43822-AC5
Liquids Spreading and Recoiling on Liquids: The Influence of Interfacial Chemical Reactions
We have been investigating problems involving interfacial films where the interfacial material affects the fluid dynamics. In a first study we focused on liquid lenses floating on a liquid interface. In a second project, we studied sedimentation of liquid drops in a second immisible liquid when a chemical reaction occurs at the interface. In a third independent study we introduced a new hypothesis for predicting the thickness of soap films.
We begin by describing the motion of a drop of liquid floating on a liquid substrate. This topic falls into the literature on spreading of thin films, which is vast due to many applications in the fields of medicine, coatings and detergency amongst others. In our work, we describe an unusual case of surface-tension-driven flow where changes in surface tension produced by a chemical reaction at an interface drive a non-volatile liquid droplet to spread into a thin film on a second immiscible non-volatile liquid. The droplet spreads rapidly, reaches a maximum radius, and then slowly recoils back to a compact liquid lens. Although work has been reported on a great variety of problems involving (im)miscible and/or (non-)volatile surfactants spreading and/or retracting, to the best of our knowledge the spreading and retraction in the same system with non-volatile immiscible fluids has not been reported.
In particular, droplets of oil containing oleic acid were observed to spread, then recoil, on an aqueous solution of sodium hydroxide. Surfactant is produced at the interface during spreading, and for reagent concentrations of order 1 mM spreading is observed to be much faster than in the absence of a chemical reaction (we have quantified the rate of spreading by measuring the radius as a function of time). After about 10 seconds, drops reach a maximum radius, which is about 3-4 times the initial radius. Spreading is faster and the maximum radius is larger for higher concentrations of reagents. The drops are then observed to recoil, which we explain by the diffusion of surfactant away from the oil/water interface, with the rate of recoil being controlled by the NaOH concentration in the subphase.
In a second project, we examined sedimentation of liquid drops in a second immisible liquid when a chemical reaction occurs at the interface. The two liquids that we are using are oleic acid, which is an anionic surfactant, and cetyl trimethylammonium bromide (CTAB), which is a cationic surfactant. Experimentally, we have observed that when CTAB solutions drops are injected in oleic acid, there is a reaction at the interface, which modifies the shape of the drop.
Some of the observations made so far are that at lower CTAB concentrations the drops (2 mM, 5 mM) deform from their spherical shapes into prolate shapes. At concentrations of 10mM CTAB the drops seem to react and vibrate as they sediment. With a 15 mM concentrated CTAB solution the drops start to release bits of “skin” every 4~5 seconds, while at 20 mM the drops release this interfacial material but form a vertical straight threadlike structure. At 30 mM, 40 mM and 50 mM the drops grow long tails (e.g. Figure 2, right) that eventually break. We have almost completed a model for our observations and are now trying to write up our results for publication.
Finally, we considered the thickness of soap films formed by withdrawal of a wire from solution. This kind of investigation has been undertaken for more than 40 years for various kinds of soap films. In this literature the model for predicting the thickness of the soap film as a function of material properties and withdrawal speed is known as Frankel’s law. We have introduced an alternative hypothesis for the thickness of the soap film by introducing the interfacial viscosity of the surfactant film into the model of the withdrawal dynamics. Our results our consistent with the available data (papers covering more than 40 years) and predict the same dependence on speed as given in Frankel’s law. We are continuing these investigations in order to understand whether our assumptions are more consistent with these experiments and systems than the assumptions in Frankel’s law.
