Enabling Soft Molds for Manufacturing Polymeric Surface Structures with Overhangs

Surface structures with overhangs are ubiquitous in nature to offer vital functions, yet reproducing them for manufacturing is challenging due to mold interlock during demolding. Soft molds have been proposed to prevent interlock, but they risk stiction-caused collapsing because their intrinsic overhanging features are susceptible to strong intermolecular forces under the microscale. To address this, we model the relationship between overhang geometries and material properties, targeting a balanced relationship between flexibility and structural integrity. We then verify our model using polydimethylsiloxane (PDMS) molds with different moduli and geometries, as well as reported soft molds in the literature. The excellent agreement between our model and all experimental data enables us to proceed with molding using various thermosetting polymers. Employing one of the robust PDMS molds, we replicate doubly re-entrant surface structures exhibiting two levels of hierarchical overhangs, which exhibit high-fidelity reproduction that successfully repels low-energy fluids without a coating. This work establishes key design principles for soft mold fabrication that prevent interlock damage and enable complex overhang formation, paving the way for large-scale manufacturing of intricate biomimetic surfaces with functional overhanging architectures.