Force measurements for the movement of a water drop on a surface with a surface tension gradient

On a solid surface with a surface tension gradient, a liquid drop is driven toward the more wettable end. To investigate the mechanism for the movement of the drop, we first measured directly the driving force of the drop, using a flexible glass microneedle. We determined that the driving force results from an imbalance of surface tension acting on the liquid-solid contact line on the two opposite sides of the drop. The hydrodynamic force, which is conventionally used as a balance force against the driving force, was not compatible with our results. Here, we show that a balanced drag force with the driving force has to be reconstructed, using a new concept of solidlike friction.