  Quantifying the response of marine mussel plaque attachment on wet surfaces
remains a significant challenge to a mechanistic understanding of plaque
adhesion. Here, we developed a customised microscopy system combined with
two-dimensional (2D) in-situ digital image correlation (DIC) to quantify the
in-plane deformation of a deformable substrate that interacts with a mussel
plaque while under directional tension. By analysing the strain field in the
substrate, we gained insight into how in-plane traction forces are transmitted
from the mussel plaque to the underlying substrate. Finite element (FE) models
were developed to assist the interpretation of the experimental measurement.
Our study revealed a synergistic effect of pulling angle and substrate
stiffness on plaque detachment, with mussel plaques anchoring to a 'stiff'
substrate at a smaller pulling angle having mechanical advantages with higher
load-bearing capacity and less plaque deformation. We identified two distinct
failure modes, i.e., shear traction-governed failure (STGF) mode and normal
traction-governed failure (NTGF). It was found that increasing the substrate
stiffness or reducing the pulling angle resulted in a failure mode change from
NTGF to STGF. Our findings offer new insights into the mechanistic
understanding of plaque and substrate interaction, which provides a general
plaque-inspired strategy for wet adhesion.
