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A Simple Method for Evaluating and Predicting Chaotic Advection in Microfluidic Slugs

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A Simple Method for Evaluating and Predicting Chaotic Advection in Microfluidic Slugs Chemical Engineering Science 65 (2010) 5382–5391 Contents lists available at ScienceDirect Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces A simple method for evaluating and predicting chaotic advection in microfluidic slugs Jayaprakash Sivasamy a,b, Zhizhao Che a, Teck Neng Wong a,Ã, Nam-Trung Nguyen a, Levent Yobas b a School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore b Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research), 11 Science Park Road, Singapore 117685, Singapore article info abstract Article history: A simple method for evaluating chaotic advection in slug micromixing is reported in this paper. We Received 24 April 2009 consider a slug moving in a slit microchannel ðwbhÞ and flow field in a plane far from the boundary Received in revised form walls is modelled as a two-dimensional low-Reynolds-number flow (Stokes flow). Analytical solution 11 June 2010 for normalised velocity field in the slug is derived. The two-dimensional analytical solution is compared Accepted 18 June 2010 with the two-dimensional slice from the three-dimensional numerical solution of the slug velocity field. Available online 8 July 2010 Boundary conditions mimicking the motion of the slugs in microchannel geometries, in Lagrangian Keywords: frame of reference, is used to track the passive tracer particles using Lagrangian particle tracking Microfluidics method. Poincare´ sections and dye advection patterns are used to analyse chaotic advection of passive Micromixing tracer particles using statistical concepts such as ‘variance’, ‘Shannon entrophy’ and ‘complete spatial Chaotic advection randomness’. Results for boundary conditions mimicking constant-velocity straight-channel flow, Slug Droplet constant-velocity normal-meandering channel flow are compared. A method for finding new channel Modelling geometries which enhance chaotic mixing is also proposed. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction gas–liquid segmented flow, the liquid-slug between two gas bubbles generates the counter rotating vortices in the Taylor flow In microscale, momentum, mass and energy transport experi- regime (Fries et al., 2008). ence laminar and Stokes flow conditions (i.e., low-Reynolds From the available literature (Aref, 1984, 2002; Stone et al., number, Reo1) (Manz et al., 1990; Whitesides, 2006). Unavail- 2004), it is well understood that chaotic advection can be ability of turbulent conditions make mixing dependent on the produced whenever the kinematic equations of motion for diffusive properties of the species involved in the process passively advected particles give rise to a nonintegrable dynami- (Gunther¨ et al., 2004). In micro-total-analysis systems (mÀTAS), cal system. Understanding the passive advection of particles in species involved in the analysis are macromolecules and biologi- the complex laminar flow is considered as a useful first step in cal species with low mass diffusivities and hence have long describing the mixing process in chaotic advection driven mixing time in laminar flow conditions. They are comprehen- microscale mixing. Channel modifications have been implemen- sively reviewed by Nguyen and Wu (2005), Hardt et al. (2005) and ted to improve the chaotic advection by promoting internal Teh et al. (2008). circulation of liquids in the vortices in droplets and slug. When a Droplet and gas–liquid segmented flow are the two kinds of droplet moves through a straight microchannel, recirculating flow flow that have been investigated for passive micromixing of of equal size is generated in each half of the droplet (Handique liquids as they are known to mix the components rapidly by and Burns, 2001). Fluids within each half of the droplet are mixed, internal recirculation (Song et al., 2003a, b; Bringer et al., 2004; but the two halves remain unmixed and separated from each Garstecki et al., 2006a, b; Tanthapanichakoon et al., 2006; Gunther¨ other. et al., 2004, 2005). Laminar flow conditions, which is prevalent in To enhance internal mixing within droplets, modifications in continuous microchannel flows, can be observed in droplet the channel geometry, by employing turns and bends, are used to micromixing (Teh et al., 2008). In droplet flows, the liquid inside create chaotic advection to fold and stretch the content of the the droplet generates two counter rotating vortices. In the case of droplet (Song et al., 2003b; Teh et al., 2008). As the droplet traverses through a curved channel, the two halves of the droplet experience unequal recirculating flows. One half of the droplet is à Corresponding author. Tel.: +65 67905587; fax: +65 67911859. exposed to the inner arc of the winding channel, a shorter channel E-mail address: [email protected] (T.N. Wong). section, and thus a small recirculating flow is generated compared 