Surface-driven actuation: Sign reversal under load and surface load-memory effect

Abstract

Motivated by suggestions that hybrid nanomaterials from nanoporous metal and aqueous electrolyte can be used as actuators, we study the impact of an external load on the actuation behavior of nanoporous gold impregnated with aqueous electrolyte. At no load, we observe the well-documented trend for a more positive electrode potential prompting elongation of the nanoporous body. For purely capacitive electrode processes we confirm that the elastic response to external load is simply superimposed on the potential-induced elongation, so that the strain per electric charge is invariant with the load. The observations so far are consistent with the expectation for surface-stress driven actuation in a linear elastic materials system. Surprisingly, however, actuation in the regime of oxygen electrosorption responds strongly to loading: as the load is increased, the strain per charge gradually drops to zero and even inverts its direction. In other words, the actuator moves backward when asked to do work against a substantial external load. Furthermore, we demonstrate that the length change in response to lifting the oxygen adsorbate layer depends on the load that was present at the instant of oxysorption. This “load memory effect” has analogies to shape-memory behavior in massive alloys. Yet, contrary to shape-memory alloys, the microscopic origin is here a surface phase transition. We argue that the observation is a signature of the reorientation of local surface domains with anisotropic surface stress, and that the required atomic transport process acts only while mobile adatoms are supplied during the deposition or lifting of the oxygen adsorbate layer.