When a bulk temperature gradient was applied to a horizontal condensing surface in Marangoni dropwise condensation, the spontaneous movement of condensate drops occurred. The characteristics of the condensate drop movement in a condensate system of water and ethanol binary vapor mixture were experimentally investigated for a wide range of bulk temperature gradients and for various mass fractions. Drops moved from the low-temperature side to the high-temperature side of the heat transfer surface. When the initial drop distance was adopted as a parameter for the Marangoni force acting on the condensate drop together with the surface tension gradient corresponding to the surface temperature of the condensing surface, the drop moving velocity correlated well as a function of both the surface tension gradient and the initial drop distance. In the range of larger initial drop distances, the condensate drop velocity increases as the initial drop distance is reduced and it subsequently decreases after the velocity reaches its maximum value under an almost constant bulk surface tension gradient.

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