0009-2509/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2010.06.017 S. Jayaprakash et al. / Chemical Engineering Science 65 (2010) 5382–5391 5383 to the other half of the droplet which is exposed to a longer and the boundary conditions mimicking the flow of the slug in channel section. The irregular motion along the walls promoted microchannel are used to get the velocity at every point of the chaos and crossing of fluid streams since the vortices of each half flow domain. Velocity field obtained from the analytical model is are assymetrical. The sharp turns also help to reorient the droplet, used to track the passive tracer particles and construct Poincare´ so that it becomes thoroughly mixed as it goes through a series of maps and dye advection pattern. To analyse the nature of chaotic stretching, reorientation, and folding (Bringer et al., 2004). advection, traditional tools such as ‘variance’, ‘Shannon entrophy’ Many of the studies in microfluidics literature are application- and ‘complete spatial randomness’ are used. Based on the 2D driven experimental studies involving complex geometries with model and the results obtained, a method to predict microchannel no theory (Shui et al., 2008). Very few researchers have tried to geometries which can produce chaotic advection in liquid-slug model the chaotic motion in slugs and droplets moving in and droplet flows is proposed. microfluidic channels. Experimental scaling of the chaotic mixing in a droplet moving through a winding microfluidic channel has been done using baker’s transformation (Song et al., 2003a). 2. Problem description However, the argument was too simple and does not provide the optimization of the channel geometry for rapid mixing through Microchannel flows are similar to Stokes flow problems because chaotic advection. A mathematical model to estimate the mixing of the inherently laminar flow conditions. Gas–liquid segmented time for slugs in a straight slit microchannel was proposed flow (often called as slug flow) can be described by Stokes (Handique and Burns, 2001). Although the model did estimate the equations, when the liquid-slug is considered as a single-phase time required for mixing, it did not provide any insight about flow in Lagrangian frame of reference, which model viscous fluids chaotic motion in the slugs. The mixing characteristics inside a in macroscales and ordinary fluids in microscales. We consider the microfluidic slug using computational fluid dynamics was done case where a slug fully occupies the channel cross-section and its by Tanthapanichakoon et al. (2006), where each slug was cross-sectional shape is determined by that of the channel. The modelled as a single-phase flow domain in two-dimensional microchannel is considered as infinitely large in one of the three (2D) as well as in three-dimensional (3D) domain. Boundary directions and therefore the flow in a plane far from the boundary conditions of the slug in Lagrangian frame of reference were used walls, in the direction, is two-dimensional. Fig. 1 shows the model to simulate the mixing characteristics. They reported that the of a liquid-slug with moving wall boundary conditions in radially arranged reactants mix more rapidly than the axially Lagrangian frame of reference in a straight rectangular arranged reactants and proposed a new dimensionless number to microchannel. The continuous phase is assumed to wet the wall estimate mixing rates. However, they did not calculate chaotic well and no-slip condition is applied on the top and the bottom of advection of the flow inside the slugs. Since droplets and slugs the slug. In the front and the rear, the slug is surrounded by an produce similar vortices, similar inferences can be made from immiscible phase with negligible viscosity such as air and therefore their flow fields when moving in various microchannel geome- no shear stress condition is assumed. Other assumptions made in tries and hence only the slug terminology will be used for this analysis are: no body force has any influence on the slug; the convenience hereafter. slug is approximately rectangular; the liquid in the slug is an In order to understand mixing in microfluidic slugs, we incompressible Newtonian fluid. Two-dimensional flow field propose a simple method to characterize the chaotic flow: numerically simulated by Fluent (ANSYS, Inc., USA) is used to Analytical solution for the two-dimensional flow field is derived compare the analytical results in the next section. Slug Movement z Moving top wall b c l w c 1 Rear Front Approx. h 3 Approx. b d 2 x 4 a d Moving bottom wall z Both top and bottom wall are moving in negative x−direction. x a y 1. Moving top wall 2. Moving bottom wall 3. Moving side wall (outer) 4. Moving side wall (inner) Fig. 1. 3D model of the slug in a slit microchannel simplified into 2D model with internal recirculating flow with appropriate boundary conditions on moving walls and no shear stress conditions on front and rear edge. The slug is moving in positive x-direction and the wall velocities are in negative x-direction.
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