Luka Pocivavsek's month with Prof. Cerda at UCACH

Luka spent the monthe of January 2009 with Prof. Enrique Cerda, continuing the productive collaboration between Cerda's group and Prof. Ka Yee Lee's group at Chicago. He reports as follows:

We indeed has a very productive one month. As I indicated in
the pre-trip e-mail, Enrique and I are moving forward with
several of the systems that we only touched upon in the
Science paper. Our analysis and experiments in the Science
paper were focused on the wrinkle to fold transition as it
occurs in elastic membranes on liquid substrates. In the same
paper, we also briefly mention that similar transitions are
observed when the substrate is not a liquid but rather a gel.
We are highly motivated to explore the transition on a gel
substrate because of the strong structural and geometric
similarities that such systems seem to have to biological
tissues including skin, arteries, and airways. 

During this month we made progress on several fronts allowing
us to quantitatively study the wrinkle to fold transition on
elastic substrates. First, we found an appropriate method for
making an elastic substrate with tunable mechanical properties
such as stiffness and also very good adhesion to latex rubber
sheets. Controlling gel stiffness while maintaining strong
adhesion between the gel and surface membrane is paramount to
our ability to study the wrinkle to fold transition in these
systems. Poor adhesion leads to blistering, a phenomenon that
complicates and competes with the focusing effects of folding.
Secondly, we designed and constructed a prototype compression
apparatus for the gel systems. Unlike for a membrane on a
liquid, where simple linear compression is easily achieved, a
gel/membrane system must be stressed by invoking curvature.
The prototype compression apparatus allows us to achieve
uniform curvatures along a membrane strip 12x3x2cm at low
strains in which the flat to wrinkled transition can easily be
studied. During the visit, we also developed ideas for a
second compression apparatus which will allow us to maintain
uniform curvatures  at higher strains where the wrinkle to
fold transition can be studied in detail. With the prototype
system and our new gels, we were able to obtain preliminary
data that 1. validates that the response of our gels is
linearly elastic during the wrinkling stage and 2. that a
clear change in amplitude scaling is seen during the
transition from wrinkles to folds which follows our arguments
from the Science paper.

Over the next several months, we will continue the development
of the second compression apparatus, take more data in the
wrinkling regime to strongly validate the lambda ~
t(Em/Es)^(1/3) law, and work on numerical simulations of the
system.