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Soft Materials and Rheology Group Research


Buckling phenomena

Most of this research is supported by the Air Force Office of Scientific Research (AFOSR).

Our interest in natural and synthetic papillae is closely linked to a broader interest in buckling phenomena and elastic instabilities.

Simulations

Simulating elastic instabilities is a significant challenge: such instabilities are accompanied by symmetry breaking, large changes in deformation, and high computational requirements. Of particular interest are situations in which eigenvalue analysis, a basic tool to predict buckling modes, does not successfully predict the observed buckling modes. We are developing a new approach of applying random perturbations and loading a structure in a step-by-step fashion. This allows the instability – with the right mode – to “emerge” without imposing any systematic perturbation. Fig. 1 illustrates how this method is applied to the famous “Lame’ problem” where an annular sheet under radial tension develops radial wrinkles.

 

Fig. 1: Left: A quarter model of a thin annular sheet with random perturbations of modulus of 5% amplitude. The tension at the periphery is fixed, whereas the inner tension is ramped up gradually. Right: Corresponding height profile showing radial buckles.

Swelling-induced fold formation

Crosslinked polymers cannot dissolve, but can swell strongly when exposed to a good solvent. We discovered that when a thin film of polymer weakly bound to a substrate is exposed to a drop of solvent, the swelling induces the formation of permanent folds (Fig. 2). The mechanism of this fold formation is a multi-step process: first the swelling region of the polymer develops a severe compressive stress and undergoes buckle delamination off the surface. Second, the delaminated region grows into a sharp, tall fold. Finally, upon evaporation of the solvent, the fold does not relax back, but becomes permanent. This is not a wrinkle-to-fold transition (in which sinusoidal precursor wrinkles turn into folds). Instead, the film goes directly from a flat to a folded state. We are now examining methods to the location and shaoe of folds.

 

Fig. 2: Left: A drop of toluene swelling a PDMS film and forming folds. Right. SEM image of folds after evaporation of solvent. Images are from Velankar, Lai and Vaia, ACS App. Mat. Int., 4, 24-29, 2012. Download. See two videos of the fold formation process.

Thin film buckling on liquid substrates

Our lab has a long-standing interest in compatibilizers added to blends of immiscible polymers. Compatibilizers are block copolymers of various architectures which are adsorb at the interface between immiscible homopolymers and strongly affect the mechanical properties of the interface. Yet, quantitatively measuring these mechanical properties is difficult since the compatibilizer films are only a few nm thick and are inextricably bound to the surrounding – very viscous – homopolymers. We seek to use buckling phenomena to obtain the mechanical properties. The basic idea is to apply a compressive stress on the interface to induce interfacial buckling and use the buckling characteristics to back out the mechanical properties of the interface. We are presently validating our methods using plastic films floating on the surface of viscous polymer fluids (Fig.3). Once successful, this approach will be applied to compatibilizers.

Fig. 3: Thin plastic film floating on a liquid buckles due to compression. The buckling wavelength is related to the modulus of the film.


Questions, Suggestions, Comments? Send e-mail to velankar@pitt.edu

 

 

 

 

 

 

 

 

 

 

Current projects

Interfacially-active particles

Natural and synthetic papillae

Buckling phenomena

Microfluidic drop flows

Other research