Abnormal wetting behavior of supercooled deep eutectic solvents

The supercooled liquid state of a deep eutectic solvent (DES) composed of a mixture of choline chloride and citric acid monohydrate (CA) is studied for different molar ratios at room temperature. The supercooled DESs are highly stable and exhibit viscosities exceeding 10⁴ mPa∙s, which increase with higher CA content. The dependence of the viscosity on temperature can be described by the Arrhenius equation, where the activation energy increases with CA content. Due to extremely high viscosity, the surface tension measurement using the pendant drop method requires a stepwise pressurization approach, and the relaxation to equilibrium is quite slow. Surface tensions were found to be considerably high, comparable to that of water, and they slightly decrease with higher CA concentration. The wetting behavior of supercooled DESs on glass is characterized by an unusually slow spreading process. The equilibrium contact angle increases with higher CA content. Moreover, wetting competition of a liquid bridge between dissimilar surfaces was also examined, and it is interesting to find the outcome dependent on the contact time, in addition to surface wettability.