Liquid marble granulation using super-hydrophobic powders

Abstract

This work analyses the forces involved in liquid marble formation, primarily the surface energy of the powder on the liquid surface and the powder gravitational force, which we have shown to oppose liquid marble formation. In order to investigate these phenomena, hydrophobic powders possessing both high density and high contact angle need to be employed. For this purpose, superhydrophobic (SH) particles based on a copper substrate were prepared by a silver deposition technique of particle sizes 9, 20, 320 and 400μm having a contact angle with water approaching 160°. Liquid marbles formed using the SH copper substrate were compared with polymer powder (e.g., PMMA) based liquid marbles. Initially, the effective surface tension of the liquid marbles was determined using the maximal height methodology, which indicated that effective surface tension was a function of powder particle size. It is probable that the phenomena of heavy particles give rise to lower effective surface tension, is caused by the weight of the particles reducing the puddle height. Moreover, it was found that in cases with large dense SH powder particles, liquid marble formation was not possible, as the additional weight of the larger particles, lowered the “puddle” to an extent that marble became unstable and collapsed. Therefore, an analysis of the energies associated with the powder particles was undertaken to quantify the physical characteristics of powders that form liquid marbles under: (i) static and (ii) dynamic conditions. In addition, we have correlated this data using a regime map analysis in which we have plotted a surface energy versus a gravitational energy to provide operating regions within the regime map of potential liquid marble formation.