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Abstract Achieving mobile liquid droplets on solid surfaces is crucial for various practical applications, such as self‐cleaning and anti‐fouling coatings. The last two decades have witnessed remarkable progress in designing functional surfaces, including super‐repellent surfaces and lubricant‐infused surfaces, which allow droplets to roll/slide on the surfaces. However, it remains a challenge to enable droplet motion on hydrophilic solid surfaces. In this work, we demonstrate mobile droplets containing ionic surfactants on smooth hydrophilic surfaces that are charged similarly to surfactant molecules. The ionic surfactant‐laden droplets display ultra‐low contact angle and ultra‐low sliding angle simultaneously on the hydrophilic surfaces. The sliding of the droplet is enabled by the adsorbed surfactant ahead of three‐phase contact line, which is regulated by the electrostatic interaction between ionic surfactant and charged solid surface. The droplet can maintain its motion even when the hydrophilic surface has defects. Furthermore, we demonstrate controlled manipulation of ionic surfactant‐laden droplets on hydrophilic surfaces with different patterns. We envision that our simple technique for achieving mobile droplets on hydrophilic surfaces can pave the way to novel slippery surfaces for different applications.
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Effect of interfacial viscosities on droplet migration at low surfactant concentrations
In this paper, we theoretically investigate the migration of a surfactant covered droplet in a Poiseuille flow by including the surface viscosities of the droplet. We employ a regular perturbation expansion for low surface Péclet numbers and solve the problem up to a second-order approximation. We represent the drop surface as a two-dimensional homogeneous fluid using the Bousinessq–Scriven law and employ Lamb's general solution to represent the velocity fields inside and outside the droplet. We obtain an expression for the cross-stream migration velocity of the droplet, where the surface viscosities are captured by the Bousinessq numbers for surface shear and surface dilatation. We elucidate the influence of the surface viscosities on the migration characteristics of the droplet and the surfactant redistribution on the droplet surface. Our study sheds light on the importance of including the droplet surface viscosities to accurately predict the migration characteristics of the droplet.
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- Award ID(s):
- 1705371
- PAR ID:
- 10255488
- Date Published:
- Journal Name:
- Journal of Fluid Mechanics
- Volume:
- 902
- ISSN:
- 0022-1120
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1017/jfm.2020.551
