Emergent Softening and Stiffening Dictate Transport of Active Colloidal Filaments.

Active semiflexible filaments are crucial for biophysical processes, yet insights into their single-filament behavior have predominantly relied on theory and simulations, owing to the scarcity of controllable synthetic systems. Here, we present an experimental platform of active semiflexible filaments composed of dielectric colloidal particles activated by an alternating electric field that induces contractile or extensile electrohydrodynamic (EHD) flows. Our experiments reveal that contractile flow-generating filaments (CFs) undergo softening, significantly expanding the range of accessible conformations, whereas extensile filaments (EFs) exhibit active stiffening. By independently tuning filament elasticity and activity, we show that the competition between elastic restoring forces and emergent hydrodynamic interactions along the filament governs conformational dynamics. Specifically, we find that the time scale of conformational dynamics governs the transport of active filaments: enhanced fluctuations lead to diffusive motion despite activity, whereas activity-induced stiffening enables directed propulsion in nonlinear filaments. Our findings highlight that conformational changes, not just geometric or chemical asymmetry, enable propulsion of flexible microswimmers. These insights are essential for designing flexible microswimmers, whose transport can be tailored through controlled activity and shape changes. Additionally, our system provides a powerful platform for gaining fundamental insights into active filament dynamics.