The effect of roughness on the impact dynamics and heat transfer of cryogen droplets impinging onto indented skin phantoms

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 μm to 50 μm. 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. NOMENCLATURE As maximum covered area of spread [mm] Ar covered area after retreat [mm] Cd drag coefficient do outer diameter of nozzle [mm] D diameter of droplet [mm] d depth of indentation [mm] g gravity [ms] r radius of indentation [mm] Ra Surface roughness [μm] Re Reynolds number (ρVD/η) t time [ms] ts spread time [ms] tr retreat time duration [ms] Tsat saturation temperature [C] V droplet velocity [m/s] Vb break up velocity of droplet [m/s] We Webber number (D ρV/σ) η dynamic viscosity [ N s/m] σ surface tension [N/m] INTRODUCTION Laser dermatological surgery is the treatment of choice for cosmetic (e.g., hair [1] and tattoo [2] removal) as well as for vascular lesions (e.g., hemangiomas [3] and port wine stain (PWS) birthmarks [4]). For many of these treatments, cryogen spray cooling (CSC) is an essential auxiliary method that protects the epidermis from excessive thermal damage during laser irradiation, while the intended 1 Copyright © 2005 by ASME target, such as hypertrophic blood vessels located 100–500 μm below the skin surface [5,6] are thermally photocoagulated. The only cryogen used for this purpose, approved by the FDA [6] is Tetrafluoroethane-1,1,1,2 (R134a), with a boiling temperature of -26 C at ambient pressure. Short cryogen spurts (20–100ms) [6] are released from a pressurized container through a spray valve/nozzle system. Well-atomized cryogen droplets with diameters ranging between 3–20 μm [7] and velocities 10–60 m/s [8] impinge onto human skin and extract heat as they spread and evaporate. The efficiency of the heat extraction during spray deposition is largely dictated by the dynamics of droplet impingement [9]. Moreover, during CSC, the momentum of the spray droplets causes skin indentations al large as 2 mm in depth. In a previous study, we investigated the effect of skin indentation on the heat extraction during CSC [11] using indented and smooth skin phantoms. It was seen that the maximum heat flux could be diminished by as much as 30% on phantoms with indentations of ≈ 2 mm in depth. Evidently, human skin surface is not smooth and its roughness (typically between 50 to 150 μm depending on anatomical location, age, race, etc. [10]) is likely to influence the efficiency of the heat extraction. Therefore, to better understand how CSC efficiency may be optimized, we decided to investigate single cryogen droplet impingement dynamics and its relationship to the heat extraction using indented and rough human skin phantoms.