Bubble Meets Droplet: Particle‐Assisted Reconfi guration of Wetting Morphologies in Colloidal Multiphase Systems

in many industrial processes, related e.g., to encapsulation, enhanced oil recovery, water purifi cation, food technology, or defoaming. [ 4–15 ] An analysis of wetting in these systems was fi rst reported in the early 1970s by Torza and Mason [ 4 ] and was revisited recently by Pannacci et al. [ 5 ] and Guzowski et al. [ 6 ] These authors found that the wetting of a liquid droplet by a second fl uid in a third immiscible fl uid medium can result in three possible wetting morphologies (non-wetting, partial engulfment, and complete engulfment of one drop by the other), which were successfully predicted based on the spreading coeffi cients of three fl uids. A fl uid i will spread spontaneously at the interface of two immiscible fl uids j and k if its spreading coeffi cient Si jk ik ij ( ) γ γ γ = − + is positive, where γ ij , γ ik , and γ ik are the respective interfacial tensions. [ 4–6 ] For the wetting behavior of a colloidal multiphase system, three cases can be distinguished: if the spreading coeffi cient of the continuous medium phase is positive, the droplets will be non-wetting and try to separate; if, by contrast, one of the droplet phases has a positive spreading coeffi cient, it will fully engulf the other droplet (complete wetting); the case DOI: 10.1002/smll.201600799 Wetting phenomena are ubiquitous in nature and play key functions in various industrial processes and products. When a gas bubble encounters an oil droplet in an aqueous medium, it can experience either partial wetting or complete engulfment by the oil. Each of these morphologies can have practical benefi ts, and controlling the morphology is desirable for applications ranging from particle synthesis to oil recovery and gas fl otation. It is known that the wetting of two fl uids within a fl uid medium depends on the balance of interfacial tensions and can thus be modifi ed with surfactant additives. It is reported that colloidal particles, too, can be used to promote both wetting and dewetting in multifl uid systems. This study demonstrates the surfactant-free tuning and dynamic reconfi guration of bubble-droplet morphologies with the help of cellulosic particles. It further shows that the effect can be attributed to particle adsorption at the fl uid interfaces, which can be probed by interfacial tensiometry, making particle-induced transitions in the wetting morphology predictable. Finally, particle adsorption at different rates to air–water and oil–water interfaces can even lead to slow, reentrant wetting behavior not familiar from particlefree systems. Wetting Behavior