The mechanism of spreading on liquids

This spreading as a thick, unstable sheet, is probably due to the motions of the molecules of the oil, diffusing in contact with the water, transmitting the outward motion to the upper layers of the oil by viscosity. 

Leslie s explanation,8 that the spreading is due to an attraction exerted by the lower liquid on molecules of the upper liquid beyond the first layer, squeezing out those in the first layer, cannot be correct, or the final state could not be a monomolecular film in equilibrium with excess liquid in drops—the drops would be squeezed out along the surface. 

The velocity of spreading on water has been observed by Brinkman,9 Cary and Rideal,10 Ramdas,11 Woog,12 and others. It is often of the order 20 cm. per 

8 With mixtures of substances, thick spread sheets may persist for a very long time (Hardy, Proc. Ray. Soc. A, 88, 316 (1913)). This is familiar when paraffin oil is spilt on puddles, and is of vital importance in killing mosquito larvae on water. 

Solids spread on liquids to a monomolecular film, as has been described in Chapter II.4 Owing to the non-mobility of the particles of a solid, the molecules which are driven out next the water cannot drag a thick layer of the solid along with them, as happens when liquids spread. 

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