Chiral surface waves — boundary-localized modes that propagate in one direction with minimal backscattering — are well-known in quantum systems (the quantum Hall effect) and engineered metamaterials (topological insulators). In those contexts, the chirality comes from broken time-reversal symmetry imposed by an external field or careful lattice design. The material must be ordered.
Disordered odd solids support them too (arXiv:2603.21312). Odd elasticity arises when internal torques break the usual symmetry between stress and strain. Active biological gels and synthetic magnetic particle assemblies exhibit odd elastic moduli. In a model of torque-driven disordered odd solids, chiral surface waves appear with stronger surface localization and more stable boundary velocity than in ordered lattice models.
The improvement from disorder is counterintuitive. In conventional elastic systems, disorder scatters waves and destroys coherent propagation. In odd solids, disorder eliminates the bulk modes that would otherwise compete with the surface wave. The surface mode doesn't survive despite disorder — it thrives because of it. Bulk dissipation from disorder suppresses the bulk channels, leaving the surface as the only propagation pathway.
The mechanism depends on the interaction between boundary torques and odd elasticity. At the surface, the material's active torques generate a non-reciprocal response that channels energy along the boundary. In the bulk, the same torques create dissipative modes that absorb energy.
The structural insight: protection of a surface mode doesn't always come from topology in the mathematical sense. Here it comes from the selective destruction of alternatives. The chiral wave propagates because everything else is damped. The boundary is special not because it supports a protected state but because the bulk is too dissipative to support anything. Robustness through the elimination of competition.