is treated with various surface treatments in order to convert it to hydrophilic. The contact angle made by the fluid with the PDMS microchannel surface is altered when the surface is converted from hydrophobic to hydrophilic. The flow rate of fluid in the microchannel is directly proportional to the hydrophilicity of that microchannel since the capillary force which is the driving force of the flow is dependent on the contact angle. Flow control mechanism of capillary driven flow in microchannel using non-mechanical forces is developed by treating the microchannel surfaces with various surface treatments. The precise control of the surface characteristics like hydrophilicity and roughness of the miocrochannel helps to control the capillary flow in microchannel. The flow rate variation with respect to the various surface treated channels are studied.   

Effect of shear-thinning behaviour on liquid-liquid plug flow in microchannels

The present work investigates the dynamics of plug formation of shear-thinning solutions in a 200 $\mu $m microchannel using a two-colour micro-PIV system. Measurements, including phase-averaged velocity fields, have been conducted both at the T-junction inlet and the main channel to enhance understanding of non-Newtonian liquid-liquid flows. Two aqueous glycerol solutions containing xanthan gum are used as the non-Newtonian fluids while 5 cSt silicone oil is the Newtonian phase. The current experimental results revealed a pronounced impact of the xanthan gum (shear-thinning behaviour) on the flow pattern transition boundaries, and enhance the fluid flowrates where plug flow occurred. The addition of polymer resulted also in different hydrodynamic characteristics such as a bullet-shaped plug and an increased film thickness between the plug and the wall. In the present work, the technique allows to capture the velocity field of both phases simultaneously. Experimental results are compared with the numerical simulations provided by the code BLUE.   

Effect of meniscus curvature on thermal transport in microchannels with ridged walls

It is well known that textured surfaces can reduce flow resistance in microchannels, but their effect on thermal transport in, e.g., direct liquid cooling of microprocessors, has only recently been considered. We investigate thermal transport in Poiseuille flow through a channel textured with periodic longitudinal ridges that are held at constant heat flux. We assume the liquid only makes contact with the tips of the ridges, reducing drag but also the area for heat transfer. Accounting for curvature of the interfaces (menisci) that bridge each cavity, we consider two asymptotic limits: (i) small meniscus deflection from flat, using boundary perturbation; (ii) channel height large compared to ridge period, using matched asymptotics. In limit (i), the problem is reduced to dual series equations. If limit (ii) is also taken, we find explicit expressions for the effective slip length and Nusselt number. A remarkable finding is that the simple slip length expressions have exponentially small errors and so are accurate even for channel heights as low as half a ridge period. Finally, limit (i) is compared against direct numerical computations using Chebyshev collocation, and the effect of arbitrary curvature on the Nusselt number is presented for the full range of channel geometries.   

Comparison of Hydrodynamic and Thermal Performance of Micro Heat Sinks with Inline and Staggered Arrangements of Cylindrical Micro Pin Fins

This computational study compares the hydrodynamic and thermal characteristics of flow inside a rectangular microchannel with different in-line and staggered arrangements of cylindrical micro pin fins (MPF). The channel dimensions are 5000 x 1500 x 100 \textmu m$^{\mathrm{3}}$ (l x w x h) while the height and diameter of MPFs are both 100 \textmu m which results in the H/D ratio of 1. Two different values of 1.5 and 3 are considered for the horizontal and vertical pitch ratios (S$_{\mathrm{L}}$/D and S$_{\mathrm{T}}$/D) among MPFs in each of the in-line and staggered arrangements which results in eight configurations. A constant heat flux of 30 W/cm$^{\mathrm{2}}$ is applied through the bottom section of microchannel as well as the liquid interacting surfaces of MPFs. The flow field is simulated at five different Reynolds numbers of 20, 40, 80, 120 and 160 using ANSYS FLUENT v.14.5. Four parameters of pressure drop, friction factor, Nusselt number and Thermal Performance Index (TPI) are used to analyze the hydrodynamic and thermal performance of micro heat sinks. Results show a great dependency of evaluating parameters on the vertical pitch ratios while minor dependencies are seen on the horizontal pitch ratio.   

Inertial migration of spherical particles in submillimeter-sized square channel flows

The distributions of neutrally buoyant spherical particles were measured at downstream cross-sections of submillimeter-sized square channels for the Reynolds number from 1 to 800. Polystyrene particles of diameter d = 30 - 70$\mu$m were suspended in water-glycerol mixture at the volume concentration of $2.5 - 11\times10^2$cm$^{-3}$, and this suspension was made to flow through square channels of width D = 400 - 800$\mu$m and length L = 50 - 600mm. The Reynolds number (Re) was defined in terms of the average flow velocity and the channel width. For the size ratio d/D = 0.075 - 0.125, we found that for Re $<$ 260, particles were focused on four equilibrium positions placed at the center of channel faces, which was in accord with previous experimental and numerical studies. For Re $>$ 450, four additional equilibrium positions were observed near the channel corners. Between these two Reynolds numbers (i.e., 260 $<$ Re $<$450), we observed new equilibrium positions located on a heteroclinic orbit connecting the channel face and corner equilibrium positions. These new equilibrium positions were shifted towards the corner equilibrium positions with increasing Re.   

Prediction and validation of concentration gradient generation in a paper-based microfluidic channel

A paper-based microfluidic channel has obtained attention as a diagnosis device that can implement various chemical or biological reactions. With benefits of thin, flexible, and strong features of paper devices, for example, it is often utilized for cell culture where controlling oxygen, nutrients, metabolism, and signaling molecules gradient affects the growth and movement of the cells. Among various features of paper-based microfluidic devices, we focus on establishment of concentration gradient in a paper channel. The flow is subject to dispersion and capillary effects because a paper is a porous media. In this presentation, we describe facile, fast and accurate method of generating a concentration gradient by using flow mixing of different concentrations. Both theoretical prediction and experimental validation are discussed along with inter-diffusion characteristics of porous flows.   

Internal convection of two interacting liquid slugs of aqueous solution placed inside a microchannel

We experimentally investigated the internal convection of two neighboring liquid slugs of aqueous NaCl solution present inside a microchannel having cross-sectional area of 1 mm $\times$ 1 mm. Micro-PIV technique is used to measure the velocity field inside the slugs. The two slugs have different solute concentration (1 M and 2 M) and the volume of each slug is equal to 2 $\mu$L. There is no physical contact between the two slugs and the slugs are separated by a distance of 680 $\mu$m. Concentration difference between the two slugs lead to different vapor pressure at the liquid-air interface of the two slugs. Slug having lower solute concentration has higher vapor pressure at the interface as compared to the slug having higher solute concentration. Hence, water evaporates from the slug having lower solute concentration and condenses on the slug having higher solute concentration. Evaporation and condensation lead to buoyancy driven Rayleigh convection inside both the slugs. Single recirculating loop is observed in both the slugs. The flow strength in both the slugs decreases with time as evaporation and condensation decreases due to reduce in concentration difference between the two slugs.   

Tailoring tails in Taylor dispersion: how boundaries shape chemical delivery in microfluidics: experiments

We present the results of an experimental investigation of the spreading of an initial dye concentration in laminar shear flow through rectangular ducts. In particular, we demonstrate the critical role that the cross-sectional aspect ratio plays in defining the longitudinal asymmetry of the resulting tracer distribution. Thin ducts (aspect ratio $\ll 1$) generate distributions with sharp fronts and tapering tails, whereas thick ducts (aspect ratio $\sim 1$) produce the opposite effect. The experimental results are shown to be in strong agreement with recent theoretical predictions. Our findings could potentially be useful in a number of microfluidic applications, some of which will be discussed.