Microdroplet Contaminants: When and Why Superamphiphobic Surfaces Are Not Self-Cleaning

Abstract

Superamphiphobic surfaces are commonly associated with superior anticontamination and antifouling properties. Visually, this is justified by their ability to easily shed off drops and contaminants. However, on micropillar arrays, tiny droplets are known to remain on pillars' top faces while the drop advances. This raises the question of whether remnants remain even on nanostructured superamphiphobic surfaces. Are superamphiphobic surfaces really self-cleaning? Here we investigate the presence of microdroplet contaminants on three nanostructured superamphiphobic surfaces. After brief contact with liquids having different volatilities and surface tension (water, ethylene glycol, hexadecane, and an ionic liquid), confocal microscopy reveals a "blanket-like" layer of microdroplets remaining on the surface. It appears that the phenomenon is universal. Notably, when placing subsequent drops onto the contaminated surface, they are still able to roll off. However, adhesion forces can gradually increase by up to 3 times after repeated liquid drop contact. Therefore, we conclude that superamphiphobic surfaces do not warrant self-cleaning and anticontamination capabilities at sub-micrometric length scales.

Keywords: adhesion; contamination; microdroplet; pinning; superhydrophobicity; wetting.

Figure 1

Optical and interference microscopy of microdroplet contamination on soot-templated superamphiphobic surfaces. Top-view SEM images at (a) low and (b) high magnification of soot-templated surfaces. (c) Sequence of video images showing the pristine surface with a sessile ethylene glycol drop (30 μL). After 2 min, the drop was removed by a tissue. The drop left a circular imprint (hazy spot) that disappeared after a minute. (d) Rolling of a 30 μL ethylene glycol drop off a tilted surface. Tilt angle: 13°. Trails are shallower compared to the spots. (e) Interference scans were performed using confocal microscopy, mapping the mean penetration depth by the liquid drop into the surface. In this case, the drop remained on the surface. From the interference patterns (f, g, insets) the penetration depth was calculated with respect to time with both (f) ethylene glycol and (g) an ionic liquid (trihexyltetradecylphosphonium bis(trifluoromethyl sulfonyl) imide). Dark regions correspond to constructive and bright regions to destructive interference using a wavelength of 633 nm. A dark-to-bright transition represents λ/4, or 158 nm.

Figure 2

Confocal microscopy imaging of an ethylene glycol drop rolling off a soot-templated superamphiphobic surface. (a) Sketch of a drop (blue) rolling over a nanoparticle-based soot-templated surface. Microdroplets (blue) remain. The particles (gray) are hydrophobized (green) to lower the surface energy. (b–e) A fluorescence-dyed ethylene glycol drop (blue) was rolled off a superamphiphobic surface while dynamically observing surface fluorescence. XZ-plane showing the vertical, Z-axial contact line of a drop. The images were taken using an inverted laser scanning confocal microscope using a 40× air objective. Ethylene glycol was dyed with ATTO 647-ester at a concentration of 10 μg/mL (blue). The bulk drop and thus microdroplets appear blue. Reflection from the interface between the glass and the superamphiphobic coating appears red. All particle spheres represented in the schematics should be considered as agglomerates instead of individual nanoparticles.

Figure 3

Imaging of the “blanket-like” coverage of ethylene glycol microdroplets on soot-templated and nanofilament surfaces. Confocal images of the XY-plane taken close to the surface–air interface of (a) soot-templated nanoparticles and (b) nanofilaments after drop removal. Surface coverages of ethylene glycol microdroplets on soot-templated nanoparticles, wet-sprayed nanoparticles, and nanofilaments are approximately 22 × 10³, 2 × 10³, and 26 × 10³ microdroplets per millimeter square, respectively. (c, d) Microdroplets were found on the accumulated XZ section at the air–surface interface. Evidently, none of the microdroplets impale deep into the nanostructured surface. (e–g) Scanning electron microscopy of the soot-templated surface after a nonvolatile (ionic liquid) drop was removed. The dark spots reflect the previous positions of the microdroplets. These imprints are composed of multiple dispersed subagglomerate zones that are approximately 1–10 μm in diameters.

Figure 4

Geometrical pinning of liquids to a spherical asperity. (a) Case 1, pinning: The contact angle and contact line are pinned at the local contact angle (θ_local) until the capillary bridge (double-sided arrow) ruptures. (b) Case 2, depinning: The contact angle and contact line reach the inherent receding contact angle (θ_rec) and the latter begins sliding. However, thinning of the capillary bridge continues (double-sided arrow), eventually rupturing and breaks before complete depinning of the contact line. In both instances, a remnant droplet is formed. However, because of experimental resolution, visualization of remnants on a single nanoparticle having a diameter of approximately 80 nm is not possible. Therefore, we are only able to visualize the remnants integrating several nanoparticles. (c) Surrounding remnant droplets merge, forming the micrometric droplets that we observe.

Figure 5

Microdroplet clusters on soot-templated surfaces: Influence on wettability. Time-resolved dynamic analysis of increasing roll-off angles with respect to three different liquids: water, ethylene glycol, and the ionic liquid [P_6,6,6,14]⁺[TFSI]⁻. (a) The roll-off angle of water and ethylene glycol remained below 5°. The roll-off angle of the ionic liquid gradually rose to 47° after 60 min. (b) The roll-off angle for ethylene glycol rose from 2° to 4°. This is accompanied by the formation of a dense white fuzzy spot on the surface (inset, at 60 min).

Figure 6

Adhesion properties of microdroplets. (a) A pristine soot-templated surface was tested using droplet-probe force microscopy. A total of 200 force curves were taken under repeated contacts of a nonvolatile ionic liquid drop with the surface on two locations. (b) The shift in the thermal noise spectra of the droplet-probe cantilever, before (black) and after 200 cycles (cyan), corresponds to a mass loss of 20 fg. (c) The maximum tip-to-sample distance decreases around 25 nm between the first and last measurement. (d) Time-dependent adhesion on an initially pristine spot. Adhesion on the pristine surface starts at approximately 200–400 nN and increases over time up to 500–550 nN after gradual contamination.