Investigating Arrangement of Composite Drops in Two-Dimensional Microchannels Using Multiagent Simulations: A Design Perspective

Abstract

Drops can self-organize in two-dimensional (2D) microchannels to form ordered arrangements. Design of microchannels that result in a particular pattern of drops for a specific purpose is difficult and nonintuitive due to the inherent complexity of the drop dynamics. We address the problem of understanding the arrangement of composite drops inside a microchannel. A multiagent modeling strategy that was recently proposed by us is employed to understand this complex design problem. We consider the design of a drop−drop contactor that results in an equal mix of A and B. We seek to find the inlet sequence of drops A and B that would result in the maximum contact between A and B in the ordered arrangement. We find that intuition-based results work well only for a single layer arrangement of drops. We attribute this anomalous behavior to the symmetry breaking instability of the drop patterns in these 2D microchannels. From the dynamic simulations, we understand why certain inlet sequences perform better than others. We then discuss the use of the simulation strategy in several possible design problems. INTRODUCTIONDrop microfluidics allows the discretization of one of the phases (drop) in another continuous phase. This two phase flow finds applications in several fields 1,2 such as reaction engineering, 3−5 chemical and biological analysis, 6−8 emulsion science, 9−11 drug discovery 1 and synthesis, 12−14 and solute extraction. 15,16 The drops can be merged, 17,18 split, 19,20 synchronized, 21−23 sorted, 24,25 and incubated 26−28 inside the microchannels through active control strategies 25,29−31 or passive design. 19,32−34 The tremendous application potential for drop-based microfluidics can be realized if efficient design strategies for optimized performance are available. Understanding the movement of drops inside the channel becomes crucial in device design. Presence of a drop complicates the flow fields making the flow problem nonlinear and sometimes multiscale, which renders the solution to the drop flow problem computationally demanding. However, from a design perspective, a simple model is preferred because it allows its incorporation into an optimization routine for generating designs for specific objectives. 32 Drop flows in one-dimensional (1D) microchannel where the drop size is of the order of the channel diameter have been modeled using a simple network model. 35−37 The simplicity of the model allowed its incorporation in a model predictive control routine 39,40 for active synchronization and sorting of drops and genetic algorithm-based optimization routine 32 for passive design of ladder networks to expand flip, contract, or synchronize drops. However, such studies are not currently available for two-dimensional (2D) channels.One could conceive of several fascinating applications for 2D channels if rational design approaches exist. As an example, one of the applications in colloidal science could be the formation of anisotropic particles, 12,41 through self-assembly of drops in a 2D microchannel. 10,11,1...

Microfluidic formation of crystal-like structures

“…The observed layering (a symmetry breaking instability) was caused by the infinitesimal local perturbations around the droplets entering the expansion channel. The results by Danny Raj et al 44 demonstrated even further the complexity of the formation of crystal-like structures in microfluidic device and called for further studies targeting a general framework to describe the observed structures.…”

“…In applications such as synthesis of anisotropic particles 41,42 or microfluidic crystals 43 formed by polymerised droplets having different properties, it is important to know how to inject the two droplets types in order to obtain the desired final structures. Numerical simulations carried out by Danny Raj et al 44 using the same geometry of Jose and Cubaud 24 studied the effect of the injection policy of two droplets types called 'A' and 'B', obtaining also some counterintuitive results. For instance, they demonstrated that an injection policy ABAB led to a microfluidic crystal where the succession of particles follow the order ABAB only when the crystal was 1D.…”