Zeolites are microporous crystalline materials widely used in catalysis and adsorption applications. The fabrication of zeolite thin films and membranes has also opened up the possibility of using zeolites in electronic devices and membrane separations. The existing approach to growing zeolite films involves exposing the substrate to a high-pH environment; however, this process is applicable to only specific types of substrates. Our group has developed the direct wet deposition of zeolites via ultrasonic nozzle spray deposition to address this issue; however, the relationship between wetting properties and thin-film quality has yet to be investigated. In this study, we prepared zeolite CHA (Si:Al:P = 3:10:20) suspensions using different solvents and surfactants in various concentrations. We then examined the relationships among the composition of the cast solution, their wetting behavior on the glass substrate, and the uniformity of the resulting thin films. We found that using ethanol as a solvent with zeolite crystals in low concentrations with added surfactant yielded zeolite films of high quality. We were also able to produce low-haze zeolite coatings on glass. The zeolite coatings with high hydrophilicity and adsorption capacity presented excellent antifogging capability.
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.
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