Mitigating cavitation erosion using biomimetic gas-entrapping microtextured surfaces (GEMS)

Abstract

Cavitation refers to the formation and collapse of vapor bubbles near solid boundaries in high-speed flows, such as ship propellers and pumps. During this process, cavitation bubbles focus fluid energy on the solid surface by forming high-speed jets, leading to damage and downtime of machinery. In response, numerous surface treatments to counteract this effect have been explored, including perfluorinated coatings and surface hardening, but they all succumb to cavitation erosion eventually. Here, we report on biomimetic gas-entrapping microtextured surfaces (GEMS) that robustly entrap air when immersed in water regardless of the wetting nature of the substrate. Crucially, the entrapment of air inside the cavities repels cavitation bubbles away from the surface, thereby preventing cavitation damage. We provide mechanistic insights by treating the system as a potential flow problem of a multi-bubble system. Our findings present a possible avenue for mitigating cavitation erosion through the application of inexpensive and environmentally friendly materials.

Fig. 1. Representative scanning electron micrographs of cuticles and fine hairs on the mesothorax of springtails (Collembola) and sea skaters (H. germanus), respectively.

(A and B) Springtails have primary granules (triangular) connected by ridges forming honeycomb patterns that prevent the intrusion of liquids on submersion. (C) Long needle-shaped hairs and tiny mushroom-shaped hairs on dorsal and ventral mesothorax of sea skaters provide robust repellency against seawater. (D) Magnified micrograph of mushroom-shaped hairs. Photo credit: Sankara Arunachalam, KAUST.

Fig. 2. Illustration summarizing how GEMS prevent damage from cavitation jets.

(A) Liquid jet from a bubble collapsing close to a solid boundary affecting the substrate and causing erosion. The time scale corresponds to a cavitation bubble of R_max ≈ 570 μm. (B) The gas entrapped inside the GEMS protrudes near the cavitation bubble and behaves as a free boundary. As a result, the liquid jet from the collapsing bubble is directed away from the substrate. The time scale shown is that of a cavitation bubble of R_max ≈ 520 μm. The time in μs and maximum bubble radius depicted in (A) and (B) are typical values observed in the experiments. (C) The gas entrapped inside the GEMS expands because of the pressure field generated by the nearby cavitation bubble. Notice that the gas contained in the GEMS bulges outward and reaches an almost hemispherical shape during the expansion of the cavitation bubble as mentioned in the text. Image credit: Xavier Pita, Scientific Illustrator, KAUST.

Fig. 3. Scanning electron micrographs of silica-GEMS.

(A) Tilted cross-sectional view (35°) of the silica-GEMS. (B) Top view of the silica-GEMS comprising circular cavities in a hexagonal distribution. (C) Cross-sectional view of the single cavity shown in (A). (D) Detailed cross-sectional view of the mushroom-shaped edge. This sharp edge stabilizes the intruding liquid meniscus and facilitates the entrapment of air inside the cavity. Photo credit: Sankara Arunachalam, KAUST.

Fig. 4. Comparison of wetting behaviors of smooth silica and silica-GEMS with water.

(A) Smooth silica surfaces are water-wet, characterized by intrinsic contact angles, θ_o < 90°. (B) Silica-GEMS robustly entrap air underwater and show apparent contact angles, θ_r > 90°. (C and D) Three-dimensional reconstructions of the air-water interface at the inlets of silica-GEMS underwater realized using confocal laser scanning microscopy. The cross-sectional view in (D) is along the dotted red line in (C). Scale bars are the same in (A) and (B), and the diameter of the cavities in (C) and (D) is D = 200 μm. Photo credit: Sankara Arunachalam, KAUST.

Fig. 5. Cavitation bubble dynamics at high standoff parameters.

(A) Side view of the bubble near a smooth silica surface, γ = 4.8 and R_max = 630 μm. The bottom black line indicates the location of the surface. (B) Side view for the silica-GEMS, γ = 5.1 and R_max = 610 μm. The arrow indicates the location of the microcavities. (C) Top view of the microcavities portrayed in (B). Scale bar, 500 μm. The numbers on all the panels refer to time in microseconds after cavitation generation. The dotted lines in (A) and (B) serve to guide the eye for the direction of the jet after the bubbles collapse. These figures are derived from movies S1 to S3. Photo credit: Silvestre Roberto Gonzalez-Avila, OVGU.

Fig. 6. Cavitation bubble dynamics on silica-GEMS at low standoff parameters.

(A) Bubble dynamics for γ = 1.8 and R_max = 530 μm. (B) Bubble dynamics for γ = 0.7 and R_max = 430 μm. Scale bars, 500 μm. The numbers on all the panels refer to time in microseconds after the generation of the cavitation bubble. These figures are derived from movies S4 to S5. Bubble dynamics on perfluorinated silica-GEMS for similar γ values can be seen in fig. S8 and movies S7 and S9. Photo credit: Silvestre Roberto Gonzalez-Avila, OVGU.

Fig. 7. Comparison between the experimental and simulated results.

(A) Selected frames of the bubble dynamics. The left half of each frame depicts a simulated result, and the right half is the experimental result from the high-speed images captured. Scale bar, 500 μm. The inset numbers on all the panels refer to time in microseconds after the generation of the cavitation bubble. (B and C) Time series results derived from the numerical simulation and experimental results as shown in Fig. 5. Panel (B) shows the bubble collapsing near a microcavity substrate. The scatter blue data points represent experimental results, and the continuous blue line denotes simulated values. Error bars are equal for each point, and only the error bars of the last data points are shown for better visualization. The scatter black data points represent the position of the bubble’s centroid, while the continuous black line portrays the position of the simulated bubble’s centroid. Panel (C) shows results of a bubble collapsing near a smooth surface. The blue and black lines are the bubble’s radius and the location of the bubble’s centroid, respectively. (D) Simulated radial dynamics of a bubble collapsing near a solid surface. (E) Resulting pressure field on the surface p(x,t). Photo credit: Silvestre Roberto Gonzalez-Avila, OVGU.

Fig. 8. Parametric plot built from the numerical results of the model showing the region where coalescence of the entrapped gas in adjacent microcavities is expected to take place.