Arrested Coalescence of Viscoelastic Droplets: Ellipsoid Shape Effects and Reorientation.

The stable configurations formed by two poroelastic, ellipsoid-shaped droplets during their arrested coalescence have been investigated using micromanipulation experiments. Ellipsoidal droplets are produced by millifluidic emulsification of petrolatum into a yield stress fluid that preserves their elongated shape. The liquid meniscus between droplets can transmit stress and instigate movement of the droplets, from their initial relative position, in order to minimize doublet surface energy. The action of the liquid meniscus causes the ellipsoidal droplets to undergo rolling and reorientation events because of their unique ellipsoid shape and associated variation in the surface curvature. The final configuration of the droplets is controlled by the balance between interfacial Laplace pressure and internal elasticity, as well as a constraint force that resists complete minimization of surface energy. Geometric and surface energy calculations are used to map the possible and most likely configurations of the droplet pairs. Experimental deviations from the calculations indicate the magnitude and potential origin of the constraint force resisting full equilibration. Droplet aspect ratio and elasticity are both shown to influence the degree of reorientation and stability of the droplets at energy extrema. Higher aspect ratios drive greater reorientation and better agreement with final doublet configurations predicted by energy minimization. Lower elasticity droplets undergo secondary deformations at high aspect ratios, further broadening the space of possible morphologies.