A computational framework based on immersed boundary-lattice Boltzmann coupling for ultrasound-excited encapsulated microbubbles

Encapsulated microbubbles (EMBs) play important roles in biomedical ultrasound applications, including diagnostic imaging, targeted drug delivery, sonoporation, and mechanotransduction, yet existing Rayleigh-Plesset-type and potential-flow-based formulations remain limited in their ability to resolve the complex viscous fluid-structure interactions and nonspherical interfacial dynamics exhibited by EMBs. In the present study, a fully resolved computational fluid dynamics (CFD) framework based on immersed boundary-lattice Boltzmann coupling is developed to simulate ultrasound-driven EMB dynamics in biologically relevant environments. The proposed methodology combines an axisymmetric multicomponent multiphase lattice Boltzmann flow solver with an immersed boundary representation of the EMB viscoelastic shell, while incorporating shell rheology through the Boussinesq-Scriven constitutive law along with an exponential elasticity model. The framework is validated through Young-Laplace equilibrium tests and comparison with solutions of a modified Rayleigh-Plesset equation for acoustically-driven EMB oscillations. The results demonstrate accurate prediction of interfacial pressure jumps, physically relevant liquid-to-gas density ratios, and EMB oscillatory dynamics under both linear and nonlinear acoustic regimes. Higher-order interpolation-spreading stencils and higher-order advection schemes are shown to improve numerical accuracy, while further simulations reveal that shell elasticity strongly modifies nonlinear oscillatory behavior and that shell dilatational viscosity introduces substantial damping and dissipative effects. Beyond radial oscillations, the proposed solver accurately reproduces jet formation and nonspherical collapse of an ultrasound-driven EMB near a membrane, a problem of particular interest in sonoporation. Altogether, the present work establishes a robust and physically resolved CFD framework capable of capturing complex multiphase fluid-structure coupling in ultrasound-driven EMBs, thereby providing a versatile computational platform for further nonspherical EMB dynamics (e.g., EMB shape mode oscillations), fully bidirectional bubble-boundary interactions, cavitation-based tissue ablation, and other biomedical ultrasound phenomena.