Microscopic active forces have mesoscopic elastic consequences

Active materials are those in which individual components convert ambient free energy into mechanical work. Of particular interest because of their analytical tractability and potential for describing biological phenomena are active systems composed of cytoskeletal filaments and molecular motors. These assemblies in sufficiently dense two-dimensional suspensions form active nematic liquid crystals (LCs). These are materials with long range spatial ordering whose components can nonetheless interchange positions. Due to the large time and length scale separations between the action of molecular motors and the flows they generate in the LC, most modelling has focused on coarse graining microscopic details in favor of mesoscale phenomenological models. The logic of this coarse graining is that the microscopic details can be subsumed by hydrodynamic parameters at this longer scale. In this work, we use a combination of experimental perturbations, machine learning, and microscopic modelling to investigate how changes in specific molecular motor properties are manifested at the scale of the system. We find that increasing the availability of fuel for our specific motors leads to a non-monotonic trend in activity that is related to the microscopic crosslinking of the motor clusters on pairs of filaments. The consequence of these microscopic differences in crosslinking is that as ATP is increased, the mechanics of the material change non-trivially alongside the activity.