Drop morphology can be manipulated by designing localized solid/liquid interactions to create a favorable interfacial energy equilibrium. A topographical surface with hierarchical roughness can be harnessed to generate complex drop morphologies, enhance uniaxial and anisotropic spreading, in a designable fashion. Here, using an active surface is proposed with a responsive roughness (wrinkle patterns) under uniaxial compression/stretching, to morph droplet shape biaxially in a continuous and reversible manner. The keys to achieve biaxial drop shaping are the in‐plane confinement from lattice hole patterns and the programmable formation of roughness, to pin and guide contact line movement in both in plane directions. The complex interplay between wetting and the patterns is elucidated by both experiments and numerical analysis. The results enrich the current understanding of shaping droplets by managing the contact line pinning/movement on an engineered elastic substrate, and providing insights for emerging applications in the areas such as droplet emicrofluidics, liquid robotics, ink‐jet printing, 3D printing and healthcare.
A new test configuration called Holed‐Cracked Square Plate (HCSP) is proposed to investigate I/II mixed‐mode fracture of brittle and quasi‐brittle materials. This specimen is a square plate containing a central hole with two radial cracks emanating from its circumference. The finite element method was used to calculate mode I and mode II stress intensity factors and T‐stress solutions for various crack lengths and hole diameters. The numerical results show that a full range of fracture mode mixities can be realized by changing the orientation angle of two radial cracks, while maintaining a very simple specimen geometry and loading requirement. A series of fracture tests were conducted on polymethylmethacrylate (PMMA) to study the practical capabilities of the HCSP specimen. These yielded fracture toughness values that are consistent with other experimental results. Other observed quantities including fracture initiation angles and fracture resistance also agree very well with mixed‐mode fracture theories.
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