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

Abstract

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 mm 2 /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.

On the Impact of Ambient Humidity and Evaporation on Demixing of Binary Mixtures

“…For instance, Marié et al and Li et al discovered that ambient moisture could condense on highly volatile droplets of another liquid (not even miscible with water) due to their strong evaporative cooling, giving rise to non-trivial effects. Wang et al, Hu et al, and Parimalanathan et al point out the role of moisture absorption (hygroscopicity) on spreading/contraction regimes of sessile droplets. Furthermore, the heat generated due to moisture condensation or absorption can also play a prominent role. , …”

Marangoni spreading and contracting three-component droplets on completely wetting surfaces

“…Other studies have investigated flower-like patterns with liquid-on-liquid spreading (19), Marangoni bursting with H 2 O/EtOH on oil (20), or even evolution of complex droplet behaviors in automated multicomponent droplet-in-oil experiments (21). Spontaneous reversal from spreading to retraction has been observed as oil/surfactant mixtures become autophobic (22) or with N,N -dimethylformamide in humid environments (23).…”