Session MM: Microfluids: General VII

Dynamics of desalination shocks in microstructures

We describe a nonlinear regime of ion transport that results from coupled conduction effects of electric double layers (surface conduction) in electrokinetic systems. Mani, Zangle, and Santiago ({\it Langmuir}, 25, 3898--3916) recently showed that sharp concentration gradients can be formed and propagate away from a microchannel/nanochannel junction, analogous to shock waves in gases. Propagation of these shocks in microchannels leaves behind a region with orders of magnitude lower salt concentration acting to desalinate the bulk electrolyte. In this talk we describe the basic dynamics of desalination shocks and present the mathematical theory of shock existence and propagation in complex microstructures. We predict that desalination shocks accelerate and sharpen in narrowing structures and decelerate and weaken, even disappear, in widening channels. We will also discuss mathematical models for propagation of desalination shocks in porous media.   

Microfluidic flow characterized by light-induced local viscosity distribution

Dominance of the viscous force in fluid flow is unique and important in a microfluidic field. The generation of property distribution, especially in viscosity, can induce the spontaneous change in flow structure. In this study, the effect of local variation of viscosity based on photothermal phenomena on the flow behavior in microchannel is examined. A light absorption in liquid causes a local change of the temperature, and it induces corresponding change in the viscosity, because of the high temperature sensitivity in liquid viscosity. The flow velocity measurement of the stable liquid flow in microchannel with the photothermally-induced viscosity variation is performed by micro-PIV technique. As a result, flow structure around the hot spot is changed by the local property variation. The origin of the change in flow behavior is investigated numerically, and it is confirmed that only the viscosity has significant effect on the fluid flow in the small domain. Additionally, optimal profile of focused light irradiation to induce the significant change in flow field is obtained.   

Irreversible gelation of wormlike micelle solutions under microfluidic flow

The formation of flow-induced gel-like structures in surfactant solutions containing wormlike micelles have previously been observed in macroscopic flow under applied shear in dilute solutions of cetyl-trimethylammonium bromide (CTAB) and sodium salicylate (NaSal). However, the observed gelation phase transition is short-lived once the applied flow is stopped and reversibly disappears. Recently, irreversible gelation was achieved by applying high shear and extensional flows within a packed bed of microbeads in a microfluidic device [1]. We present here a further investigation of the irreversible flow-induced gelation of dilute solutions of CTAB/NaSal in microfluidic devices with microfabricated arrays of microposts with varying post diameters and inter-post spacing. The onset of gelation at various surfactant concentrations and flow rates (both shear and extension rates) will be examined to determine the extent of this phenomenon. \\[4pt] [1] Vasudevan, M., et al., Irreversible nanogel formation in surfactant solutions by microporous flow. Nat Mater, 2010. 9(5): p. 436-441.   

Temporally resolved 3D3C velocity measurements using confocal volumetric scanning

A diagnostic platform for measuring three dimensional velocity fields in whole microscopic volumes is presented. The imaging system is based on Nipkow spinning disk confocal microscopy. The confocal system provides optical sectioning using pinhole spatial filtering which rejects light originating from out of focus objects. Volumetric scanning is obtained by rapid translation of the high numerical aperture objective using a piezo stage. High speed optical sectioning and volumetric scanning of microscopic volumes can be used for real time visualization and velocimetry of three dimensional micro flows in applications such as 3D3C particle tracking velocimetry (PTV) and volumetric quantitative fluorescence imaging. Temporally resolved 3D3C velocity measurements of microchannel flow are presented at near video rates (10-20 Hz) using the scanning confocal system. Little post processing is required because only a single objective is used and no complex algorithms are needed to recover the depthwise velocity component nor to reconstruct the particle images.   

Self-consistent unstirred layers in osmotically driven flows

It has long been recognized, that the osmotic transport characteristics of membranes may be strongly influenced by the presence of unstirred concentration boundary layers adjacent to the membrane. Previous experimental as well as theoretical works have mainly focused on the case where the solutions on both sides of the membrane remain well-mixed due to an external stirring mechanism. We investigate the effects of concentration boundary layers on the efficiency of osmotic pumping processes in the absence of external stirring i.e. when all advection is provided by the osmosis itself. This case is relevant in the study of intracellular flows, e.g. in plants. For such systems, we show that no well-defined boundary layer thickness exists and that the reduction in concentration can be estimated by a surprisingly simple mathematical relation across a wide range of geometries and P\'eclet numbers. This work is accepted for publication in Journal of Fluid Mechanics.   

