Magnetocontrollable droplet mobility on liquid crystal-infused porous surfaces

Yang Xu; Yuxing Yao; Weichen Deng; Jen-Chun Fang; Robert L Dupont; Meng Zhang; Simon Čopar; Uroš Tkalec; Xiaoguang Wang

doi:10.1007/s12274-022-5318-y

Magnetocontrollable droplet mobility on liquid crystal-infused porous surfaces

Nano Res. 2023;16(4):5098-5107. doi: 10.1007/s12274-022-5318-y. Epub 2022 Dec 21.

Authors

Yang Xu¹, Yuxing Yao², Weichen Deng¹, Jen-Chun Fang¹, Robert L Dupont¹, Meng Zhang¹, Simon Čopar³, Uroš Tkalec^{4

5

6}, Xiaoguang Wang^{1

7}

Affiliations

¹ William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, OH 43210 USA.
² Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125 USA.
³ Department of Physics, Faculty of Mathematics and Physics, University of Ljubljana, 1000 Ljubljana, Slovenia.
⁴ Institute of Biophysics, Faculty of Medicine, University of Ljubljana, 1000 Ljubljana, Slovenia.
⁵ Department of Physics, Faculty of Natural Sciences and Mathematics, University of Maribor, 2000 Maribor, Slovenia.
⁶ Department of Condensed Matter Physics, Jožef Stefan Institute, 1000 Ljubljana, Slovenia.
⁷ Sustainability Institute, The Ohio State University, Columbus, OH 43210 USA.

Abstract

Magnetocontrollable droplet mobility on surfaces of both solids and simple fluids have been widely used in a wide range of applications. However, little is understood about the effect of the magnetic field on the wettability and mobility of droplets on structured fluids. Here, we report the manipulation of the dynamic behaviors of water droplets on a film of thermotropic liquid crystals (LCs). We find that the static wetting behavior and static friction of water droplets on a 4'-octyl-4-biphenylcarbonitrile (8CB) film strongly depend on the LC mesophases, and that a magnetic field caused no measurable change to these properties. However, we find that the droplet dynamics can be affected by a magnetic field as it slides on a nematic 8CB film, but not on isotropic 8CB, and is dependent on both the direction and strength of the magnetic field. By measuring the dynamic friction of a droplet sliding on a nematic 8CB film, we find that a magnetic field alters the internal orientational ordering of the 8CB which in turn affects its viscosity. We support this interpretation with a scaling argument using the LC magnetic coherence length that includes (i) the elastic energy from the long-range orientational ordering of 8CB and (ii) the free energy from the interaction between 8CB and a magnetic field. Overall, these results advance our understanding of droplet mobility on LC films and enable new designs for responsive surfaces that can manipulate the mobility of water droplets.

Electronic supplementary material: Supplementary material (further details of the stability of LCIPS against water-induced dewetting, the interfacial tension and contact angle measurement using a goniometer, the estimation of the thickness of LC wrapping layer at air-water interface on droplets, SEM measurements, the average sliding velocity of a water droplet on 5CB, E7, silicone oil, and mineral oil films with and without a magnetic field, representative force diagram (F_d versus time) of a 3-µL water droplet moving at a speed of 0.1 mm/s on a nematic 8CB film, F_dynamic acting on 3 µL water droplets moving at speeds of 0.1-1 mm/s on an isotropic 8CB film, the calculated magnetic coherence length as a function of the magnitude of the magnetic field applied to the nematic LCIPS, and the apparent advancing and receding contact angles of a moving water droplet on nematic LCIPS as a function of time, and polarized light micrographs (top view) of a nematic 8CB film between two DMOAP-functionalized glass slides before and after applying a horizontal magnetic field) is available in the online version of this article at 10.1007/s12274-022-5318-y.

Keywords: droplet mobility; liquid crystals; lubricated surfaces; magnetic field; wettability.