Under-water superoleophobicity of fish scales

Abstract

Recent surge in the development of superhydrophobic/superoleophobic surfaces has been motivated by surfaces like fish scales that have hierarchical structures, which are believed to promote water or oil repellency. In this work, we show that the under-water oil repellency of fish scales is entirely due to the mucus layer formation as part of its defense mechanism, which produces unprecedented contact angle close to 180°. We have identified the distinct chemical signatures that are responsible for such large contact angle, thereby making fish scale behave highly superoleophobic inside the water medium. In absence of the mucus layer, it is found that the contact angle decreases quite dramatically to around 150°, making it less oleophobic, the degree of such oleophobicity can then be contributed to its inherent hierarchical structures. Hence, through this systematic study, for the first time we have conclusively shown the role of the fish's mucus layer to generate superoleophobicity and negate the common notion that hierarchical structure is the only reason for such intrinsic behavior of the fish scales.

Figure 1. Contact angle measurement on fish scales.

Static contact angle of oil and water drops on fish scales with and without mucus layer for both in air and water media.

Figure 2. FTIR data for fish scales with and without mucus along with key chemical signatures.

Figure 3. Tilapia fish used for quantification of wetting characteristics of its scales.

The red marked parts of the skin was sectioned out for further analysis. (a) Sectioned out skin from the fish. (b) Scanning Electron Microscopy (SEM) image of fish scales with mucus and without mucus.

Figure 4. Measurement of contact angle on fish scales.

(a) Silicon oil drops, used as model fluid, resting on fish scales kept under water in a glass cuvette. (b) Technique used to achieve under-water needle-free drop deposition of silicon oil on soft deformable fish scale surface. (c) Commonly used procedure of contact angle measurement which inaccurately quantifies the static contact angle by using normal DSA 100 set-up due to unresolved drop base contact area. (d) Accurate quantification of contact angle using an additional illumination arrangement and modified experimental set-up. This figure was drawn by PRW.