Micro-rheology of soft glassy materials: Collective phenomena in concentrated emulsions

In previous studies, non local effects have been evidenced and modeled [1, 2]. The fluidity is the number of rearrangement of droplets by unit of time, and appears to be a critical parameter to describe experimental data. In the present work, flows of flocculated and non-flocculated concentrated emulsions within Hele-Shaw cells are investigated. Fast confocal imaging is used to determine fluidity and cooperative length, and the Kinetic Elasto-Plastic model is challenged for systems with different interdroplet attractions\\[4pt] [1] J. Goyon, A. Colin, G. Olvarlez, A. Ajdari, and L. Bocquet, Nature 454 (2008) 84-87.\\[0pt] [2] L. Bocquet, A. Colin, and A. Ajdari, PRL 103 (2009).   

Topology Optimization of Solid Oxide Fuel Cells

We present a free form optimization of Solid Oxide Fuel Cell models, using a high-level implementation of topology optimization according to [1]. As a first step towards the cell's full optimization, we focus in the design of the interconnect. The interconnect is a key element of the whole cell as it is responsible for separating the anode of one cell from the cathode of its next one in a stack of cells, thus being responsible for the supply of fuel and cooling gases, securing in the same time the efficient conductance of electrons through the stack. Modeling the steady-state operation of a fuel cell incorporates the coupling between different physics, such as reaction, electronic, ionic, thermal and fluid phenomena, and is adequately described in two dimensions. Different objective functions, guiding the optimization method, are being investigated, such like the cell's and interconnect's efficiency, heat convection rate and the inlet flowrates of fuel and cooling gases. \\[4pt] [1] L.H. Olesen, F. Okkels, and H. Bruus, Int. J. Num. Meth. Eng. 65, 975 (2006).   

Traffic jams and intermittent flows in microfluidic networks

We investigate both experimentally and theoretically the traffic of particles flowing in microfluidic obstacle networks. We show that the traffic dynamics is a non-linear process: the particle current does not scale with the particle density even in the dilute limit where no particle collision occurs. We demonstrate that this non-linear behavior stems from long range hydrodynamic interactions. Importantly, we also establish that there exists a maximal current above which no stationary particle flow can be sustained. For higher current values, intermittent traffic jams form thereby inducing the ejection of the particles from the initial path and the subsequent invasion of the network. Eventually, we put our findings in the broader context of the transport proccesses of driven particles in low dimension.   

Efficiency of a minimal micro-pump in a viscoelastic fluid

We perform a numerical simulation of a simple micro-pump, and compare its performance in a newtonian and in a viscoelastic fluid. The device consists of two beads which are moved by means of elastic forces. While one bead is held by a static potential, the second one is moved by a pair of potentials alternating periodically. Such system was shown in [1] (where it was named the ``dimer'') to be capable of forcing a newtonian fluid. We investigate the behaviour of the dimer in a viscoelastic fluid, studying how the rheology of the fluid affects the efficiency of pumping.\\[4pt] [1] M. Leoni, B. Bassetti, J. Kotar, P. Cicuta, and M. Cosentino Lagomarsino, {\em Phys. Rev. E}, {\bf 81}, 036304 (2010)   

Compliant synthetic cilia induce deposition of solid particles

Using computational modeling, we examine flow of neutrally buoyant micrometer-sized particles in a fluid-filled microchannel lined with regularly-spaced compliant cilia. The flow is driven by a pressure gradient along the channel. Our simulations reveal that non-motile synthetic cilia can be harnessed to regulate deposition of solid particles. We show that elastic cilia, deflected by the flow, create circulatory secondary flows that direct solid particles towards the ciliated channel wall, thereby inducing their rapid deposition. Our results suggest that synthetic ciliated surfaces could be harnessed for hydrodynamic separation, trapping, and filtration of microscopic biological and synthetic particles in microfluidic devices. The results are also useful for the understanding of the function of certain suspension-feeders that use ciliary filters to capture food particles from streaming water.