Laser-induced thermocapillary flows on a flowing soap film

We propose a focused laser heating method to study wave properties of flowing soap films. The laser-induced thermocapillary flows lead to symmetric disturbances in film thickness. The shock waves originating from the propagating symmetric elastic waves, which have remained elusive despite considerable experimental efforts, are thus stimulated on flowing soap films with low surfactant concentrations, or appreciable elasticities. Interestingly, on soap films with high surfactant concentrations, or vanishingly small elasticities, the laser-induced disturbances in film thickness remain unchanged and flow with the film without propagating, creating ``laser-engraving'' on free liquid films.