“Nanoscale physics”   
Specificity of boundary conditions for laser-stimulated destructive surface treatment without melting
L.V. Shmeleva, A.D. Suprun, S.M. Yezhov , V.V. Datsyuk
 Department of Theoretical Physics, Faculty of Physics, Taras Shevchenko National University of Kyiv. Volodymyrska Street, 64/13, Kyiv-01601, Ukraine. 
E-mail: lshmel@univ.kiev.ua
Irradiation of a solid surface by a laser pulse of high intensity can result in the removal of the substance from the irradiated surface. In the process of laser ablation, the substance evaporates or sublimates in the form of a plasma formed over the surface of the sample. When the density of the laser radiation exceeds the threshold of the ablation mode, a microexplosion occurs with the formation of nanostructured crater cavities on this surface. Under conditions when the liquid phase is not formed on the surface, special boundary conditions arise between the solid and plasma phases, taking into account the crater formation process. Exactly such a problem was considered in the research. 
The problem of surface destruction by laser pulsed radiation is based on a substantially inhomogeneous system of equations of continuum mechanics. The inhomogeneity is associated with the presence of volumetric sources and sinks in the gas phase during a phase transition solid - plasma. For their inclusion, the sources-sinks function of mass R was introduced into the system of equations, which has the dimension of mass per unit volume per unit of time and the mass density of the force f, which acts on the gas flow from the side of irradiation and has the dimension of acceleration. The function R and the force f are specified by the specific formulation of the problem and their presence is another feature of the problem under consideration. 
The boundary conditions the surface destruction problem determined at the boundary line of the solid - plasma. Therefore, the following procedure was applied to correctly determine the boundary conditions: the integration of the equations of the original system along the volume covering the surface of the phase separation in the processing region, was carried out. Then, the volume of integration was tightening to the surface in such a way that in the direction perpendicular to the surface, this volume became infinitely narrow. This approach allows us to ignore time variables. All boundary conditions are formulated: of the balance of mass flow, momentum flux balance and energy flow balance. These conditions are formulated in the most general form in a local coordinate system.