Journal of Colloid and Interface Science 332 (2009) 455–460 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis Chasing drops: Following escaper and pursuer drop couple system Prashant Bahadur, Preeti S. Yadav, Kumud Chaurasia, Aisha Leh, Rafael Tadmor ∗ Department of Chemical Engineering, Lamar University, Beaumont, TX 77710, USA article info abstract Article history: Received 30 July 2008 Accepted 17 December 2008 Available online 25 December 2008 Keywords: Marangoni flow Drop couples We study experimentally six different systems in which Marangoni flow is induced by two chemically different drops on a solid surface in air. In such systems one drop seems to chase away the other. We show that in all the systems studied, the Marangoni flow is induced at the solid–vapor interface as opposed to the air–liquid interface. This is true even for the case of water drop and alcohol drop on a glass surface (which corresponds to the “tears of wine” classical case). Thus we explain the drop motion as a result of an interfacial tension gradient which takes place primarily at the air–surface region and less, if at all, at the two other interfaces in the problem: the liquid–substrate or liquid–air interfaces. Then we follow the motion of drops on surfaces and find that it is discontinuous, i.e. characterized by stops and jumps as in a stick slip mechanism. We explain this behavior by an increase in the Laplace pressure that creates a higher anchoring pinning effect at the front edge of the moving drop. The understanding of this process has implications for passively separating mixed liquids.  2008 Elsevier Inc. All rights reserved. 1. Introduction The Marangoni effect [1–3] is a general term for a family of flows induced by some interfacial tension gradient associated with either chemical [4] or thermal [5] gradients with numerous appli- cations and ramifications [5–14]. In this study we focus on chem- ically induced Marangoni flow, for which one can distinguish be- tween two different but related systems: (1) Binary solution: A solution of two materials from which drops migrate along a wall (often climb a vertical wall) due to the different properties (volatilities and interfacial tensions) of the two components composing the solution [15]. (2) Drop couples: The two drops used are of two different mate- rial compositions. In this setup one drop seems to be pushing the other (again as a function of the different volatilities and surface tensions). Bangham and Saweris [16] studied three systems of drop cou- ples using mica surface as a substrate. Though their paper is pri- marily a phenomenological report, they suggested that the phe- nomenon is induced by a very thin invisible film that spreads on the mica surface by one of the drops and pushes the other drop. The case in which the more volatile drop that seemed to be push- ing the less volatile drop was more common than vice versa. * Corresponding author. E-mail address: rafael.tadmor@lamar.edu (R. Tadmor). Carles and Cazabat [17], on the other hand, suggested a dif- ferent mechanism. They repeated the experiments of Bangham and Saweris and added yet another system. They argued that the drop motion is induced by an air–liquid surface tension gradient along the front–rear drop axis caused by the volatile substance adsorbing from air preferentially on the moving drop’s air–liquid interface regions that are closer to the volatile drop (as opposed to a thin invisible film at the solid–vapor interface). This conclu- sion emanated mainly from their results with a system in which it was the volatile component that had a higher surface tension. Specifically they used trans-decaline (TD), with surface tension γ TD = 26.3 mN/m and the essentially non-volatile polydimethyl- siloxane oil (PDMS) with surface tension γ PDMS = 20 mN/m on a glass surface. They believed that the mechanism is as follows: the volatile TD evaporates and condenses on the PDMS drop. Since γ TD > γ PDMS , the adsorption of TD on the PDMS drop’s surface causes the surface tension of the mainly-PDMS-drop to rise es- pecially in the side closest to the TD source (i.e. TD drop). This, in turn, induces a surface tension gradient in the mainly-PDMS– liquid–air interface, and this gradient, Carles and Cazabat argued, induces a stream of liquid in the bulk of the mainly-PDMS-drop that causes it to move along the surface. Thus they suggested that the process takes place via the atmosphere and not via a thin invisible film [18] as suggested by Bangham and Saweris half a century earlier. Carles and Cazabat also proposed that the phe- nomenon is due to changes at the air–liquid interface and not at the air–solid interface (which is the Bangham and Saweris model). In this study we investigate a few drop couple systems includ- ing the system used by Carles and Cazabat [17] and a few addi- 0021-9797/$ – see front matter  2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2008.12.050