Phonon Interface Scattering: Kapitza Boundary Resistance

Heat carried by phonons rarely flows across a material interface unimpeded — some fraction is reflected back, producing the thermal boundary (Kapitza) resistance that dominates heat flow at nanoscale contacts. This simulation fires a continuous stream of phonons from a hot reservoir toward a real interface plane between two elastic media, and decides transmission versus reflection using the two standard textbook models: the specular acoustic mismatch model (Snell's law refraction, an impedance-mismatch Fresnel coefficient, and true total-internal-reflection above a critical angle) or the diffuse mismatch model (angle-independent transmission with fully randomised outgoing directions). Live readouts track the theoretical normal-incidence transmission coefficient, the critical angle, and the actually measured transmission fraction accumulated over every interface encounter so far.