Stability of a compound sessile drop at the axisymmetric configuration

• Published in: Journal of colloid and interface science Vol. 462; pp. 88 - 99
• Main Authors: Zhang, Ying, Chatain, Dominique, Anna, Shelley L., Garoff, Stephen
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 15.01.2016; Elsevier
• Subjects: Bond number; Compound drop; Condensed Matter; Fluid mechanics; Interfacial tension; Materials Science; Mechanics; Partially miscible fluids; Physics; Sessile drop; Surface tension; Compound drop; Sessile drop; Partially miscible fluids; Surface tension; Bond number; Interfacial tension
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2015.09.043

[Display omitted] •Experimental realization of stable axisymmetric compound sessile drops in air.•Criterion established for stable configurations in the zero Bond number limit.•Small density ratio required for large interfacial tension system to be stable.•Stability transition demonstrated using both experiments and simulations. The equilibrium configuration of compound sessile drops has been calculated previously in the absence of gravity. Using the Laplace equations, we establish seven dimensionless parameters describing the axisymmetric configuration in the presence of gravity. The equilibrium axisymmetric configuration can be either stable or unstable depending on the fluid properties. A stability criterion is established by calculating forces on a perturbed Laplacian shape. In the zero Bond number limit, the stability criterion depends on the density ratio, two ratios of interfacial tensions, the volume ratio of the two drops, and the contact angle. We use Surface Evolver to examine the stability of compound sessile drops at small and large Bond numbers and compare with the zero Bond number approximation. Experimentally, we realize a stable axisymmetric compound sessile drop in air, where the buoyancy force exerted by the air is negligible. Finally, using a pair of fluids in which the density ratio can be tuned nearly independently of the interfacial tensions, the stability transition is verified for the axisymmetric configuration. Even though the perturbations are different for the theory, simulations and experiments, both simulations and experiments agree closely with the zero Bond number approximation, exhibiting a small discrepancy at large Bond number.