Experimental Study On The Effects Of Surface Roughness On Drop Collision Dynamics

The phenomenon of drop impacting on a solid surface is ubiquitous and has great significance for many technical applications.Since there is no perfectly smooth surface in practice,the drop collision dynamics is affected unavoidably by roughness structure on the substrate.For example,wood is a common combustible material is the building fire,its surface profile could affect the interfacial interactions between the substrate and the drop of extinguishing agent and then affect the fire extinguishing mechanism.However,previous research has less concerned about the influences of surface topography in the drop impingement and lacked for the quantitative understanding of the issue.While in fact,studying the influence of solid surface can not only enrich the knowledge of this interfacial flow problem,but also provide reference for further revealing microscopic mechanisms in engineering applications like spray cooling and water mist fire extinguishing.On this basis,this work adopts different rough substrates respectively with random textures and etched micro-structures to conduct drop impact experiments,intending to reveal the effects of surface topography on drop collision dynamics.According to the experimental results of drop impacting on ordinary rough surfaces,such as wood and glass surfaces,both the number of finger-like perturbations appeared on the drop edge and the maximum spreading diameter are negatively correlated with roughness.The corresponding empirical formula is established through dimensionless analysis and fitting approach.The mechanism of roughness effects on drop behavior can be explained from the perspective of solid-liquid interaction.And based on the drop impact experiments on various hydrophilic square-latticed micropillar arrayed substrates,it turns out that the lamellar shape formed during the inertial spreading stage depends on all the three elements,including drop impact velocity,surface structural feature,and solid wettability.By defining the shape factor and establishing correspondence between shapes and numerical values,now it is available to obtain the pattern of shape formation and development decided by the above-mentioned factors,which are:an increasing initial kinetic energy tends to make the lamellar shape diamond-like,while the wetting tendency of the liquid on the substrate will promote a square shape;and the maximum spreading extent on the hydrophilic micro-structured surface greatly depends on the solid-liquid contact area and the corresponding viscosity loss.Therefore,it decreases with the increase of the impact velocity,but increases with the decreasing packing density of micropillars;moreover,the hemi-wicking phenomenon no longer follows its pattern in a wetting process.Specifically,the length of the fringe film inside the structures does not increase with the square root of time.