Peculiar Wetting of N, N-Dimethylformamide: Expansion, Contraction, and Self-Running

The nonvolatile N,N-dimethylformamide (DMF) droplet can display peculiar wetting behavior on some substrates such as poly(methyl methacrylate), flame-treated brass, and sapphire. Similar to the expansion of a water drop on a total wetting surface, the DMF droplet shows a spontaneous spreading initially but its spreading dynamics is beyond Tanner's law. The spreading droplet exhibits a ridge near the rim whose height is greater than that of the center. Contrary to typical spreading, the DMF droplet stops its outward expansion at some point and begins inward contraction. Eventually, the droplet shrinks to a spherical cap with a low contact angle within 5 min. This phenomenon may be attributed to the increment of surface tension caused by the adsorption of ambient water vapor. It is interesting to find that, upon addition of surface-active agents, the droplet performs the self-propelled motion after spreading-contraction. The trajectory is random and can be described as the diffusive motion with the diffusivity ∼0.005 to ∼0.01 mm²/s. Unlike self-propulsion driven by reactive wetting, the DMF droplet can cross the trail left by itself. This self-propulsion can be attributed to the effects of ultralow contact angle hysteresis and Marangoni stress. Based on those results, a mechanism explaining the contraction and self-propelled droplet motion is proposed.