On-the-Fly Formation of Polymer Film at Water Surface

The self-propulsion of bodies floating in water is of great interest for developing new robotic and intelligent systems at different scales, and whenever possible, Marangoni propulsion is an attractive candidate for the locomotion of untethered micro-robots. Significant cases have been shown using liquid and solid surfactants that allow an effective propulsion for bodies floating on water to be achieved. Here, we show for the first time a strategy for activating a twofold functionality where the self-propulsion of a floating body is combined with the formation of a polymer thin film at the water surface. In fact, we demonstrate that by using polymer droplets with an appropriate concentration of solvent and delivering such drops at specific locations onto freely floating objects, it is possible to form “on-the-fly” thin polymer films at the free water surface. By exploiting self-propulsion, a polymer thin film can be formed that could cover quite extensive areas with different shapes depending on the motion of the floating object. This intriguing twice-functionality activated though a single phenomenon, i.e., film formation and related locomotion, could be used in perspective to perform complex operations at water surfaces, such as dynamic liquid packaging, cleaning, and moving away floating particles, monolayer films, or macro-sized objects, as discussed in the text.

Mini-Generator of Electrical Power Exploiting the Marangoni Flow Inspired Self-Propulsion

The mini-generator of electrical energy exploiting Marangoni soluto-capillary flows is reported. The interfacial flows are created by molecules of camphor emitted by the "camphor engines" placed on floating polymer rotors bearing permanent magnets. Camphor molecules adsorbed by the water/vapor interface decrease its surface tension and create the stresses resulting in the rotation of the system. The alternative magnetic flux in turn creates the current in the stationary coil. The long-lasting nature of rotation (approximately 10-20 h) should be emphasized. The brake-specific fuel consumption of the reported generator is better than that reported for the best reported electrical generators. Various engineering implementations of the mini-generator are reported.

Spreading of Oil-in-Water Emulsions on Water Surface

This work presents the spreading behavior of oil-in-water (o/w) emulsions on the water surface recorded using a high-speed photography method. We study a series of o/w emulsions with two different droplet sizes of 4.50 and 0.75 μm and volume fractions of the oil phase in the 20-80% range. Results show that for all the emulsions a rapid spreading occurs upon the collision with the water surface, which then forms a thin film expanding with time. Appearance of a dry spot in the center of collision is observed in the spreading of the emulsions in midvolume fraction range that induces a bursting-like spreading. For the highly concentrated emulsions, the deliberation of decompression energy from the deformed oil phase droplets inhibits the bursting, increases the equilibrium propagation radius, and reduces the dissipation time. The role of viscoplasticity (existing of the yield stress) is considered and a model describing the propagation step of the emulsion spreading is presented. The model shows that the peculiarities of the spreading are determined by the competition between yielding, plastic viscosity, and interfacial tension. By comparing the model prediction and experimental results, it is suggested that the spreading behavior of the emulsions is not only a consequence of the surface tension gradient but also the coalescence of the oil droplets during spreading.