Tuning the Charge of Sliding Water Drops - PubMed (original) (raw)

Tuning the Charge of Sliding Water Drops

William S Y Wong et al. Langmuir. 2022.

Abstract

When a water drop slides over a hydrophobic surface, it usually acquires a positive charge and deposits the negative countercharge on the surface. Although the electrification of solid surfaces induced after contact with a liquid is intensively studied, the actual mechanisms of charge separation, so-termed slide electrification, are still unclear. Here, slide electrification is studied by measuring the charge of a series of water drops sliding down inclined glass plates. The glass was coated with hydrophobic (hydrocarbon/fluorocarbon) and amine-terminated silanes. On hydrophobic surfaces, drops charge positively while the surfaces charge negatively. Hydrophobic surfaces coated with a mono-amine (3-aminopropyltriethyoxysilane) lead to negatively charged drops and positively charged surfaces. When coated with a multiamine (_N_-(3-trimethoxysilylpropyl)diethylenetriamine), a gradual transition from positively to negatively charged drops is observed. We attribute this tunable drop charging to surface-directed ion transfer. Some of the protons accepted by the amine-functionalized surfaces (-NH2 with H+ acceptor) remain on the surface even after drop departure. These findings demonstrate the facile tunability of surface-controlled slide electrification.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1

Figure 1

Concept: bi-layered surface functionalization. To inhibit wetting, (a) plain soda lime glass was functionalized with a primary layer of (b) hydrocarbon (trichloro(propyl)silane, TCPS) or (c) fluorocarbon (trichloro(1_H_,1_H_,2_H_,2_H_-perfluorooctyl)silane, PFOTS). Thereafter, secondary layers of (d,e) (3-aminopropyl)triethoxysilane, APTES, or (f) _N_-(3-trimethoxysilylpropyl)diethylenetriamine, NTDET, were added. The relevant functional molecular groups are hydrocarbon: green, fluorocarbon: purple, amine: blue, and unreacted hydroxyls: orange, respectively. The (+) and (−) symbols indicate the nature of polarity during slide electrification.

Figure 2

Figure 2

Tunable slide electrification. Slide electrification is achieved by sequentially dropping-and-sliding liquid water drops on tilted, functionalized glass substrates. (a) The drop current is measured using an electrode, which is then amplified for analysis. (b) Depending on the surface, the charges collected can be positive (hydro/fluorocarbon, as shown: PFOTS-functionalized) or negative (amines, as shown: PFOTS-APTES-functionalized). (c) The drop charge saturation for all variants is achieved within 500 drops (10 repeat runs, presented as average ± standard deviation). Both pure hydrophobic variants (PFOTS, purple, and TCPS, green) show positive initial and saturation charges. Secondary functionalization of the mono-amine (APTES) with both hydrophobic variants (PFOTS-APTES, blue, and TCPS-APTES, cyan) results in negative drop charging, showing both negative initial and saturation charges.

Figure 3

Figure 3

Adaptive charging phenomenon (PFOTS-NTDET). Sequential drop deposition (45 μL, 2 s interval) shows an (a) initial positive drop charge but adapts and saturates at a negative drop charge equilibrium. (b) The positive charge persists for ca. 3 drops before flipping toward a negative charge. The drop charge saturation is achieved, albeit first showing a minor overshoot, within 50 drops (10 repeat runs, presented as average ± standard deviation). (c) The positive-to-negative drop charging adaptation in PFOTS-NTDET is reversible (dark blue), with a longer drop interval (top _x_-axis, in seconds) enabling reversal back to a positive charging behavior. (d) Drop sliding velocity was analyzed using a conductive silicon wafer substrate that removes the influence of charging on drop mobility. Without the influence of electrostatics, and over the course of 1000 sequential drops (33 μL), drop mobility adapts (i.e., changes), indicative strongly of surface chemistry adaptation. (c, d) PFOTS (purple) is included as a control for comparative purposes.

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