To protect the epidermis during selected dermatological laser surgeries, Cryogen Spray Cooling (CSC) is used as an auxiliary procedure to remove the heat generated in the epidermis by laser irradiation. The impingement of cryogen droplets onto the skin determines not only the effective amount of cryogen deposited on the surface, but also the dynamic behavior of the cryogen layer that forms on the skin surface and thus, it strongly affects the heat extraction process. Recently, it has been shown that the force of the spray alters the skin surface producing an indentation that in turn affects CSC efficiency. To achieve a better understanding of cryogen spray deposition, the impingement of a single cryogen (R-134a) droplet onto indented skin phantoms was experimentally studied. We first built an experimental pressure chamber where the pressure may be raised slightly above the saturation pressure of R-134a (600 kPa), with the objective to suppress and control the rapid evaporation that cryogen droplets undergo as they are released from the nozzles into ambient pressure. This allows us to focus on individual droplet impingement. Two nozzles of different outer diameters were used to generate droplets of different outer diameters were used to generate droplets of different sizes, which fall by gravity onto various indented skin phantoms. To maintain similarity, the range of Weber (We) and Reynolds (Re) numbers measured at the point of impact for typical cryogen spurts were matched with the We and Re of the individual droplets under study. A high-speed digital camera (500 fps) recorded the droplet impingement on indented surfaces resembling in vivo skin indentation measurements. Different indentation geometries were used since studies have shown that skin deformation depends on factors such as nozzle-to-skin distance, skin type and anatomical location. Experimental results showed different droplet impingement dynamics for droplets of two different sizes but same velocity. For We = 130 and Re = 25000 the amount of cryogen left at the bottom of the indentation after two droplets impinged consecutively was equivalent to the total mass of one droplet, whereas for We = 96 and Re = 18500, the amount of cryogen left at the bottom of indentation was twice as much as that after first droplet impingement. The effect of the depth and radius of curvature of the indentations appeared to have small effect on the droplet impingement behavior, compared to that associated with the droplet size (We and Re), as confirmed by the high speed videos.

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