An experimental study of Marangoni flows in horizontal layers of tin based metallic
alloys heated from the side is carried out, based on the observation of the motion of tracer
particles at the surface associated with a direct measurement of temperatures in the melt. A key
feature of the procedure is that the experiments are performed in a high vacuum environment
to limit as much as possible the pollution of the liquid vapour interface. In pure tin, depending
on the intensity of the driving force measured by the Reynolds-Marangoni number, either
viscous or boundary layer flow regimes are found. In addition, the onset of temperature
oscillations is found to be consistent with the hydrothermal wave instability mechanism
proposed by Smith and Davis. In tin-bismuth alloys, the observed flow directions and velocities
are interpreted in terms of a non linear dependence of the surface tension on temperature, and
successfully compared with the dependence predicted by interpolating the available
experimental surface tension data with the help of a simple thermodynamic model. During the
directional solidification of the layer, the concentration gradient in the solute rich boundary
layer ahead of the growth front is found to locally modify the flow pattern. For instance, when
thermal and solutal driving forces act in the same direction, temperature oscillations are
observed at low values of the Reynolds-Marangoni number. As for opposing driving forces,
the situation is that a local reversal of the flow direction is observed in correlation with the
morphological instability of the growth front.