  Active stresses, which are collectively generated by the motion of
energy-consuming rod-like constituents, generate chaotic autonomous flows.
Controlling active stresses in space and time is an essential prerequisite for
controlling the intrinsically chaotic dynamics of extensile active fluids. We
design single-headed kinesin molecular motors that exhibit optically enhanced
clustering, and thus enable precise and repeatable spatial and temporal control
of extensile active stresses. Such motors enable rapid, reversible switching
between flowing and quiescent states. In turn, spatio-temporal patterning of
the active stress controls the evolution of the ubiquitous bend-instability of
extensile active fluids and determines its critical length dependence.
Combining optically controlled clusters with conventional kinesin motors
enables one-time switching from contractile to extensile active stresses. These
results open a path towards real-time control of the autonomous flows generated
by active fluids.
