Simulation method of microscale fluid-structure interactions: Diffuse-resistance-domain approach

We propose the Diffuse-Resistance-Domain (DRD) approach --- a thermodynamically consistent DNS method for microscale fluid-structure interactions (mFSI) in multicomponent multiphase flows. By unifying interfacial dynamics through Onsager's variational principle and smooth interpolation of resistance coefficients, DRD naturally enforces complex boundary conditions --- like dynamic contact angles --- without ad hoc assumptions. It avoids evolving solid-phase fields, cutting computational cost while enabling high-fidelity simulations of moving or deforming boundaries. DRD accurately captures interfacial phenomena in microfluidics, active matter, and porous media, showing strong agreement with experiments.