Journal of Colloid and Interface Science 265 (2003) 409–421 www.elsevier.com/locate/jcis The effect of surfactant on the efficiency of shear-induced drop coalescence Steven D. Hudson, a,∗ Alex M. Jamieson, b and Brian E. Burkhart c a Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8544, USA b Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106-7202, USA c The Goodyear Tire & Rubber Co., Akron, OH 44305, USA Received 14 August 2002; accepted 9 April 2003 Abstract The volume-averaged shear-induced drop-coalescence efficiency ε v is measured by in situ videomicroscopy of blends of poly(propylene glycol) and poly(ethylene glycol), emulsified with poly(ethyleneglycol-b-propyleneoxide-b-ethyleneglycol) block copolymer surfactant. Ad- sorption of copolymer to the immiscible blend interface is indicated by a reduction in the interfacial tension, measured by the drop retraction method. The effects of temperature, copolymer molecular weight, copolymer concentration, and capillary number Ca are explored. At small Ca, ε v is essentially independent of shear rate and drop size, and depends mainly on the solubility, diffusivity, and surface pressure of the surfactant, indicating that drop trajectories during flow are perturbed by surfactant Marangoni stresses that are controlled by the diffusion- limited sorption of surfactant. At larger Ca, ε v approaches zero. This arrest of coalescence is associated with the onset of slight deformation of the drops during their collision, and drainage of a film of continuous fluid between them. The effect of the surfactant, though signifi- cant, saturates even while the amount of surfactant adsorbed to the interface is quite small. Governing dimensionless parameters, associated material parameters and the behavior of more insoluble surfactants are discussed.  2003 Elsevier Inc. All rights reserved. Keywords: Coalescence; Adsorption; Surfactant; Emulsions; Drop collisions; Film drainage; Peclet; Marangoni 1. Introduction Since it is often essential to control droplet size in emul- sions, manipulation of the rate of coalescence is necessary. The ability of surfactants or block copolymers to influence drop breakup and coalescence phenomena during the mixing of immiscible fluids is well known and has great commer- cial importance, but the mechanism by which they achieve this remains obscure. Here, by determining the appropriate governing dimensionless parameters (see Table 1), we deter- mine this mechanism (for soluble surfactants) in regimes of both low and moderate capillary number. Basic parameters are introduced in this section. As two drops approach one another in flow, two effects determine the possibility of coalescence. First, the drop tra- jectories curve away from one another; hydrodynamic in- teractions that arise from the viscosity of the continuous fluid cause drops to repel one another, so that collisions * Corresponding author. E-mail address: steven.hudson@nist.gov (S.D. Hudson). may be avoided, even while the drops remain essentially undeformed [1]. “Collisions” are defined to occur when the distance between drop centers is less than or equal to the sum of their radii; smaller separations are possible if the collision force is strong enough to cause drop deformation in the near contact region. Such deformation may or may not be suffi- cient to prevent drop contact and coalescence: collided drops remain nearly in contact for a finite “interaction time,” as the pair rotates in the flow, while a thin film of continuous fluid of thickness h separating the drops drains [2]. Film drainage is the second effect that decides whether two drops will coa- lesce or will ultimately be convected away from one another. These two processes have been discussed recently by Yang et al. [3]. A useful parameter that characterizes this film drainage process can be obtained by nondimensionalizing the thin- film force balance for glancing-angle collisions [4,5]. This and related so-called dimensionless Hamaker parameters implicitly define relevant length scales for film drainage: the drop radius a cr at the onset of the importance of film drainage [5,6] and the final film thickness h c before it rup- 0021-9797/$ – see front matter  2003 Elsevier Inc. All rights reserved. doi:10.1016/S0021-9797(03)00396-5