Visualization and Experimental Characterization of Wrapping Layer Using Planar Laser-Induced Fluorescence

Abstract

Droplets on nanotextured oil-impregnated surfaces have high mobility due to record-low contact angle hysteresis (∼1-3°), attributed to the absence of solid-liquid contact. Past studies have utilized the ultralow droplet adhesion on these surfaces to improve condensation, reduce hydrodynamic drag, and inhibit biofouling. Despite their promising utility, oil-impregnated surfaces are not fully embraced by industry because of the concern for lubricant depletion, the source of which has not been adequately studied. Here, we use planar laser-induced fluorescence (PLIF) to not only visualize the oil layer encapsulating the droplet (aka wrapping layer) but also measure its thickness since the wrapping layer contributes to lubricant depletion. Our PLIF visualization and experiments show that (a) due to the imbalance of interfacial forces at the three-phase contact line, silicone oil forms a wrapping layer on the outer surface of water droplets, (b) the thickness of the wrapping layer is nonuniform both in space and time, and (c) the time-average thickness of the wrapping layer is ∼50 ± 10 nm, a result that compares favorably with our scaling analysis (∼50 nm), which balances the curvature-induced capillary force with the intermolecular van der Waals forces. Our experiments show that, unlike silicone oil, mineral oil does not form a wrapping layer, an observation that can be exploited to mitigate oil depletion of nanotextured oil-impregnated surfaces. Besides advancing our mechanistic understanding of the wrapping oil layer dynamics, the insights gained from this work can be used to quantify the lubricant depletion rate by pendant droplets in dropwise condensation and water harvesting.

Keywords: lubricant-impregnated surfaces (LIS); oil-infused surfaces; planar laser-induced fluorescence (PLIF); slippery liquid-infused porous surfaces (SLIPS); spreading coefficient; wetting ridge; wrapping layer.

Figure 1

Contact angle. Apparent contact angle of a millimetric water droplet on (a) a Glaco-treated surface and (b) a nanotextured oil-impregnated surface. The apparent contact angle of a water droplet on the Glaco-coated microscope glass slide was ∼110° with an advancing/receding contact angle pair of 113/100° and ∼13° contact angle hysteresis. When the nanotextured glass slide was impregnated with silicone oil, water droplets became highly mobile with an advancing/receding contact angle pair of 95/94° with ∼1° contact angle hysteresis.

Figure 2

Confocal visualization of the wrapping oil layer. Confocal images of the wetting ridge and wrapping layer when the nanotextured surface was impregnated with (a–c) silicone oil and (d) mineral oil. The silicone oil was dyed using BODIPY (C₁₄H₁₇BF₂N₂) while the mineral oil was dyed using Nile Red (C₂₀H₁₈N₂O₂), both at 0.1 wt % concentration. The wetting ridge and the intercalated oil film became visible when the lubricant oil was labeled by using a fluorescent dye. Unlike mineral oil, silicone oil cloaks the water droplet and forms a wrapping layer.

Figure 3

PLIF setup. (a) Optical layout diagram. The experimental setup consists of a laser source, a cylindrical lens, a reflecting mirror, a long-pass filter, and an sCMOS detector. The laser passes through the cylindrical lens, reflects off of a 45° mirror, and passes through the substrate before reaching the water droplet. The emitted light from the dye was allowed to pass through a filter to isolate the emission wavelength from the scattered light. (b) Illustration of droplet geometry, the pixel raster grid, and the line spread function.

Figure 4

PLIF visualization of the wrapping oil layer. Ensemble-averaged and background-subtracted PLIF image of a water droplet on a transparent glass substrate impregnated with (a) silicone oil and (b) mineral oil. The bright strip on top of the droplet in (a) indicates the presence of a wrapping oil layer. On the other hand, the droplet in (b) does not have a bright strip, indicating the absence of a wrapping layer. The radius of the droplet is 1.0 mm in (a) and 1.1 mm in (b). The radius is measured by circle fitting the droplet spherical cap by a self-developed MATLAB script.

Figure 5

Wrapping layer thickness. (a) PLIF intensity (ξ) image of a water droplet on a silicone oil infused surface. (b) PLIF image shifted to align the wrapping oil film. (c) Corrected intensity profile across the region of interest with the line-spread function (LSF) contribution shaded in red and background (BG) shaded in blue. The total intensity, which is proportional to film thickness, is proportional to the area of the red-shaded region. (d) Mean and range of measured film-thickness profiles calculated from 30 time-lapse images. The horizontal position is the i direction in Figure 5b, with the droplet center denoted as the origin i = 0. The oil film thickness is nonuniform spatially. (e) Histogram and Gaussian kernel-density estimate (KDE) of film thickness measurements over the entire region of interest. The average oil film thickness is near 48 nm. (f) Temporal variation of the wrapping layer thickness. The error bars represent a 95% confidence interval about the mean. The radius of the droplet was 1.3 mm, and the oil viscosity was 10 cSt.