Shedding of Multiple Sessile Droplets

A droplet which is placed on a surface and is exposed to an airflow, can be shed, if the drag force overcomes the droplets adhesion force. Presence of other sessile droplets, in proximity, changes the drag force, so the minimum airflow velocity required to shed the droplets (Ucr) can vary. In this thesis, an experimental study on shedding of the multiple sessile droplets was performed on both hydrophilic and hydrophobic surfaces. The effects of the droplets arrangement type, and the spacing on Ucr were elucidate. For a pair of sessile droplets, a model was proposed to predict the Ucr based on droplets size, spacing, arrangement, and surface wettability. For three, or four sessile droplets arranged in triangle, square, reversed triangle, and diamond configurations, the effects of the droplets interaction on variation of the Ucr, was clarified. A critical value for spacing was determined beyond which multiple sessile droplets shed independently

Emulsion characterization via microfluidic devices : A review on interfacial tension and stability to coalescence

Emulsions have gained significant importance in many industries including foods, pharmaceuticals, cosmetics, health care formulations, paintings, polymer blends and oils. During emulsion generation, collisions can occur between newly-generated droplets, which may lead to coalescence between the droplets. The extent of coalescence is driven by properties of dispersed and continuous phases, e.g. density, viscosity, ion strength and pH, and system conditions, e.g. temperature, pressure or any external applied forces. In addition, the diffusion and adsorption behaviors of emulsifiers which govern the dynamic interfacial tension of the forming droplets, the surface potential, and the duration and frequency of the droplet collisions, contribute to the overall rate of coalescence. An understanding of these complex behaviors, particularly those of interfacial tension and droplet coalescence during emulsion generation, is critical for the design of an emulsion with desirable properties and the optimization of the processing conditions. However, in many cases, the time scales over which these phenomena occur are extremely short, typically a fraction of a second, which makes their accurate determination by conventional analytical methods extremely challenging. In the past few years, with advances in microfluidic technology, many attempts have demonstrated that microfluidic systems, characterized by micrometer-size channels, can be successfully employed to precisely characterize these properties of emulsions. In this review, current applications of microfluidic devices to determine the equilibrium and dynamic interfacial tension during the droplet formation, and to investigate the coalescence stability of dispersed droplets applicable to the processing and storage of emulsions, are discussed.Peer reviewe