Hierarchical fractal assemblies from dual-anisotropic building blocks fabricated via magnetic field-assisted thermo-mechanical deformation

Precisely controlling particle anisotropy is essential for designing building blocks for colloidal assembly, yet independently programming multi-directional anisotropic particle interactions remains a key challenge in constructing hierarchical architectures. Here, we introduce dual-anisotropic ferromagnetic polystyrene (PS) particles with independently tunable shape and magnetic response. Using a magnetic field-assisted thermo-mechanical deformation platform, shape anisotropy is controlled by mechanically stretching PS particles embedded in a polyvinyl alcohol (PVA) film above their glass transition temperature, while magnetic anisotropy is independently programmed by applying a directional magnetic field during deformation. Under a unidirectional magnetic field, individual batches of these particles assemble into basic head-to-tail chains or side-by-side stacks. Importantly, co-assembling a binary mixture of these distinct building blocks triggers the formation of orthogonal T-junctions, as supported by numerical simulations. These programmable local junctions serve as structural nodes that transcend simple aggregation, driving the emergence of scale-invariant fractal networks. Fourier-space analysis confirms that these unit configurations statistically govern large-scale assembly organization. Fractal and multifractal analyses further reveal that particle-level anisotropy fundamentally controls aggregate connectivity and multiscale structural complexity within assembled networks. This strategy provides a scalable route for constructing fractal assemblies through programmable particle anisotropy, expanding opportunities for designing functional materials with controlled hierarchical architectures.