(6je) Using X-Ray Science to Study Structure and Ultrafast Dynamics in Liquids

4th Year Postdoctoral Researcher

Research Interests:

My research
interests are in the field of Chemical Physics with a specialization in liquids
and aerosols. Aerosols enable us to reach metastable conditions which are
difficult to achieve in bulk samples and consequently, materials can be studied
far from equilibrium by aerosolizing them. I am interested in studying
structure of materials and how they explain thermodynamic and kinetic
properties of aqueous systems relevant to modern chemical engineering. A novel
way to do this is by performing experiments at state-of-the-art ultrabright
x-ray sources. E.g., X-ray scattering of micron-sized water droplets can reveal
structural changes on supercooling; Ultrafast
pump-probe experiments can help us observe dynamics on a femtosecond timescale.
Such experiments are now possible due to X-ray Free Electron Laser (XFEL) sources which were not available a decade ago. I would also
like to complement these studies with molecular modeling results by collaboration
with theoreticians.

Successful
proposals: I have written proposals to ultrabright x-ray facilities like SACLA
in Japan, ESRF in France and APS in USA to perform experiments related wide
angle and small angle x-ray scattering of supercooled bulk water.

Postdoctoral
Project: Building an experimental set-up to study supercooled water and
performing experiments at various ultrabright x-ray facilities to study its
unique properties.

Principal
Investigator: Dr. Anders Nilsson, Department of Physics, Stockholm
University.

PhD Dissertation:
Nucleation and Droplet Growth During Co-condensation of Nonane and D₂O
in a Supersonic Nozzle

Under the
guidance of Dr. Barbara Wyslouzil, Department of Chemical and Biomolecular
engineering, The Ohio State University (OSU).

Research Experience:

My research
career started when I was admitted for a M.S.+Ph.D. Program
at the Chemical Engineering Department at OSU in September 2008. I studied
nucleation and droplet growth of water and n-alkane nanodroplets and their
mixtures. Supersonic nozzles are an effective way for dehydration of natural
gas without the use of chemicals or excess energy. My research involved producing
nanodroplets (nucleation) by condensation of vapors in a supersonic nozzle and
following their growth. The vapors consisted of pure water, pure n-alkanes or
mixtures of them. I used three experimental techniques to monitor this process-
Pressure trace measurements (PTM), Fourier transform infrared (FTIR)
spectroscopy and Small angle x-ray scattering (SAXS). I was also trained in the
National school of Neutron and x-ray scattering school at Argonne national Lab
and Oak Ridge National Lab to study the principals of scattering theory.

I joined Dr.
Anders Nilsson for post-doctoral research in March 2015 where I along with a
PhD student, built a set-up for supercooling microdroplets of water in vacuum
to study structure and dynamics of supercooled water and answer questions about
the anomalous behavior of this critical compound. The resulting set-up has been
used at different X-ray synchrotron and XFEL sources like European Synchrotron
Research Facility (ESRF), APS at Argonne National Lab (ANL) in USA, Linac
Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory in
USA, SACLA XFEL in Japan and PAL-XFEL in Korea. I have also led three of these
beamtimes at SACLA, APS and ESRF. The experiments done with this set-up have
resulted in high impact publications e.g. in Science (where I was the 2^nd
out of 12 authors) and Nature Communications.

I
also led a project on correlating water tetrahedrality based on the peak split
in the first peak of water's structure
factor . I worked with theoreticians like Dr. Lee-Ping Wang and Dr. Jeremy
Palmer on this project and became familiar with the different complexities of
molecular modelling.

Teaching Interests:

I have a teaching philosophy
that encourages students on asking questions and keeps the classroom
environment more interactive. Since my teaching experience in Freshman engineering at OSU, I have interacted with students
from different backgrounds. Based on my subsequent assignments as teaching
supervisor/assistant, I have developed the following views on teaching:

Fundamental
concepts:  By the end of the course, the students
should be able to grasp the fundamental concepts of the course. I designed
tutorials that helped the students achieve this goal. A relevant example is
scaling laws during transport phenomenon courses. The concept of thermal
diffusivity in a micron-sized film and a thick wall of a house can be described
by the same equation with differing time scales. Such universal applicability
of any phenomenon makes students appreciate the subject and helps them retain
it better.

Communication
skills:  According to Richard Feynman, whether you
really understand a topic is based on whether you can explain that topic on a
freshman level. I have realized that this is an important skill for anybody
with higher education and requires good communication skills.

