Arrested coalescence of viscoelastic droplets with internal microstructure

Abstract

There are many new approaches to designing complex anisotropic colloids, often using droplets as templates. However, droplets themselves can be designed to form anisotropic shapes without any external templates. One approach is to arrest binary droplet coalescence at an intermediate stage before a spherical shape is formed. Further shape relaxation of such anisotropic, arrested structures is retarded by droplet elasticity, either interfacial or internal. In this article we study coalescence of structured droplets, containing a network of anisotropic colloids, whose internal elasticity provides a resistance to full shape relaxation and interfacial energy minimization during coalescence. Precise tuning of droplet elasticity arrests coalescence at different stages and leads to various anisotropic shapes, ranging from doublets to ellipsoids. A simple model balancing interfacial and elastic energy is used to explain experimentally observed coalescence arrest in viscoelastic droplets. During coalescence of structured droplets the interfacial energy is continuously reduced while the elastic energy is increased by compression of the internal structure and, when the two processes balance one another, coalescence is arrested. Experimentally we observe that if either interfacial energy or elasticity dominates, total coalescence or total stability of droplets results. The stabilization mechanism is directly analogous to that in a Pickering emulsion, though here the resistance to coalescence is provided via an internal volume-based, rather than surface, structure. This study provides guidelines for designing anisotropic droplets by arrested coalescence but also explains some observations of "partial" coalescence observed in commercial foods like ice cream and whipped cream.

Arrested coalescence of viscoelastic droplets: triplet shape and restructuring

“…Previous work showed that droplets containing an elastic network of crystals can initiate but not finish coalescence, creating a range of anisotropic intermediate droplet shapes, [1,10] but only recently has a physical model of the process for uniform [7] and non-uniform [8] droplet pairs been developed. Essentially the interfacial pressure driving two droplets to minimize their area must be offset by the internal elasticity to stably arrest coalescence at an intermediate state: a liquid doublet rather than a sphere.…”

“…Essentially the interfacial pressure driving two droplets to minimize their area must be offset by the internal elasticity to stably arrest coalescence at an intermediate state: a liquid doublet rather than a sphere. [7] In such cases the exact doublet shape represents one of a number of intermediate points between the beginning state of two discrete droplets and a completely coalesced single sphere and is determined by the level of solids in the droplet and the point at which coalescence is arrested. [7] We extend that work here by examining the dynamic behavior of an aggregate of three arrested droplets, a triplet, as multiple connections are the basis for many fluid microstructures formed during the manufacture of food, coatings, and material templates like bijels.…”

“…[7] In such cases the exact doublet shape represents one of a number of intermediate points between the beginning state of two discrete droplets and a completely coalesced single sphere and is determined by the level of solids in the droplet and the point at which coalescence is arrested. [7] We extend that work here by examining the dynamic behavior of an aggregate of three arrested droplets, a triplet, as multiple connections are the basis for many fluid microstructures formed during the manufacture of food, coatings, and material templates like bijels. [11] Unlike aggregated solid particles, however, arrested droplets are permeated by a liquid oil phase that can adhere easily to other surfaces and can form a neck that bridges the two droplets.…”

“…Our ultimate goal is to build on previous work on local deformations of structured emulsion droplets [7,8] to realistically simulate the formation of full-scale emulsion networks [12,13] and more complex coalescence phenomena. [13] We increase the complexity of scrutiny here by studying triplets of structured droplets as these break the symmetry of the doublets previously examined, exhibiting new mechanisms of shape-change and construction.…”

“…Coalescence can be prevented by a number of mechanisms, but one common to many complex fluids like foods is the arrest resulting from colloids that provide an offsetting rheological resistance. [1][2][3] Arrest occurs once coalescence is initiated but can not complete because of a halt in the ability to reduce droplet surface area [4][5][6] or increase deformation [7,8] to minimize system energy. While both types of arrest play a role in practical microstructures, the surface, or Pickering, type can have significant limits on the number of connections formed between droplets [9] because the end result is essentially a solid interfacial shell.…”