Unraveling stability of a floating liquid marble, its opening and resulting collapse patterns

Abstract

Liquid marbles (LMs) are droplets of liquid enwrapped by hydrophobic particles. LMs can float on a liquid substrate but collapse after a certain time period. In this work, stability of LMs on a liquid substrate has been investigated. LMs were formed by rolling water drops over polytetrafluorethylene (PTFE) powder bed. Collapse patterns and times recorded for these LMs were found to be a function of the core liquid and supporting liquid substrate. Flower-shaped patterns were obtained when there was no surface tension gradient between the core liquid of LM and supporting liquid substrate. However, in case of a finite surface tension difference between the two, extremely porous patterns were formed. In order to capture process of LM collapse, aqueous solutions of methylene blue (MB) were used to form LMs. Surprisingly, these LMs made out of MB solutions exhibited very high stability against collapse. Typically, LMs collapse within a few seconds to a few minutes but LMs made out of MB solutions exhibited stability up to 6 h. The stability of LMs floating on a liquid substrate was found to be directly proportional to the effective surface tension (γeff) or more specifically to the capillary interaction between particles signified by γint.