AN ANALYSIS OF THE EFFECT OF SURFACTANT ON THE LEVELING BEHAVIOR OF ATHIN LIQUID COATING LAYER

A linear analysis, employing the lubrication approximation, is present ed for the leveling history of a thin layer of Newtonian liquid. When the surface tension is taken to be constant, the classic result of Orc hard is reproduced. As expected, the presence of surfactant, with resu lting surface-tension-gradient forces, slows the rate of leveling of a n initial train of periodic ripples. For very weak surfactants, initia l leveling rates are unaffected, but ripple amplitude can be lowered o nly to a ''plateau'' level which persists for long times. Surprisingly , critical surfactant concentrations exist, depending on other system parameters such as the coating thickness and ripple wavelength, for wh ich leveling is maximally retarded. To the extent that this retardatio n is generally undesirable, it is suggested that these critical values be avoided, even by increasing the amount of surfactant present. This anomalous effect is maximized for an assumed nondiffusing, insoluble surfactant and is mitigated by either surface or bulk diffusion of sur factant.