Controlling the morphologies and properties of the surface and/or interface of bimaterials consisting of soft polymers provides new opportunities in many engineering applications. Crease is a widely observed deformation mode in nature and engineering applications for soft polymers where the smooth surface folds into a region of self-contact with a sharp tip, usually induced by the instability from mechanical compression or swelling. In this work, we explore the competition mechanisms between surface and interface creases through numerical simulations and experimental studies on bilayer hydrogels. The surface or interface crease of the bilayer hydrogels under swelling is governed by both the modulus ratio (M₂/M₁) and the height ratio (H₂/H₁). Through extensive numerical simulations, we find that the interface crease of the bilayer hydrogels can only occur at a moderate modulus ratio (24 < M₂/M₁ < 96) and a large height ratio (H₂/H₁ ≥ 8). Guided by this phase diagram, our experiments confirm that both surface and interface creases can be generated by swelling triggered instability, and the transition of surface to interface creases occurs at the critical value of the height ratio (H₂/H₁) between 5 and 10. Such an observation is in good agreement with our numerical predictions. Fundamental understandings on the switching between the surface and interface creases provide new insights into the design of highly tunable soft materials and devices over a wide range of length scales.

中文翻译：

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双层水凝胶中表面界面折痕的转变

控制由软聚合物组成的双材料的表面和/或界面的形貌和特性在许多工程应用中提供了新的机会。折痕是在软聚合物的自然和工程应用中广泛观察到的变形模式，其中光滑的表面折叠成与尖锐的尖端自接触的区域，通常是由于机械压缩或膨胀引起的不稳定性所致。在这项工作中，我们通过数值模拟和双层水凝胶的实验研究来探索表面和界面折痕之间的竞争机制。溶胀下双层水凝胶的表面或界面折痕受模量比（M ₂ / M ₁）和高度比（H₂ / H ₁）。通过广泛的数值模拟，我们发现，双层水凝胶的接口折痕只能发生在中等模量比率（24 <中号₂ /中号₁ <96）和大的高度比（ H ^ ₂ / ħ ₁ ≥8）。在此相图的指导下，我们的实验证实，膨胀和触发的不稳定性都可能产生表面和界面折痕，并且表面到界面折痕的过渡发生在高度比的临界值（ H ₂ / H _1）处。）在5到10之间。这样的观察与我们的数值预测非常吻合。对表面折痕和界面折痕之间转换的基本理解为在各种长度范围内的高度可调的软材料和设备的设计提供了新的见解。