Destruction of waves and formation of rivulets on the surface of a heated liquid film at Re = 10

The mechanism of transformation of developed naturally formed waves into thermocapillary-wave structures in the vertical flow of a heated water film was studied experimentally. The motion of large waves whose width significantly exceeds the distance between the thermocapillary rivulets (transverse inhomogeneities of thickness and temperature), which have already formed in the residual film layer in front of the incoming large wave was studied in detail. It is shown that combination of thermocapillary and inertial effects in a wavy liquid film leads to deformation of large waves and their decay into the waves of a smaller width. Various scenarios of large wave destruction are considered. It is found out that when medium and small waves move, the thermocapillary forces change the distance between their crests, adjusting them to the distance between the forming structures in the residual layer. The regime maps, where the flow is divided into two zones: the region of deformation of three-dimensional waves and the region of developed rivulet flow, have been plotted for different heat fluxes. Various scenarios of changes in three-dimensional waves occur in the region of their deformation. In the region of developed rivulet flow, the motion of waves obeys the general laws. Waves propagate only along the rivulets with the average inter-rivulet distance equal to five capillary constants. Between the rivulets, the film is smooth. The amplitude of waves (the maximal film thickness) in the rivulets and their velocity increase as they move along the heater.