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43822-AC5
Liquids Spreading and Recoiling on Liquids: The Influence of Interfacial Chemical Reactions
We have been investigating several problems involving chemical reactions at the interface between two liquids, where the reaction causes a significant modification to the flow. In the first part of our study we have focused on liquid lenses floating on a liquid interface. In the second part of our study we have examined sedimentation of liquid drops.
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 (see figure 1). 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 that we have only recently started, we are examining 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 because of the flow, modifies the shape of the drop (see Figure 2).
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 (Figure 2, left). At 30 mM, 40 mM and 50 mM the drops grow long tails (e.g. Figure 2, right) that eventually break. We are continuing our work on the sedimentation problem and measuring the influence of the interfacial chemical reaction.
This research support has been valuable to my students and my group. The first project was performed and lead by my graduate student Ernst van Nierop who continues to work on problems involving thin film that are coupled to chemistry. In the second project most of the preliminary experiments were conducted by an undergraduate student who worked with my research group for the summer 2007.
