Effect of surface roughness on single cryogen droplet spreading

Cryogen spray cooling is an auxiliary procedure to dermatologic laser surgery, which consists of precooling the superficial skin layer (epidermis) during laser irradiation of subsurface targets to avoid nonspecific epidermal thermal damage. While previous studies have investigated the interaction of cryogen sprays with microscopically smooth human skin models, it is important to recognize that real human skin surface is far from smooth. With the objective to provide physical insight into the interaction between cryogen sprays and human skin, we study the effect of surface roughness on the impact dynamics of single cryogen droplets falling on skin models of various roughnesses (0.5-70 mu m). We first develop a theoretical model to predict the maximum spread diameter (D(m)) following droplet impingement based on a similarity approximation to the solution of a viscous boundary layer that incorporates friction as the major source of viscous dissipation on a rough surface. Then, we measure the droplet diameter, impact velocity, and D(m) of cryogen droplets falling by gravity onto skin models. Experimental data prove that the proposed model predicts D(m) with good accuracy, suggesting that the effects of surface roughness and friction on D(m) are properly taken into account for the range of surface roughness studied herein.