Effect of surface coating on droplet generation in flow-focusing microchannels
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ORIGINAL ARTICLE
Effect of surface coating on droplet generation in flow‑focusing microchannels Bryan Palogan1 · Ranganathan Kumar1 · Samik Bhattacharya1 Received: 24 May 2020 / Accepted: 9 August 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Different stages of droplet generation are reported in this paper with two immiscible liquids, silicone oil and deionized water, inside a flow-focusing device for hydrophobic and hydrophilic channel walls. Hydrophobic and hydrophilic channels of identical geometry are compared. In this first set of experiments, the efficacy of the hydrophobic channel is compared with a square cross junction for a continuous oil phase with low viscosity. In the hydrophobic case, the flow-focusing design with a diverging outlet delays jetting and allows for the use of higher flow rate ratios in the squeezing regime. For the hydrophilic case, stable and well-structured droplet and slug generation can be achieved using oil and water, resulting in an inverse emulsion. However, the morphology of the fluid interface displays an unusual behavior compared to that of a hydrophobic microchannel. The droplet generation in the hydrophilic channel occurs following the formation of single and double T-junctions, a phenomenon hitherto unreported in the literature. The results demonstrate that the uncoated hydrophobic channels generate monodisperse droplets at a higher capillary number when compared to the hydrophilic channels. Keywords Flow focusing · Hydrophobic · Hydrophilic · Droplet formation
1 Introduction Over the past few decades, drop generation in microfluidics has been a prime area of research due to its wide array of chemical and biological applications. For instance, drops have been used for micro-mixing due to its low dispersion and rapid mixing (Tice 2003). The versatility and efficiency of microfluidic devices in generating monodisperse droplets have especially garnered attention in medical implementation. These characteristics make drops effective tools in applications such as clinical diagnostics and controlled drug delivery (Srinivasan et al. 2004; Kaler and Prakash 2014; Zhao 2013). Oil emulsions have various applications that range from food applications to pesticide creation (McClements and Decker 2000; Wang 2007). * Samik Bhattacharya [email protected] Bryan Palogan [email protected] Ranganathan Kumar [email protected] 1
Department of Mechanical and Aerospace Engineering, University of Central Florida, Orlando, USA
Generally, droplets are generated when a dispersed phase is injected into an immiscible carrier fluid, or continuous phase. Shear stresses are imparted on the dispersed phase by the continuous phase until the interfacial tension is overcome and the drop is formed. Channel designs vary at the junction and take varying approaches in optimizing the interface interaction such that the production rate and monodispersity of droplets are maximized. Numerous experimental papers have been published that investigate various g
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