PMMA particles on silk fibers

PMMA colloids (more or less Plexiglass) are small spherical particles that are used as model systems. I was using these PMMA particles to create soft gel-like structures in the hopes of providing experimental evidence for some of the computational results found by Jader Colombo and Emanuela Del Gado in a recent-ish paper.

However, this side project hit the point where it either became a side project, or it needed to be put on the shelf. But I kept these particles, supplied by Itai Cohen’s group at Cornell, for a rainy day and decided to do something fun with them.

I dipped these silk fibers from a cocoon into a solution with the 1.5 micrometer fluorescent particles and imaged them with a confocal microscope at different magnifications. Its cool to see that the particles really like the fibers and really highlight the disordered fiber network.



If you zoom in a bit, you can actually resolve how the particles pack onto the curved surface of the silk fiber! The video here shows the 3D structure. Useful? Probably not.



Rheology in the news: Workshop recap

Great workshop. While the topics certainly covered the friction vs hydrodynamics argument that I expected to see, I was happy to see more work on stress propagation, Wyart-Cates theory, and even shear banding. I’m a little disappointed that, despite good discussion, it didn’t appear that people were converging on an agreement for the mechanism for shear thickening. Maybe I was too optimistic.

To me, the next big problem that needs to be addressed in the field is: what does colloidal friction mean? This is certainly a loaded question, because the mechanism behind macroscopic friction isn’t so well defined. But showing that two colloids can have a frictional contact without irreversibly aggregating would answer a lot of questions.

Georgetown University wrote a little summary about the workshop, and it even includes a quote or two from me. Here is the news story.



ACS Colloids meeting recap

Another great ACS Colloids meeting has come and gone. In addition to my talk being well attended and having nice questions, I saw some really great science presented.  Here were my some of my favorites (in no particular order):

Chinedum Osuji posed a very interesting problem that I had never considered. A bulk material made of complex particles can relax stress either from structural rearrangements or by dissipative modes in the particles themselves. And you can tune these two timescales to compete with each other.

Jeff Morris provided a very clear explanation of a difficult subject, stressing the importance of frictional contacts in shear thickening suspensions. Some of his results can be found here where frictional and frictionless simulations are compared. I am excited to hear more later this week when Georgetown hosts the seminar on dense colloidal suspensions. The program is now available, and it looks awesome!

John Crocker addressed the fact that power-laws are seen everywhere in the rheology of soft materials. The origin of some of these power-laws has eluded researchers, but he suggested to think about these problems in terms of the free energy landscape instead of in real space. Particularly noteworthy was his mapping of the problem onto the features of the Colorado river running through the Grand Canyon.

I’m looking forward to the 2017 ACS Colloids meeting at the City College of NY!