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Soft Materials and Rheology Group Research 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
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Current projects Interfacially-active
particles Natural and
synthetic papillae
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