Observations of solitary wave dynamics of film flows

Experimental results are reported on non-stationary evolution and interacti ons of waves forming on water and water-glycerol solution flowing along an inclined plane. A nonlinear wave generation process leads to a large number of solitary humps with a wide variety of sizes. A fluorescence imaging met hod is applied to capture the evolution of film height in space and time wi th accuracy of a few microns. Coalescence - the inelastic interaction of so litary waves resulting in a single hump - is found to proceed at a timescal e correlated to the difference in height between the interacting waves. The correlation indicates that waves of similar height do not merge. Transient phenomena accompanying coalescence are reported. The front-running ripples recede during coalescence, only to reappear when the new hump recovers its teardrop shape. The tail of the resulting solitary wave develops an elevat ed substrate relative to the front, which decays exponentially in time; bot h observations about the tail confirm theoretical predictions. In experimen ts with water, the elevated back substrate is unstable, yielding to a tail oscillation with wavelength similar to that of the front-running ripples. T his instability plays a key role in two complex interaction phenomena obser ved: the nucleation of a new crest between two interacting solitary humps a nd the splitting of a large hump (that has grown through multiple coalescen ce events) into solitary waves of similar size.