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Here, we trap and control the position of droplets to study their dynamicsusing hydrodynamic forces alone without an external field. The hydrodynamictrap is adapted from a previously implemented Stokes trap by incorporating adrop-on-demand system to generate droplets at a T-junction geometry on the samemicrofluidic chip. We then study confined droplet dynamics in response toperturbation by applying a millisecond-pressure pulse to deform trappeddroplets. Droplet shape relaxation after cessation of the pressure pulsefollows an exponential decay. The characteristic droplet shape relaxation timeis obtained from the shape decay curves for aqueous glycerol droplets ofvarying viscosities in the dispersed phase with light and heavy mineral oils inthe continuous phase. Systems were chosen to provide similar equilibriuminterfacial tensions (5-10 mN/m) with wide variations of viscosity ratios. Itis found that the droplet shape relaxation shows a strong dependence on dropletradius, and a weak dependence on the ratio of dispersed to continuous phaseviscosity. The relaxation time is smaller for the highest viscosity ratios,potentially indicating that the dominant viscosity controls the droplet shaperelaxation time in addition to the interfacial tension and droplet size.Droplet shape relaxation time can be used inform the response of droplets in anemulsion when subjected to transient flows in various processing conditions.Finally, an application of this platform for directly visualizing individualdroplet coalescence in a planar extensional flow is presented. The microfluidicfour-channel hydrodynamic trap can thus be applied for studying fundamentalphysics of droplet deformation and droplet-droplet interactions on themicro-scale to provide an enhanced understanding of emulsion behavior on anindividual droplet level.

A 4-channel microfluidic hydrodynamic trap for droplet deformation and coalescence in extensional flows