Interfacial stability and shape change of anisotropic endoskeleton droplets

Abstract

The delivery of suspended active ingredients to a surface is a central function of numerous commercial cosmetic, drug, and agricultural formulations. Many products use liquid droplets as a delivery vehicle but, because interfacial tension keeps droplets spherical, these materials cannot exploit the benefits of anisotropic shape and shape change offered by solid colloids. In this work, individual droplet manipulation is used to produce viscoelastic droplets that can stably retain non-spherical shapes by balancing the Laplace pressure of the liquid-liquid interface with the elasticity of an internal crystalline network. A stability criterion is developed for idealized spherocylindrical droplets and shown to agree with experimental data for varying droplet size and rheology. Shape change can be induced in the anisotropic droplets by upsetting the balance of droplet interfacial tension and internal rheology. Using dilution to increase the interfacial tension shows that external stimuli can trigger collapse and shape change in these droplets. The droplets wrap around substrates during collapse, improving contact and adhesion. The model is used to develop design criteria for production of droplets with tunable response.

Temperature-Induced Collapse, and Arrested Collapse, of Anisotropic Endoskeleton Droplets

“…28 The resulting viscoelastic droplets possess a hybrid structure that can be shaped into numerous anisotropic shapes because of an internal elastic network of solids but still behave externally as a liquid droplet by wetting and adhering to compatible surfaces. Such droplets can take on a wide range of shapes, like a solid colloid, while retaining the liquid surface and dissolution properties of a droplet.…”

Arrested coalescence of viscoelastic droplets: triplet shape and restructuring

“…[12,14,15] This work also demonstrates a new aspect of responsiveness and shape-change: restructuring of an underlying elastic framework using the strong driving force of a liquid interface. [16][17][18][19][20][21] We show that numerous complex shapes can be self-assembled even from simple droplet building blocks larger than the thermal limit.…”

“…An example of this was seen previously when the strain of a doublet oscillated between two deformation states because of changes in the local interfacial tension, [8] demonstrating significant degrees of reversibility and responsiveness. [19,20] A key feature of the formation of the stable triplet in Figure 2 is the dynamic behavior of the liquid neck bridging the droplets. It is instructive to examine in closer detail what sorts of changes occur to understand the driving force behind structure formation.…”

Arrested coalescence of viscoelastic droplets: polydisperse doublets

“…It is likely that some restructuring took place within the doublet, or the step change in stress caused an irreversible deformation, whereas more reversible behaviour might be expected for less brittle structures like hydrogels. Figure 11 demonstrates the responsiveness, at small deformations, of viscoelastic structures to subtle changes like dilution but also suggests an approach for triggering an irreversible shape and structural change at larger deformations [12,13]. It also suggests the need for more detailed study of individual droplet response to deformations experienced at different strain levels.…”

“…It has also been suggested to play a role in gas cell stabilization of doughs and batters [8,9] and be a possible mechanism for stabilizing thin liquid films and coatings [10]. The robustness of arrested coalescence also allows creation of shaped droplets by assembly of spherical drops [3,11] or by moulding techniques to produce unique structures that change shape due to external triggers [12,13]. The formation of anisotropic droplet shapes is also relevant to the production of solid particles with non-spherical shapes, e.g.…”