MMFLVPS

Objective:

Friction, lubrication, and viscoelasticity between solid substrates represent a challenging multiscale problem, involving spatial features ranging from nanometers to the continuum scale. In particle suspensions, dissipative interactions between lubricated beads may arise from purely hydrodynamic effects, mixed lubricated/contact forces, or dry sliding friction, particularly at high applied stresses when substrates are brought into physical contact. Molecular details of the wet surface -such as morphology (e.g., roughness) or the non-Newtonian properties of the interstitial lubricating layer (e.g., free or tethered polymers)- can significantly alter the effective interfacial frictional dynamics and the associated relative mobility of closely interacting particles. For instance, tethered molecules on the surface can modify local hydrodynamic behavior (e.g., inducing flow slippage), while freely suspended molecules may produce non-Newtonian (e.g., viscoelastic) lubrication interactions. These effects can lead to measurable macroscopic changes, as observed in experiments with dense particle systems. The goal of this subproject is to address this multiscale problem hierarchically. At the microscopic level, we will numerically investigate the effects of different nanoparticle surface functionalizations, including single tethered molecules and/or homogeneous polymer layers lubricating solid substrates, on the resulting suspension dynamics. Additionally, we will study frictional/viscoelastic lubrication and boundary condition modifications (e.g., heterogeneous full/partial surface slip) and link these effects to macroscopic changes, such as altered diffusion in externally actuated suspensions or friction-induced non-Newtonian rheology, including the well-known Discontinuous Shear Thickening (DST) phenomenon observed in dense suspensions. A key objective of this subproject is, however, to bridge the microscopic physics of particle-laden and molecular suspensions with the continuum flow phenomena observed at larger spatial scales. The presence of solid particles and/or suspended molecules induces microstructural conformation changes in response to flow, generating additional macroscopic stresses that interact with the flow itself in highly non-linear ways. This coupling makes the development of accurate fluid dynamics models particularly challenging. One approach to address this is through theoretical coarse-graining, which involves developing novel bottom-up microstructural constitutive models for dense suspensions. These models, formulated as partial differential equations linked to microscopic interfacial friction dynamics, should be designed to capture key phenomena such as Discontinuous Shear Thickening (DST). The second approach involves new direct numerical coupling of different simulation methods operating simultaneously at micro- and macro-scales. This concurrent coupling should enable accurate modeling of the transient interactions between microstructure and flow in fluids with complex dispersed phases. With this overarching goal in mind, both of these methodological approaches will be systematically explored in this subproject.