Teaching experience: I have
been involved in the following courses as a Teaching Assistant at OSU:

Experience as a supervisor: When
I was studying for my PhD at The Ohio State University, I was a
supervisor/mentor to an undergraduate student. I introduced him to concepts of
nucleation theory and aerosol technology. I also trained him with hands-on work
in the laboratory. I also supervised a group of 4 students where they used
equipment in our lab for their Bachelors degree project. In total, I have
supervised bachelor degree projects for 4 students and acted as a mentor for 1
student. During my employment as a post-doctoral researcher at Stockholm
University, I was designated as an assistant supervisor to Doctoral student who
joined in September 2015.

In the future, I
would like to teach students about thermodynamics with specialization in
metastable states and phase transitions.

Future Direction:

I would like to
study dynamics and thermodynamics of supercooled water by building novel
experimental set-ups because it is experimentally challenging. Even though
water is one of the most important compounds for life, it is not yet understood
completely. My plan is to modify the experimental set-up which I built during
my postdoctoral research and achieve colder temperatures and higher pressures (as
illustrated in figure 1) and investigate the "fragile to strong" transition
of water and possibly the liquid-liquid critical point of water which is like
the holy-grail in this topic. I would also work
together with theoreticians and other experimentalists to interpret the results which I get from different experiments like x-ray
scattering and spectroscopy.

Figure 1: Phase
diagram of water. Figure adapted from Poole et al. The blue arrows show the
different experimental conditions which I propose to explore. The hypothesized
liquid-liquid critical point (LLCP) is shown as a ? and is yet to be explored
experimentally.

I. Flat sheet
set-up to study deeply supercooled water at ambient pressure:

My plan is to
build a set-up to form flat sheets (1 µm or less) of water to probe
temperatures below the Widom line (229 K at 1 atm). With such small sample
volume, I can probe even colder temperatures by delaying homogenous freezing. I
have made preliminary attempts to make these sheets work and the results look
promising.

II.
Aqueous-organic ideal mixtures:

I would also
like to study the effect of pressure on water. One way to study the effect of
pressure on water is by addition of a second component. Although addition of
some solutes delays the crystallization, they may also dilute the anomalous
behavior. However, the right kind of solute can delay crystallization and still
preserve its anomalies. This ideal mixture can then be delivered in the form of
supercooled microdroplets or flat sheets.

Selected Publications:

1.     Kyung
Hwan Kim*, Alexander Spaeh*, Harshad
Pathak, Fivos Perakis, Daniel Mariedahl, Katrin Amann-Winkel, Jonas A
Sellberg, Jae Hyuk Lee, Sangsoo Kim, Jaehyun Park, Ki Hyun Nam, Tetsuo Katayama
and Anders Nilsson. Maxima in the thermodynamic
response and correlation functions of deeply supercooled water, Science
2017, 358, 1589-1593.

2.     Harshad Pathak, Jeremy Palmer, Daniel Schlesinger,
Kjartan Thor Wikfeldt, Jonas A Sellberg, Lars GM Pettersson, and Anders Nilsson. The structural validity of various thermodynamical
models of supercooled water, Journal
of Chemical Physics 2016, 145, 134507.

3.     Andrew Amaya, Harshad Pathak, Viraj P Modak,
Hartawan Laksmono, N Duane Loh, Jonas A Sellberg, Raymond G Sierra, Trevor A
McQueen, Matt J Hayes, Garth J Williams, Marc Messerschmidt, Sebastien Boutet,
Michael J Bogan, Anders Nilsson, Claudiu A Stan, and Barbara E. Wyslouzil. How cubic can ice be?, Journal of Physical Chemistry
Letters, 2017, 8(14), 3216-3222.

4.    
Harshad Pathak, Judith Woelk, Reinhard Strey
and Barbara Wyslouzil. Co-condensation of
Nonane and D₂O in a supersonic nozzle, Journal of Chemical
Physics 2014, 140, 034304.

5.     Harshad
Pathak,
Gerald Wilemski, and Barbara Wyslouzil. The
structure of D₂O-nonane nanodroplets, Journal of Chemical
Physics, 2014, 140, 224318

6.    
Harshad Pathak, Kelley Mullick, Barbara
Wyslouzil, and Shinobu Tanimura. Nonisothermal
Droplet Growth in the Free Molecular Regime, Aerosol Science and Technology
2013, 47, 1310-1324.