Dynamic behavior of cryogen spray cooling: effects of spurt duration and spray distance

Cryogen spray cooling (CSC) is used to minimize the risk of epidermal damage during laser dermatologic surgery. Since optimization of CSC permits the safe use of higher light doses, which improves therapeutic outcome in many patients with superficial skin lesions, studies have focused on understanding spray-surface interactions and cooling dynamics. The objective of this study is to measure accurately temperature variations at the sprayed surface and the effects of spurt duration (deltat) and nozzle-to-sprayed surface distance (L) on cooling dynamics during CSC.

A fast-response temperature measurement sensor is built using thin (20 microm) aluminum foil placed on top of a poly methyl-methacrylate resin (Plexiglass) with a 50 microm bead diameter thermocouple positioned in between. Liquid film residence time (t(r)) and minimum surface temperature (T(min)) are systematically measured as a function of deltat and L.

Two distinct spray-surface interaction mechanisms are recognized. The transition between them occurs at a critical length L(c) approximately 25-30 mm. Noticeable characteristics include: (1) for spurts at L < L(c), shorter t(r), and lower T(min) are reached as compared to L > L(c), T(min) is dependent on deltat and L, while t(r) is a function of deltat only; (2) for spurts at L > L(c), T(min) still depends on L but not on deltat, while t(r) becomes a function of both deltat and L. Finally, for all deltat, t(r) reaches a maximum at L = 40 mm.

Based on our results, a good choice to achieve low T(min) and t(r) for the treatment of superficial skin lesions may be met by using deltat of approximately 30-50 milliseconds and the shortest spray distance that is tolerable by the patient. Spurt durations (deltat) of more than 30-50 milliseconds at spray distances (L) greater than L(c) lead to higher T(min) and longer t(r). These parameters may be appropriate for laser therapy of deeper targets.