The effect of roughness on the impact dynamics and heat transfer of cryogen droplets impinging onto indented skin phantoms
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abstract
Laser dermatological surgery (LDS) is the preferred therapeutic modality for various dermatoses, including port wine stain (PWS) birthmarks. LDS is commonly used in conjunction with cryogen spray cooling, which is an auxiliary procedure that pre-cools the superficial skin layer (epidermis) prior to laser irradiation to avoid non-specific and excessive epidermal heating. Clinical observations show that skin indents markedly during spray deposition due to the large momentum of cryogen droplets. Furthermore, the human skin surface is far from smooth. Therefore, with the objective to provide some insight into the interaction between cryogen sprays and the rough and deformable human skin surface, the impingement dynamics and heat transfer induced by single cryogen droplets falling on rough and indented skin phantoms are present in this paper. Epoxy skin phantoms with a constant semispherical indentation of depth and radius of 2.44 mm and 6.34 mm, respectively, were used to simulate indented skin. Each phantom had a different surface roughnesses varying from 0.5 m to 50m. The experiments were carried out within a pressurized chamber to control or eliminate droplet evaporation. A high-speed camera and the temperature sensors placed on the upper surface of the skin phantoms were synchronized to record the impact dynamics and temperature changes as cryogen droplets fell on them. The results show that the surface roughness affects the impact dynamics and heat transfer during single droplet impingement. As the surface roughness (Ra) increasing, the heat flux decrease.
