title: Formation of surface nanodroplets under controlled flow conditions.
abstract: Nanodroplets on a solid surface (i.e., surface nanodroplets) have practical implications for high-throughput chemical and biological analysis, lubrications, laboratory-on-chip devices, and near-field imaging techniques. Oil nanodroplets can be produced on a solid-liquid interface in a simple step of solvent exchange in which a good solvent of oil is displaced by a poor solvent. In this work, we experimentally and theoretically investigate the formation of nanodroplets by the solvent exchange process under well-controlled flow conditions. We find significant effects from the flow rate and the flow geometry on the droplet size. We develop a theoretical framework to account for these effects. The main idea is that the droplet nuclei are exposed to an oil oversaturation pulse during the exchange process. The analysis shows that the volume of the nanodroplets increases with the Peclet number Pe of the flow as ∝ Pe(3/4), which is in good agreement with our experimental results. In addition, at fixed flow rate and thus fixed Peclet number, larger and less homogeneously distributed droplets formed at less-narrow channels, due to convection effects originating from the density difference between the two solutions of the solvent exchange. The understanding from this work provides valuable guidelines for producing surface nanodroplets with desired sizes by controlling the flow conditions.
authors.affiliation: School of Civil, Environmental and Chemical Engineering, Royal Melbourne Institute of Technology, Melbourne, VIC 3001, Australia; Physics of Fluids Group, MESA+ Institute, and J. M. Burgers Centre for Fluid Dynamics, University of Twente, 7500 AE Enschede, The Netherlands; xuehua.zhang@rmit.edu.au.
authors.forename: Xuehua
authors.initials: X
authors.lastname: Zhang
authors.affiliation: School of Civil, Environmental and Chemical Engineering, Royal Melbourne Institute of Technology, Melbourne, VIC 3001, Australia;
authors.forename: Ziyang
authors.initials: Z
authors.lastname: Lu
authors.affiliation: School of Civil, Environmental and Chemical Engineering, Royal Melbourne Institute of Technology, Melbourne, VIC 3001, Australia; Physics of Fluids Group, MESA+ Institute, and J. M. Burgers Centre for Fluid Dynamics, University of Twente, 7500 AE Enschede, The Netherlands;
authors.forename: Huanshu
authors.initials: H
authors.lastname: Tan
authors.affiliation: School of Civil, Environmental and Chemical Engineering, Royal Melbourne Institute of Technology, Melbourne, VIC 3001, Australia;
authors.forename: Lei
authors.initials: L
authors.lastname: Bao
authors.affiliation: Centre for Biodiscovery and Molecular Development of Therapeutics, College of Science, Technology Engineering, James Cook University, Townsville City, QLD 4811, Australia.
authors.forename: Yinghe
authors.initials: Y
authors.lastname: He
authors.affiliation: Physics of Fluids Group, MESA+ Institute, and J. M. Burgers Centre for Fluid Dynamics, University of Twente, 7500 AE Enschede, The Netherlands;
authors.forename: Chao
authors.initials: C
authors.lastname: Sun
authors.affiliation: Physics of Fluids Group, MESA+ Institute, and J. M. Burgers Centre for Fluid Dynamics, University of Twente, 7500 AE Enschede, The Netherlands;
authors.forename: Detlef
authors.initials: D
authors.lastname: Lohse
keywords: Peclet number; flow condition; heterogeneous nucleation; solvent exchange; surface nanodroplet