Effective surface tension of liquid marbles using controllable nanoparticle monolayers

Abstract

The effective surface tension (γeff) of liquid marbles is a frequently used, but perplexing concept and clarifying this tension has been challenging, mainly due to difficulties in controlling affecting factors from marble surface particles, including particle agglomeration and areal particle density. In this study, we addressed this issue using water marbles covered by nanoparticle monolayers, which produced a simple physical model with controlled particle coverage. The results from sessile and pendant drop measurements reveal that under high particle density, γeff depends on very specific conditions including the measurement method and all liquid marble parameters such as the shape, smoothness, and coverage ratio. The contradictory deviation of γeff from water surface tension with the two methods implies that γeff can reflect only the macroscopic properties of the marbles rather than interparticle forces as was previously assumed. The surface solidity produced by local or overall particle jamming, which is a common, underappreciated phenomenon, is believed to account for this limitation. The concept of equivalence quality is proposed herein, which describes the degree of shape similarity between a liquid marble and a naked droplet, and is thought to be important when addressing the γeff of liquid marbles.