Flow-Directed Hierarchical Assembly of Filamentous Viruses toward Filler-Free Soft Thermal Films

Cross-plane heat transport in flexible and polymer-based devices remains a bottleneck for thermal management. Conventional strategies for enhancing heat transport in polymeric materials mainly rely on incorporating inorganic fillers; however, these approaches often compromise flexibility and interfacial integrity. In polymeric and soft materials, intrinsic structural disorder and weak interchain interactions further inhibit phonon transport, particularly in the cross-plane (through-thickness) direction. Here we present a strategy to program the mesoscale structure of M13 bacteriophage films by harnessing internal convective flow during solution casting, thereby yielding filler-free, biobased, and highly ordered soft films with enhanced cross-plane thermal diffusivity. By tuning the footprint at a fixed apparent contact angle in the pinned regime, the films developed a hierarchical alignment while preserving the interphage hexagonal lattice and showing only modest changes in the small-angle X-ray scattering (SAXS)-derived ensemble orientation. Polarized optical microscopy, atomic force microscopy (AFM) with phase-binarized boundary-angle analysis, and quantitative retardation mapping revealed that smaller footprints suppress the macrodomain boundary density and tilt disorder along the perimeter. Under pinned conditions, the cross-plane thermal diffusivity increased substantially without a monotonic dependence on thickness; at a fixed footprint, depinning reduced the diffusivity despite a comparable thickness, underscoring the role of the contact-line mode and flow topology. A comparison with magnetically aligned controls, which exhibit a higher ensemble orientation but lower diffusivity and persistent boundaries, confirms that the suppression of mesoscale boundaries, rather than the magnitude of SAXS-derived ensemble orientation or lattice densification, emerges as the structural feature most consistently associated with cross-plane transport. This geometry-programmed, convection-mediated assembly provides an ambient design principle for biobased, filler-free soft thermal films relevant to thermal management in flexible and printed electronics.