Fast spectral solver for viscoelastic structures under oscillatory flow in free space or wall-bounded domains: Applications to quartz crystal microbalance and force spectroscopy.

We present a fast spectral solver for the linear response of viscoelastic structures under oscillatory flow either in free space or close to a flat moving wall. The scheme works in the frequency domain (using phasors) and couples the oscillatory Stokes equation with rigid or flexible structures, modeled by viscoelastic networks of immersed boundary kernels. The fluid-structure coupling can be solved by two routes. One route calculates the hydrodynamic mobility matrix required to solve the equation for the structure deformation rate in matrix form. The second route iteratively solves the coupled fluid-structure equations: fluid-induced forces on the structures create a tension field, which is then transferred to the fluid, until convergence. The resulting fixed-point problem is solved iteratively using the Anderson acceleration method. The mobility route is optimal when dealing with one or a few structures, while the iterative scheme is preferred for denser dispersions. In any case, the flow resulting from the body forces is solved by a recently developed scheme [R. P. Peláez, P. Palacios-Alonso, and R. Delgado-Buscalioni, J. Fluid. Mech. 1010 A57 (2025)], which is spectral in space and time and deals with doubly periodic open domains (either free-space or wall-bounded) where meshing is restricted to the region of interest around the structures. We test the present scheme in two applied contexts: quartz-crystal-microbalance (QCM) of spheres, suspended, adsorbed, or tethered to viscoelastic linkers; and force spectroscopy (via atomic force microscopy) reproducing the power spectra of vibrating microparticles near a solid boundary. In all cases, comparisons with analytical, numerical, and experimental results show excellent agreement. We conclude by discussing new routes the scheme opens in force spectroscopy and QCM analyses of soft objects.