Water is one of the most critical substances on earth for life. Many important biological and geological reactions important for life take place in water. It has various anomalous properties like increased specific heat capacity and isothermal compressibility on cooling from ambient conditions. These become more pronounced on cooling below the freezing point i.e. supercooling. These thermodynamic properties are a result of the unique H-bonding structure in liquid water. Despite its importance, there is still debate about the intermolecular structure of water. One hypothesis to explain water’s anomalies is its two phase structure- enthalpy favored low density liquid (LDL) where molecules are arranged in a tetrahedral fashion and entropy favored high density liquid (HDL) where the tetrahedral structure is collapsed. Water molecules continuously fluctuate between HDL and LDL forms and Widom line is a line on the Pressure-Temperature surface where fluctuations between these two structures reach a maximum.
The experimental challenge to supercool water below its homogenous freezing limit (≈233 K at 1 atm) has meant that there is little data at such supercooled conditions. Ours was the first research groups to cool bulk water below its homogenous freezing limit. We have an experimental set-up where we evaporate ≈15 µm diameter water droplets in vacuum.The resulting evaporative cooling enables us to reach temperature as low as ≈227 K with still around 1% of droplets remaining unfrozen. We study these droplets with femtosecond x-ray pulses and measure the scattering pattern by ‘diffract before destroy’ technique. From measurements of wide angle and small angle x-ray scattering of water, we show the first experimental evidence of observing a maximum in thermodynamic response functions- a signature of the Widom line. Our results are consistent with the hypothesis that water has a liquid-liquid critical point (LLCP) deep in the supercooled liquid phase around ≈800+250 bar and 207+6 K. The difference between the LLCP values for H2O and D2O indicate the importance of nuclear quantum effects.
Harshad is a researcher in the Chemical Physics division at Stockholm University. His research involves studying molecular structure and dynamics of water. Harshad received his Bachelors’ degree in Chemical Engineering from Institute of Chemical Technology in Mumbai in 2008. Following this, he joined the M.S. + Ph. D. program at the Chemical engineering department at The Ohio State University, USA where he studied nucleation and aerosol science using nanodroplets of water and alkanes and earned his PhD in 2013. He later worked in ‘Humic Growth Solutions’ – a Fertilizer Industry for a year where he set-up a wet chemistry lab and was in-charge of quality control of the plant. Since 2015, he has been working at Stockholm University as a researcher and using x-rays to understand molecular structure and dynamics of supercooled water to shed more light on the uniqueness of the inter-molecular structure of water and its importance to life.
This seminar is compulsory for students registered for course CL 702 or CL 704.