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.

/pdf/on-the-fly-formation-of-polymer-film-at-water-surface-112x0fwk.pdf

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

While various attempts have been made to understand the mechanism of spontaneous emulsification within the scopes of equilibrium and nonequilibrium thermodynamics, the local processes underlying spontaneous emulsification still remain elusive. In this research, we investigate the local processes that involve the transfer of surfactants as well as water from an aqueous phase to oil, which results in the formation of a water-in-oil emulsion in the organic phase. Subsequently, these emulsions invert into the oil-in-water emulsion rather abruptly as they cross the phase boundary. Studies based on ultraviolet (UV) spectroscopy and nonequilibrium molecular dynamics simulations indicate that the crossing of the phase boundary may involve catastrophic explosions and subsequent assembly of the resulting fragments to other organized structures. These processes lead to the fluctuation of the component concentrations below the interface that also becomes evident in the fast (athermal) diffusion of the emulsion droplets from the interfacial region farther into bulk water. Spontaneous emulsification is found to be amplified in mixed solvents, but it can be arrested with additives that reduce solubility or inhibit the transfer of water and surfactants in the organic phase. 

Spontaneous Emulsification: Elucidation of the Local Processes

Micro and/or Nano sized emulsions are formed when an organic liquid gently comes in contact with water in the presence of a surfactant, where no external agitation is required. Many years of research made it clear that the driving force for spontaneous emulsification arises from the differences of the chemical potentials of various components in the organic and aqueous phases, which triggers diffusion coupled hydrodynamic fluctuation. While extraordinary theoretical developments have taken place that attempted to describe these processes within the scopes of equilibrium and non-equilibrium thermodynamics, the local processes underlying the spontaneous emulsification, however, still remain elusive. In this research, we investigate the local processes that involve the transfer of surfactant as well as water from one phase to another (i.e. water to oil), which results in the formation of water-in-oil emulsion in the organic phase and, subsequently. Thes emulsions invert into oil-in-water emulsion, rather abruptly, as they cross the phase boundary. Studies based on UV spectroscopy and molecular dynamics indicate that these processes may involve explosive events and subsequent assembly of the fragments to other organized structures which are reminiscent of cusp catastrophe proposed earlier by Dickinson. These processes lead to either to a strong or a weak fluctuation of the component concentrations below the interface that also becomes evident in the fast (athermal) diffusion of the emulsion droplets from the interfacial region farther into the bulk water. These events can be arrested suitably with polymeric additives.

pdf/spontaneous-emulsification-elucidation-of-the-local-2u053c13.pdf

Micro and/or Nano sized emulsions are formed when an organic liquid gently comes in contact with water in the presence of a surfactant, where no external agitation is required. Many years of research made it clear that the driving force for spontaneous emulsification arises from the differences of the chemical potentials of various components in the organic and aqueous phases, which triggers diffusion coupled hydrodynamic fluctuation. While extraordinary theoretical developments have taken place that attempted to describe these processes within the scopes of equilibrium and non-equilibrium thermodynamics, the local processes underlying the spontaneous emulsification, however, still remain elusive. In this research, we investigate the local processes that involve the transfer of surfactant as well as water from one phase to another (i.e. water to oil), which results in the formation of water-in-oil emulsion in the organic phase and, subsequently. Thes emulsions invert into oil-in-water emulsion, rather abruptly, as they cross the phase boundary. Studies based on UV spectroscopy and molecular dynamics indicate that these processes may involve explosive events and subsequent assembly of the fragments to other organized structures which are reminiscent of cusp catastrophe proposed earlier by Dickinson. These processes lead to either to a strong or a weak fluctuation of the component concentrations below the interface that also becomes evident in the fast (athermal) diffusion of the emulsion droplets from the interfacial region farther into the bulk water. These events can be arrested suitably with polymeric additives. 

/pdf/spontaneous-emulsification-elucidation-of-the-local-24vgjh6g.pdf

Multicomponent counter mass transfer in liquid-liquid extraction in presence of spontaneous interfacial convection

Detection and characteristics of spontaneous interfacial convection

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Multicomponent counter mass transfer in liquid-liquid extraction in presence of spontaneous interfacial convection

Detection and characteristics of spontaneous interfacial convection