Tuning Mixing within a Droplet for Digital Microfluidics

TL;DR

This paper presents a robust method for controlling mixing inside droplets in digital microfluidics by tuning the parameters of sinusoidal rotation to generate resonances, enhancing mixing efficiency and control.

Contribution

It introduces a novel approach to control internal droplet mixing by tuning flow resonances through periodic rotation parameters, demonstrating robustness against rotation shape variations.

Findings

Resonance tuning effectively controls mixing regions.

Shape of rotation function has minor impact on mixing.

Mixing strategy is robust and experimentally feasible.

Abstract

The design of strategies to generate efficient mixing is crucial for a variety of applications, particularly digital microfluidic devices that use small "discrete" fluid volumes (droplets) as fluid carriers and microreactors. In recent work, we have presented an approach for the generation and control of mixing inside a translating spherical droplet. This was accomplished by considering Stokes' flow within a droplet proceeding downstream to which we have superimposed time dependent (sinusoidal) rotation. The mixing obtained is the result of the stretching and folding of material lines which increase exponentially the surface contact between reagents. The mixing strategy relies on the generation of resonances between the steady and the unsteady part of the flow, which is achieved by tuning the parameters of the periodic rotation. Such resonances, in our system, offer the possibility of controlling both the location and the size of the mixing region within the droplet, which may be useful to manufacture inhomogeneous particles (such as Janus particles). While the period and amplitude of the periodic rotation play a major role, it is shown here by using a triangular function that the particular shape of the rotation (as a function of time) has a minor influence. This finding demonstrates the robustness of the proposed mixing strategy, a crucial point for its experimental realization.