title: Spontaneous self-coating of a water drop by flaky copper powders: critical role of the particle shape.
abstract: The self-coating process of solid particles over a liquid drop is important for the formation of a liquid marble. Generally, some external forces such as rolling or flipping are used to cover a drop by small particles. In this work, it is observed that flaky copper powders can spontaneously spread over the planar water surface and form a dense flat cluster with a fractal dimension of 2. Moreover, flaky copper powders can cover the water pendant and sessile drops spontaneously and rapidly. This powder-coated drop can roll on an inclined plane at a relatively high speed. However, spontaneous self-coating disappears for spheroidal copper powders. To explain our observations, the shape factors of particles are introduced into the spreading coefficient S for powders on the liquid surface. The flaky powders have the lowest shape factors and therefore spontaneous self-coating formation, with S > 0.
authors.affiliation: Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan. yjsheng@ntu.edu.tw.
authors.forename: Cheng-Chung
authors.initials: CC
authors.lastname: Chang
authors.forename: Cyuan-Jhang
authors.initials: CJ
authors.lastname: Wu
authors.forename: Yu-Jane
authors.initials: YJ
authors.lastname: Sheng
authors.forename: Heng-Kwong
authors.initials: HK
authors.lastname: Tsao