The growth and shrinkage of water droplets at the oil-solid interface

Ran Zhang; Wei Liao; Yunpeng Wang; Yao Wang; D Ian Wilson; Stuart M Clarke; Zhongqiang Yang

doi:10.1016/j.jcis.2020.09.102

The growth and shrinkage of water droplets at the oil-solid interface

J Colloid Interface Sci. 2021 Feb 15:584:738-748. doi: 10.1016/j.jcis.2020.09.102. Epub 2020 Oct 6.

Authors

Ran Zhang¹, Wei Liao¹, Yunpeng Wang¹, Yao Wang¹, D Ian Wilson², Stuart M Clarke³, Zhongqiang Yang⁴

Affiliations

¹ Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China.
² Department of Chemical Engineering and Biotechnology, Philippa Fawcett Drive, University of Cambridge, Cambridge CB3 0AS, UK. Electronic address: diw11@cam.ac.uk.
³ Department of Chemistry and BP Institute, Madingley Rise, University of Cambridge, Cambridge CB2 1EW, UK. Electronic address: stuart@bpi.cam.ac.uk.
⁴ Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China. Electronic address: zyang@tsinghua.edu.cn.

PMID: 33317712
DOI: 10.1016/j.jcis.2020.09.102

Abstract

Hypothesis: The mechanism for the spontaneous formation of water droplets at oil/solid interfaces immersed in water is currently unclear. We hypothesize that growth and shrinkage of droplets are kinetically controlled by diffusion of water through the oil, driven by differences in chemical potential between the solid substrate and the aqueous reservoir.

Experiments: The formation, growth and shrinkage of water droplets at an immersed oil/solid interface are investigated theoretically and experimentally with three silicone oils. The surface is hydrophobic and the droplets formed are truncated spheres with radius, a, less than 10 μm. The expansion and contraction of the droplets can be controlled by adjusting the difference in chemical potential. The growth kinetics are modelled in terms of water migration through the oil layer which predicts a²∝t.

Findings: This is the first study of possible mechanisms for the formation of such interfacial droplets. Several possible causes are shown to be unfavourable, negligible, or are eliminated by careful experiments controlling key parameters (such as oil viscosity, substrate chemistry). The rate constant for mass transport is proportional to difference in chemical potential and an estimate shows dissociation of surface groups on the substrate provides a driving chemical potential of the right magnitude.

Keywords: Chemical potential; Diffusion; Droplet growth; Droplet shrinkage; Interfacial droplets; Oil-solid interface; Osmotic pressure; Surface speciation; Viscosity.