UC Berkeley

Effective interactions in many soft and active materials appear to violate action-reaction symmetry as a result of coarse-graining over hidden degrees of freedom such as solvent flows, chemical fields, or feedback-controlled environments. We refer to this emergent action–reaction asymmetry as nonreciprocity. This dissertation investigates nonreciprocity as a minimal and tunable route to nonequilibrium behavior in multicomponent mixtures, focusing on how interspecies force asymmetry reshapes phase coexistence, stability, and transport in the absence of equilibrium free-energy structure. My work adopts a mechanical perspective grounded in species-resolved density conservation and momentum balance, derived via Irving-Kirkwood coarse-graining, and uses this framework to formulate nonequilibrium coexistence and transport directly in terms of forces, fluxes, and steady-state responses.Using a binary-mixture model with tunable interspecies force asymmetry, I show that increasing nonreciprocity produces a diverse nonequilibrium phase phenomenology that includes strongly segregated coexistence, multiphase regimes, traveling coexistence states, and homogeneous clustering. Across these regimes, equilibrium-inspired descriptions based on effective interactions or effective temperatures capture only limited aspects of the behavior, underscoring the need for principles that do not rely on free-energy structure. To address this, I develop a general mechanical route to multicomponent coexistence in flux-free steady states and identify when nonequilibrium mixtures nonetheless admit bulk “state-function” coexistence criteria—generalizing the role of chemical potentials and pressure—versus when coexistence intrinsically depends on interfacial or gradient-scale structure.To connect macroscopic stability and pattern formation to microscopic driving, I formulate a mechanical theory of multicomponent linear transport beyond local equilibrium. In this framework, the Onsager transport tensor is defined by a steady-state resistance response. Applying these results to nonreciprocal systems, I demonstrate that action-reaction asymmetry can generate transport signatures forbidden in equilibrium, including diffusion modes with complex-conjugate eigenvalues that manifest as underdamped, oscillatory relaxation of density fluctuations and spatially “odd” transport that generates flux response transverse to density gradients. More broadly, the dissertation establishes how nonreciprocity reshapes the mechanical constraints for forces and fluxes, thereby altering both the criteria for coexistence and the dynamical pathways by which homogeneous mixtures lose stability.Overall, this work provides a unified mechanical foundation for predicting and interpreting nonequilibrium phase behavior and transport in multicomponent mixtures with nonreciprocal interactions, with implications for both natural and engineered active, nonreciprocal materials.