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!

My statement on climate change

As the lone scientist in many aspects of my life, I feel like I am obligated to make a statement on climate change for my friends and family. The purpose of this is not to dive into politics, but to provide you with my own perspective as a scientist on the issue. Additionally, I would like to offer an analogy that can easily be used in conversation.

I am a member of the American Physical Society (does that make you jealous?) and the society has an official position that “[t]he evidence is incontrovertible: Global warming is occurring” as a result of “human activities.”1 And after I used a dictionary to understand what “incontrovertible” meant, over 51,000 physicists stand by the statement that global warming is real and caused by us. There are many other science organizations that have similar positions (American Chemical Society, American Association for the Advancement in Science, American Meteorological Society, etc.).

Climate change skeptics most often use the scientific result from the 1990s that measured the temperature of Earth’s troposphere with satellites.2 Remember that guy from Red Bull that jumped out of a hot air balloon basically from space? Well, the last 10 miles he fell before reaching Earth was the troposphere. In this paper, they found that while the surface temperature of the Earth was increasing, the temperature of the troposphere was actually getting colder. Not surprisingly, people who disagree with climate change point to this paper as evidence. In fact, the U.S. Representative serving as the chairman for the Committee on Science, Space and Technology (though he is not a scientist) recently alluded to this paper.

Although that paper was a good first attempt, researchers in the early 2000s found a flaw.3 The new calculations, when done correctly, showed a continual increase in temperature matching the expected change from global warming. This is typically how science works; research grows on other research, and the new results either verify the old ones, or the old ones are no longer valid.

True academic consensus can only be determined when scientific contributions are taken as a whole. Focusing on one (blemished) result is taking science out of context. An analogy to this is as follows: you are a casual tennis player and you lose 0-6, 0-6, 0-6. If this is the only information I have, it would be reasonable for me to conclude that you are a horrible tennis player. But if I learn that this score is from a match you played with Novak Djokovic, I can’t necessarily conclude that anymore. I can’t choose to focus on only the score when I have additional information to consider.

The same goes for science; I can’t focus on one result, I need to take all results into account before I make a conclusion. Unfortunately, this is difficult because we aren’t all scientists, so we are liable to being exposed to scientific results taken out of context. But when almost every scientist agrees that global warming exists and is a result of human actions, it seems unreasonable to point out extraneous contributions and give them equal weight in an argument against the whole scientific body.


I hope this is enlightening, thought provoking, and not enough to get me assassinated,

Pasha Tabatabai



2) Christy, Spencer, and McNider, J. Climate, 8, 888–896 (1995)

3) Mears and Wentz, Science, 309, 1548-1551 (2005)


A PDF of this statement is available here: Climate Statement

Silk in the News: Silk fibers stay taught

Imagine taking a spare shoelace, one end in each hand, and holding it out in front of you so that the shoelace is nice and tight. What happens when you bring your hands closer together? The shoelace sags, forming a U-shape. Now return your hands to the original position. What happens if you pulled harder? You’d break the shoelace.

Researchers in both France and the UK have found that silk fibers from orb spiders don’t act like your shoelace in either of those scenarios! When you compress the silk, it stays tight by transforming some of the fiber into liquid droplets and avoids sagging. When you extend it, it also stays tight without snapping because of the molecular structure. They also developed a synthetic system that mimics silk in this way. This is explained in their recent paper and in the movie below.

Upcoming Talk: Extending colloidal aggregation to proteins

I will be giving a talk at the American Chemical Society’s Colloid and Surface Science Symposium on June 7. My talk is titled “Extending colloidal aggregation to proteins” and represents a good chunk of my graduate work.

Aggregation and gelation have been studied extensively by using colloids as a model system. And even though there is still much to be done with colloids, there is also the need to extend this research to biological materials. We have used denatured silk protein solutions as our “colloidal suspension” because they are isotropic, stable, and we can work at high concentrations. We studied the aggregation behavior caused by a change in the pH using the framework established with the colloidal literature. While our results show some similarities, the major difference between colloid and silk protein aggregation is that the associations between two protein cause structural changes in the protein. In real life, protein structural changes upon aggregation are important to understand because they can be associated with diseases (think amyloid fibrils in Alzheimers).

If you are going to this conference, I’d appreciate if you checked out my talk.

Rheometer on TV

While lying on the couch I saw something that I have grown custom to seeing in the lab but never before on TV: a rheometer! At the 19 second mark of the video, you see a rheometer tool slowly compressing some toothpaste. Who said Soft Matter Physics isn’t all glitz and glamour?

Friction Thickens Fluids

Congratulations to my colleague John Royer for not only getting his work published in Physical Review Letters, but also for getting some media attention! His work on shear thickening suspensions, which will be discussed at the previously mentioned workshop, bridges the gap between two different models of shear thickening: hydroclusters and frictional contacts.

The media blurb can be seen here!

And here is John summarizing his results:

Upcoming Workshop at Georgetown

On June 16-17, Georgetown will be hosting a workshop on the rheology of dense particulate suspensions. Among others, my advisor Dan Blair and our soft matter colleague Emanuela Del Gado are the organizers for a workshop that I see as proof that our department is becoming more impactful in the field.

The topics covered in the workshop include non-monotonic flow curves, organization under flow, and last but not least discontinuous shear thickening. Colloquially, this workshop is being referred to as the shear thickening workshop, because of the ongoing debate on the origin of shear thickening in dense suspensions. (Shear thickening is more-or-less when something acts ‘solid-like’ when you try and deform it quickly. Think silly putty.)

The major discussion point is whether shear thickening comes from hydrodynamics or if the constituent particles get pushed together and friction plays a role. Maybe after June 17 we will have an agreement…

Active Microtubule Network

A friend of mine, Dan Chen, works on active microtubule systems and the results from his work have spawned a new project in the lab I work in. Microtubules are rigid rod-like filaments (think PVC pipes) that make up the “train tracks” for cargo transport inside the cell. Cargo is moved along the microtubules by motor proteins that more or less look like stick-figures that step along the tubes. If you attach two motors by their heads, instead of walking down the tubes, the double motor will make the two attached tubes slide past each other to make a chemically driven network that moves in a pretty mesmerizing way. Here is a video he captured using confocal microscopy! The microtubules are the bright rods that bundle together, and the motor proteins are too small to be seen.