Our recent Cover Art in Soft Matter
Our recent Cover Art in Soft
Matter
I am not a worshiper of Large-N. Large-N
worshipers refers to those scientists
who believe that scientific output is proportional to the number of published
manuscripts, Hence their ambition is simply to publish as many papers as
possible. As an experimental research group, we believe experimental repeatability and robustness are sacred cornerstones of research. In this yagna1
of research, we believe that our tapasya2 cannot be measured by the
quantity of publications, but rather by the quality of our manuscripts. So, as
2017 draws to a close, let me tell you the story of our only peer-reviewed
manuscript published this year.
Our research group works
at the intersection of physical and biological sciences, and over the last few
years we have concentrated significant research effort on a class of
biophysical problems related to bacteria. As you may already be aware, bacteria
represent one of the most ancient life forms on our planet. In 2011, we
stumbled upon the phenomena of bacterial streamers - streamers are thin
filamentous biofilms that tend to form a spider-web like structure in systems
with sustained hydrodynamic flow. These streamers consist of bacterial cells
encased in extra-cellular polymeric matrix (EPS). More information relating to this
phenomenon can be found in my blog article
(Bacterial Streamers).
So, if this is a
problem, it should be easy to stop … right? Well, no. When we first discovered
streamers we had very little idea as to how they formed. The road-block to
imaging these structures early in their inception stage was the fact that the
EPS secreted by bacteria has the same refractive index as
water, so in a liquid environment the threads were close to invisible. Thus,
streamers could only be imaged after they formed. Moreover, we did not know whether
the phenomenon was biological, or physical, or biophysical in origin.
In the absence of
concrete data, I had to rely on my intuition. I surmised that it was the EPS
matrix that was playing a key role in the formation of the streamer and that
the entire phenomenon was actually dictated by driven by fluid mechanical stresses. Based on this
hunch, I asked Mahtab Hassanpourfard (my PhD student) to extract the bacterial
EPS. She began this work sometime in early 2014 and spent several months
culturing bacteria to extract the EPS. After weeks of work, she would manage to
extract only a few milliliters of EPS. We then tried pushing the EPS fluid
first into our device and allowing it to wet the microfluidic channel before pushing
in a bacterial suspension. We saw formation of a few streamers using this
process, but the results were not repeatable and we could not arrive at any
firm conclusion regarding the inception mechanism. After about a year of
unsuccessful experimentation we abandoned this approach. Solving the bacteria
streamer riddle was proving to be quite challenging.
During the second-half
of 2015, I decided to try a new approach. I asked Nandini Debnath (another PhD
student) to use polyacrylamide (PAM) to make an EPS like fluid. PAM is a
polymer molecule with a large molecular weight and when a small amount of PAM
powder is mixed in water, it yields a colorless viscoelastic fluid much like
our bacterial EPS. As surrogates for bacterial cells, we used polystyrene
particles. We flowed particle laden PAM through a microfluidic channel and,
bingo, we had artificial streamers forming in our device exactly like the
biological counterparts. This was a huge breakthrough. Several research groups
all over the world had been chasing this research question, and we were able to
provide concrete data that the entire phenomenon is fluid mechanical in origin.
The non-linearity in the system comes from the polymeric nature of the fluid.
We were the first research group in the world to have shown that the
biophysical phenomenon of streamers can be generalized to the physical domain.
Our setup allowed us to perform several experiments that would have been almost
impossible with bacterial cells. The idea we started testing in 2014
culminated in our Soft Matter publication in 2017<sup>3</sup>.
Why I am especially proud of this manuscript: Science derives its strength
from the fact that it is able to generalize results performed from myriad
experiments into (often) universal or near universal ideas. Yet, as a scientist
one does not observe such a generalization often. Our work showed that the
phenomena of bacterial streamer formation can reach beyond the biophysical
realm, into the realm of complex fluids. The fact that this generalization came
from our lab and is the fruit of years of work is what makes me extremely
happy. Nandini's manuscript was selected to be part of the Back Cover Art of
Soft Matter (image below). The cover art shows a floc of nanoparticles, which
have aggregated together due to the bridging effect of long-chained polymer
molecules. The figure below is a false colored scanning electron microscopy (SEM) image, where भगवा (Saffron) depicts the particles, while हरा (Green) regions depict the
polymer.
Finally, this article is aimed primarily at
students interested in research. Remember, quality research cannot be rushed.
It can be a slow and painstaking undertaking, but the resulting discoveries can
be very sweet.
References:
1. Yagna (यग्न) - Sanskrit word roughly meaning sacrifice.
2. Tapasya (तपस्या) - Sanskrit word roughly meaning asceticism
3. N. Debnath, M. Hassanpourfard, R. Ghosh, J. Trivedi, T. Thundat, Mohtada
Sadrzadeh,A. Kumar, “Abiotic streamers in a microfluidic system”, Soft
Matter,13, 8698-8705 DOI: 10.1039/c7sm01771e, (2017) Web-Link
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