A single coalescence event in a 2D concentrated emulsion in a microchannel can trigger an avalanche of similar events that can destabilize the entire assembly of drops. The sensitive dependence of the process on numerous parameters makes the propagation dynamics appear probabilistic. In this article, a stochastic simulation framework is proposed to understand this collective behavior in a system employing a large number of drops. We discover that the coalescence propagation dynamics exhibit a critical behavior where two outcomes are favored: no avalanche and large avalanches. Our analysis reveals that this behavior is a result of the inherent autocatalytic nature of the process. The effect of the aspect ratio of the drop assembly on the propagation dynamics is studied. We generate a parametric plot that shows the region of the parameter space where the propagation, averaged over the ensemble, is autocatalytic: where the possibility of near destabilization of the drop assembly appears. © The Royal Society of Chemistry.

Raghunathan Rengaswamy

Department Of Chemical Engineering

M. Danny Raj

Aspect ratio

Dynamics

Emulsification

Microchannels

Stochastic models

Stochastic systems

AUTOCATALYTIC

Collective behavior

CONCENTRATED EMULSION

critical behavior

Parameter spaces

PROPAGATION DYNAMICS

Stochastic simulations

Drops

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IIT Madras

Soft Matter

Droplets, as they flow inside a microchannel, interact hydrodynamically to result in spatio-temporal patterns. The nature of the interaction decides the type of collective behaviour observed. In this context, we study the application of droplet microfluidics in the area of complex-shape particle synthesis. We show how the dynamics of droplet motion, the steady-state characteristics, the short and long-range hydrodynamics, the dependence on inlet conditions etc. are all related to the features that characterize a device like the functionality (producing many shapes) and robustness (insensitivity to fluctuations). Two primary operating regimes are identified, one where long-range interactions are dominant and the other where they are short-range. In the former, the shapes formed by droplets are steady-state solutions to the governing equations, while in the latter they are a function of how the droplets enter the channel (frequency of entry). We show that identifying the inlet conditions for producing a particle of the desired shape requires the use of a systematic approach to design which involves solving an optimization problem (using genetic algorithms) to identify the optimal operating strategy. With the knowledge of the hydrodynamics between the droplets, we demonstrate how one can reduce the complexity of the design process. We also discuss the control strategies required if one were to realize the identified operating strategy experimentally. This journal is © The Royal Society of Chemistry.

On the role of hydrodynamic interactions in the engineered-assembly of droplet ensembles

A two-dimensional concentrated emulsion exhibits spontaneous rapid destabilization through an avalanche of coalescence events which propagate through the assembly stochastically. We propose a deterministic model to explain the average dynamics of the avalanching process. The dynamics of the avalanche phenomenon is studied as a function of a composite parameter, the decay time ratio, which characterizes the ratio of the propensity of coalescence to cease propagation to that of propagation. When this ratio is small, the avalanche grows autocatalytically to destabilize the emulsion. Using a scaling analysis, we unravel the relation between a local characteristic of the system and a global system wide effect. The anisotropic nature of local coalescence results in a system size dependent transition from nonautocatalytic to autocatalytic behavior. By incorporating uncertainty into the parameters in the model, several possible realizations of the coalescence avalanche are generated. The results are compared with the Monte Carlo simulations to derive insights into how the uncertainty propagates in the system. © 2017 American Physical Society.

Physical Review E

Averaged model for probabilistic coalescence avalanches in two-dimensional emulsions: Insights into uncertainty propagation

We propose a modeling strategy to simulate drop movement in a two-phase flow inside a 2-D diverging-converging microchannel. These are planar channels that allow 2-D movement of drops. The increasing cross-sectional area of the diverging section decelerates the drop, and the decreasing cross-sectional area of the converging section accelerates it. These drops as they slow down approach each other and start to interact hydrodynamically and form 2-D arrangements inside the microchannel. We propose interacting drop-traffic models, that are phenomenological in nature, to characterize the different interactions of a drop with the neighboring drops, continuous phase and the channel walls. By incorporating these models into a multi-agent simulation, that employs Newton's second law of motion along with the creeping flow approximation, we are able to predict the positions and velocities of all the drops inside the microchannel. The time evolution of the dynamic 2-D patterns formed by the drops inside the microchannel is investigated. We are able to qualitatively understand the features in a microchannel that aid the formation of the 2-D patterns. The simulation strategy helps us to understand the layering phenomena that results in the formation of the 2-D structures near the diverging section and the breaking patterns of drops near the converging section of the microchannel. © 2014 Springer-Verlag Berlin Heidelberg.

Microfluidics and Nanofluidics

Understanding drop-pattern formation in 2-D microchannels: A multi-agent approach

Physical Review Letters

Hydrodynamic Fluctuations in Confined Particle-Laden Fluids

AIChE Journal

Microdroplet coalescences at microchannel junctions with different collision angles

Physics Reports

The physics of 2D microfluidic droplet ensembles

Droplet arrangement and coalescence in diverging/converging microchannels

Coalescence of drops in a 2D microchannel: Critical transitions to autocatalytic behaviour

Journal	Data powered by TypesetSoft Matter
Publisher	Data powered by TypesetRoyal Society of Chemistry
ISSN	1744683X
Open Access	No