Optical control of surface forces and instabilities in foam films using photosurfactants

Abstract

Molecular interactions in thin liquid films, such as the disjoining pressure, are involved in interfacial phenomena such as emulsion and foam stabilization. In this article we show that through light stimulation we can control remotely the disjoining pressure in a thin liquid film stabilized by a photosurfactant. We stabilize a horizontal thin liquid film using a cationic photosurfactant, AzoTAB, bearing an azobenzene moiety on the hydrophobic tail which can switch from a trans to a cis conformation upon light stimulation. As the film is illuminated at specific wavelengths the AzoTAB molecules switch continuously their conformation and consequently their interface affinity. The main consequence of stimulating the film with light is increasing the ratio of cis in the film. This provokes a desorption flux, and an increase in the concentration of free surfactants, as the CMC of the cis isomer is higher than that of the trans isomer. Therefore the electrostatic repulsion between the surfactant layers that stabilize the film decreases, inducing an instability in the film thickness. For films with a thickness between 20 nm and 60 nm, we observe the formation of spherical caps up to 100 μm wide, whose shape is controlled by the competition between surface tension and disjoining pressure. The motion of these caps in the film is restrained by the surface viscosity of the surfactant layers. In addition, for thicknesses below 40 nm and depending on light intensity, we can observe flat stratified islands up to 100 μm wide, with thickness steps corresponding to the size of a surfactant micelle. We suggest that this second instability is due to the oscillation of the disjoining pressure isotherm under light.

A multi-scale perspective of gas transport through soap-film membranes

“…As a representative example, a thin film pressure balance (TFPB) represents a convenient system where nanoscale properties can be precisely controlled and investigated in a relatively easy manner as shown in numerous studies. [47][48][49][50][51][52] In this work, we have focused on the gas transport across soap films by means of predictive multiscale numerical models. In particular, we used a combination of molecular and continuum models to characterise and study the transport of gases through soap film membranes.…”

Self‐Assembly of Photoresponsive Molecular Amphiphiles in Aqueous Media

“…The results obtained from experiments with macroscopic foams showed that the light‐induced flow can slow down the foam drainage at the early stage of UV light irradiation, while a rapid rupture of foams was subsequently observed. Next, the influence of illumination on disjoining pressures of thin liquid films in identical foam systems was investigated, suggesting that the UV‐light‐induced rupture of foams was due to a decrease of electrostatic repulsion in the thin foam film as well as the oscillation of the disjoining pressure isotherm [132] . This systematic study of an amphiphile‐based photoresponsive system at the air–water interface emphasizes the prospects for application in remote control (by light) of foam stability.…”

Reversible Destabilization of UV‐Responsive Polymer Particles (Latex) using a Photoresponsive Surfactant

“…The above methods are either energy intensive or require chemical additives or specific monomers. There is also a significant body of work describing photo‐switchable surfactants . Recently, Kondo and coworkers demonstrated that UV light can be used to trigger demulsification of oil droplets dispersed in water stabilized by diazene‐based surfactants .…”

“…There is also a significant body of work describing photoswitchable surfactants. [19][20][21][22][23][24] Recently, Kondo and coworkers demonstrated that UV light can be used to trigger demulsification of oil droplets dispersed in water stabilized by diazenebased surfactants. [25,26] Diazene-based surfactants are able to undergo trans-cis isomerization when irradiated by UV-light (Scheme 1A) -the trans form is a more effective surfactant than the cis form.…”