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

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.

Quantitative Spectroscopic Characterization of Near-UV/visible E. coli (pYAC4), B. subtilis (PY79), and Green Bread Mold Fungus Fluorescence for Diagnostic Applications.

Ultraviolet (UV)-excited visible fluorescence is an attractive option for low-cost, low-complexity, rapid imaging of bacterial and fungal samples for imaging diagnostics in the biomedical community. While several studies have shown there is potential for identification of microbial samples, very little quantitative information is available in the literature for the purposes of diagnostic design. In this work, two non-pathogenic bacteria samples (E. coli pYAC4, and B. subtilis PY79) and a wild-cultivated green bread mold fungus sample are characterized spectroscopically for the purpose of diagnostic design. For each sample, fluorescence spectra excited with low-power near-UV continuous wave (CW) sources, and extinction and elastic scattering spectra are captured and compared. Absolute fluorescence intensity per cell excited at 340 nm is estimated from imaging measurements of aqueous samples. The results are used to estimate detection limits for a prototypical imaging experiment. It was found that fluorescence imaging is feasible for as few as 35 bacteria cells (or [Formula: see text]30 µm3 of bacteria) per pixel, and that the fluorescence intensity per unit volume is similar for the three samples tested here. A discussion and model of the mechanism of bacterial fluorescence in E. coli is provided.