Droplet Dynamics After Impacting On Hydrophilic Powder Beds Composed Of Uniform And Non-Uniform Size Particles

An experimental investigation is carried out to study the dynamics of a water droplet after its impact on two types of hydrophilic Cu powder beds (with the contact angle of 64 degrees +/- 10 degrees), composed of uniform and non-uniform size particles, respectively. It is found that a droplet can infiltrate into a hydrophilic Cu powder bed composed of non-uniform size particles, and its penetration time is independent of the Weber (We) number. A droplet cannot infiltrate into a hydrophilic Cu powder bed composed of uniform size particles. Instead, the droplet bounces from the powder bed of uniform size particles (owing to the non-stick particle-coated surface) and forms a liquid marble. Maximum spread factors xi(max) on Cu powder beds are found to be smaller than that on a Cu substrate. Based on experimental data, the maximum spread factor on uniform and non-uniform hydrophilic powder beds can be scaled as xi(max) similar to We(1/10) for 1 < We < 10 and as xi(max) similar to We(1/5) for 10 < We < 40, respectively. It is also found that a hydrophilic powder bed, composed of small uniform size Cu particles (with an average diameter of 2.3 mu m), behaves almost like a "super-hydrophobic" substrate, where the droplet can jump repeatedly after its impact on the powder bed. The droplet contact time after impacting on this powder bed is equal to the first-order oscillation period of the droplet.