Capillary forces on a small particle at a liquid-vapor interface: Theory and simulation
YF Tang and SF Cheng, PHYSICAL REVIEW E, 98, 032802 (2018).
We study the meniscus on the outside of a small spherical particle with radius R at a liquid-vapor interface. The liquid is confined in a cylindrical container with a finite radius L and has a contact angle pi/2 at the container surface. The center of the particle is placed at various heights along the central axis of the container. By varying L, we are able to systematically study the crossover of the meniscus from nanometer to macroscopic scales. The meniscus rise or depression on the particle is found to grow as ln(2L/R) when R << L << K-1 with K-1 being the capillary length and saturate to a value predicted by the Derjaguin- James formula when R << K-1 << L. The capillary force on the particle exhibits a linear dependence on the particle's displacement from its equilibrium position at the interface when the displacement is small. The associated spring constant is found to be 27 pi gamma ln(-1) (2L/R) for L << K-1 and saturate to 2 pi gamma ln(-1) (3.7K(-1)/R) for L >> K-1. At nanometer scales, we perform molecular dynamics simulations of the described geometry and the results agree well with the predictions of the macroscopic theory of capillarity. At micrometer to macroscopic scales, comparison to experiments by Anachkov et al. Soft Matter 12, 7632 (2016). shows that the finite span of a liquid-vapor or liquid- liquid interface needs to be considered to interpret experimental data collected with L similar to K--(1).
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