1. It is important to note that the surface flow mechanism does not require physical melt layer runoff; a slow creep is sufficient. The Teflon experimental data correlated in Figure 9 have typical calculated surface flow rates in the range of 3 x 10-3 inches/second to 7 x 10-3 inches/second. Clearly, in the few seconds representative of typical test exposures, the material will not appear to move at all. Also, the viscosity-temperacuTVes of Reference 7 show that many materials do not have a discrete "melt point" accompanied by a discontinuous change in viscosity, rather only a trend of continuously decreasing visc 1;sity with increasing temperature. Nevertheless, it is readily visualized that such materials can develop a "de formation layer" wherein the viscosity at the surface is lower than in the substrata. This suggests that the terms "de formation layer" and "liquid layer" are synonymous in the general c<1nte'ltt of a surface flow with increasing subsurface viscosity determined by a temperature gradient.
2. Experimental results on crosshatched ablation patterns