Experimental And Theoretical Studies On Water Droplet Impacting Dry Solid Surfaces

The interaction between solid surface and impacting droplet is involved in many practical processes such as internal combustion, spray cooling, deposition of pesticide and ink-jet printing, and the impinging results are directly related to the wettability of solid surface and dimensional parameters (Weber, Reynolds, Capillary and Ohnesorge numbers). Also, because of its fundamental points in free surface flow, the expression for dynamic contact angle as a boundary condition as well as non-integrated shear force singularity in the vicinity of triple line, of particular interest is the study on droplet impact on various surfaces. In this paper, the experimental and theoretical study on droplet impact was carried out on different solid surfaces which were fabricated referring to the public literature.Three kinds of surfaces were fabricated, which are polished Cu surface, hydrophilic SiO2 coatings surface and superhydrophobic surface, respectively. Dynamic behavior was examined by the collision of water droplet on the polished copper surface with a wider range of wettability and heated at different temperatures. The experimental results showed the maximum height of the retracting droplet increased with the surface temperature increasing. Moreover, the differences in maximum heights of recoiling droplet were ascribed to surface tension gradient and analysed in the context of lubrication approximation. However, the same law is not ubiquitously accessible for all solid surfaces. In the case of water droplet impinging on hydrophilic surface, the pinned triple line considerably suppressed the additional flow resulting from surface tension gradient and the maximum height of receding droplet is free of heat transfer. For impacting events on superhydrophobic surface, the entrapped air between rigid surface and water droplet effectively prevented heat transfer from the heated surface to the impacting droplet. As a consequence, the maximum receding height of impacting droplet on superhydrophobic surface is also independent of the surface temperature.We fabricated a series of surfaces with wettability gradient in radial/axisymmetric direction by controlling vapor phase diffusion of n-Octyltrichlorosilane (OTS) on common glass slides. The interaction between wettability gradient surfaces and water droplet released at a given height was investigated. Experimental results revealed the maximum spreading diameter was independent of surface wettability. However, surface wettability had important bearings on the receding motion of triple line. In this regard, the receding velocity of triple line responded to wettability gradient distribution. As a consequence, the wettability distribution was responsible for the eventual profile of water droplet after impingement. The dynamic of receding triple line conforms to inertial dewetting.By combination of photolithographic and dry etching, we have fabricated a series of textured silicon surfaces decorated by square arrays of pillars whose radius and pitch can be adjusted independently. The height of the pillars was set at 40μm by controlling etching time. These surfaces displayed a hydrophobic/superhydrophobic property after silanization reaction. The dynamic behavior of water droplet impacting on these structured surfaces was examined using a high-speed camera. Experimental results illustrated that a remaining liquid film on the pillars top gave rise to a wet surface instead of a dry surface as water droplet began to recede off textured surfaces. The remaining liquid film can account for the receding contact angle and contact angle hystersis. The synergistic effect of hysteresis force per unit length (F=σLG (cosθr-cosθY) and the gas-liquid interface instability can be responsible for the occurrence of liquid film. Also, experimental results demonstrated the difference in contact time was ascribed to the solid fraction defined as the ratio of the actual area contacting with liquid to its projected area on textured surface. Since the mechanism by which residual liquid film emerges on the pillars top is essentially ascribed to the pinch-off of the liquid threads, we further addressed the changes in contact time in terms of characteristic time of pinch-off of an imaginary liquid cylinder whose radius is related to solid fraction and the maximum contact area. The agreement of the theoretical analysis and the experimental results substantiates the assumption that the pinch-off of liquid thread should be responsible for the difference in contact time.Likewise, the textured surfaces were fabricated with square arrays of microposts whose height was set at 20μm. After silanization reaction, these textured surfaces displayed hydrophobic state and tended to undergo a wetting transition from Cassie regime to Wenzel regime. The axisymmetric wetting behavior was analysed according to the modified Wenzel equation taking account of the surface roughness in terms of line fraction instead of contact area. Furthermore, the dynamic behavior of droplet impacting on these textured surfaces was also examined. The theoretical analysis, taking consideration of the viscous dissipation and surface roughness, is in a good agreement with experimental results.