Simulating Contact Instability in Soft Thin Films through Finite Element Techniques

When a thin film of soft elastic material comes in contact with an external surface, contact instability triggered by interaction forces, such as van der Waals, engenders topologically functionalized surfaces. Innumerable technological applications such as adhesives; microelecromechanical systems (MEMS), and nanoelectromechanical systems (NEMS) demand understanding of the physics behind the mechanical contact, relationship between the morphologies, and detachment forces in such films. Indentation tests are important experimental approach toward this; there also exist many simulation procedures to model the mechanical contact. Both atomistic level and analytical continuum simulations are computationally expensive and are restricted by the domain geometries that can be handled by them. Polymeric films also particularly demonstrate a rich variety of nonlinear behavior that cannot be adequately captured by the aforementioned methods. In this chapter we show how finite element techniques can be utilized in crack opening and in contact-instability problems

Contact instability of a soft elastic film bonded to a patterned substrate

A linear stability analysis is presented for the contact instability of a soft thin elastic film which is rigidly bonded to a physically patterned substrate, and is in adhesive contact with a smooth rigid contactor. Increasing roughness by enhancing the substrate-amplitude produces increasingly smaller instability length-scales. The smallest wavelengths obtainable are 0.3*h, an order of magnitude smaller than that observed with films on flat substrates (3*h). Instability length-scales are found to be largely independent of substrate length-scales. For van der Waals interaction, increase in substrate roughness increases the energy penalty and, consequently, requires smaller gap distances (< 1nm for stiff films) to engender instabilities. When an externally controllable long-range electric field is employed instead, instabilities can be initiated at very low critical voltages (˜32 V) even in relatively stiff films, making it a more suitable route to produce miniaturized instability patterns

Rheotaxis of spheroidal squirmers in microchannel flow: Interplay of shape, hydrodynamics, active stress, and thermal fluctuations

Microswimmers exposed to microchannel flows exhibit an intriguing coupling between propulsion, shape, hydrodynamics, and flow which gives rise to distinct swimming behaviors. We employ a generic coarse-grained model of prolate spheroidal microswimmers, denoted as squirmers, exposed to channel flow to shed light onto their transport properties. The embedding fluid is implemented by the multiparticle collision dynamics approach (MPC), a particle-based mesoscale simulation method, which includes thermal fluctuations. Specifically, the influence of swimmer shape—spherical vs spheroidal—, active stress—pusher, ciliate, puller—, and thermal fluctuations on their rheotactic behavior is analyzed. The microswimmers accumulate at the confining walls at very low flow rates. With increasing flow strength, squirmers are depleted from the walls, and at high flow rates are also depleted from the channel center. The squirmers show pronounced cross-channel swimming between the confining walls with mixed oscillating and rotational motions due to thermal fluctuations. This strongly affects their rheotactic behavior. In particular, spherical pullers and ciliates swim upstream, whereas spherical pushers essentially swim downstream. The anisotropic shape of spheroidal squirmers enhances wall and center depletion and the alignment of the propulsion direction parallel to the flow, which leads to preferred downstream swimming for all active stresses. This emphasizes the importance of swimmer shape and hydrodynamic wall interactions on the transport properties of a microswimmer such as Volvox and Opalina, for example