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.