Oliver Richard Gittus

Particles (molecules, colloids, polymers, etc.) in solution migrate along a temperature gradient in a process called thermophoresis, thermodiffusion, or the Soret effect. Experiments have revealed many striking behaviors in dilute polymer solutions, including sign changes and extrema in the Soret coefficient ST as a function of solvent quality. We investigate the phenomenology of dilute-regime polymer thermophoresis using a minimal particle-based model that interpolates between a swollen coil and a compact globule, simulating various chain lengths. Our model exhibits sign changes in ST as a function of solvent quality, from thermophilic (ST < 0) to thermophobic (ST > 0) behavior, in both the good-solvent and poor-solvent regimes. To our knowledge, the latter has not been previously reported and occurs when increasing the monomer–monomer attraction. We correlate the increase/decrease of ST with that of the negative solvation entropy. Furthermore, our results highlight that the difference between the thermophoretic and drift velocities can be large, even for relatively large polymers approaching the long-polymer limit. We observe the counter-intuitive situation of a net drift against the direction of particle accumulation for portions of the good-, Θ-, and poor-solvent regimes.

Water is arguably the most important liquid on Earth. Consequently, its anomalous properties have been intensely investigated for over 50 years. However, water's thermal conductivity maximum (TCM) remains hitherto unexplained. Beyond its substantial fundamental interest, this problem is critical because many natural (e.g., climate regulation), industrial and chemical processes in which water appears as solvent at near-standard conditions correspond to the anomalous heat transport regime of water. We use all-atom and minimal coarse-grained models to isolate the TCM's thermodynamic fingerprint, and subsequently demonstrate its thermodynamic and microscopic origin: (1) the depopulation of librational modes due to nuclear quantum effects and (2) the balance of two interconverting molecular arrangements, the high density and low density liquid states, that coexist in water. We systematically investigate tetrahedral liquids modeled with Stillinger-Weber potentials, which allows the interpolation between simple liquids and low coordination materials such as carbon. We show that the TCM is not exclusive to water, but an anomalous behavior shared by pure liquids with intermediate tetrahedrality. Our work provides a thermodynamic explanation for the TCM of water and tetrahedral liquids in general.

The thermal conductivity of water features a maximum (TCM) as a function of temperature at constant pressure. By examining why molecular force fields succeed or fail to reproduce the maximum and interpreting our results using the Bridgman equation, we show that water's TCM is connected with its compressibility minimum. Using Stillinger-Weber potentials for tetrahedral liquids, we interpolate between the behaviour of simple liquids and highly tetrahedral materials such as carbon. Together with two vanishing limits at low/high tetrahedrality, we identify three regimes for the TCM: when it originates from either the compressibility minimum or density maximum, or both. Thus, the TCM exists in a "Goldilocks Zone" of tetrahedral order. We provide a thermodynamic explanation for the TCM of not only water, but tetrahedral liquids in general.

Temperature gradients induce mass separation in mixtures in a process called thermal diffusion and are quantified by the Soret coefficient ST. Thermal diffusion in fluid mixtures has been interpreted recently in terms of the so-called (pseudo-)isotopic Soret effect but only considering the mass and moment of inertia differences of the molecules. We demonstrate that the first moment of the molecular mass distribution, the mass dipole, contributes significantly to the isotopic Soret effect. To probe this physical effect, we investigate fluid mixtures consisting of rigid linear molecules that differ only by the first moment of their mass distributions. We demonstrate that such mixtures have non-zero Soret coefficients in contrast with ST = 0 predicted by current formulations. For the isotopic mixtures investigated in this work, the dependence of ST on the mass dipole arises mainly through the thermal diffusion coefficient DT. In turn, DT is correlated with the dependence of the molecular librational modes on the mass dipole. We examine the interplay of the mass dipole and the moment of inertia in defining the isotopic Soret effect and propose empirical equations that include the mass dipole contribution.

We show that simple non-polar liquid binary mixtures feature a minimum in the Soret coefficient at a specific composition and explain the microscopic mechanism determining such minima.

The widescale importance and rich phenomenology of water continue to motivate the development of computational models. ReaxFF force fields incorporate many characteristics desirable for modeling aqueous systems: molecular flexibility, polarization, and chemical reactivity (bond formation and breaking). However, their ability to model the general properties of water has not been evaluated in detail. We present comprehensive benchmarks of the thermophysical properties of water for two ReaxFF models, the water-2017 and CHON-2017_weak force fields. These include structural, electrostatic, vibrational, thermodynamic, coexistence, and transport properties at ambient conditions (300 K and 0.997 g cm⁻³) and along the standard pressure (1 bar) isobar. Overall, CHON-2017_weak predicts more accurate thermophysical properties than the water-2017 force field. Based on our results, we recommend potential avenues for improvement: the dipole moment to quadrupole moment ratio, the self-diffusion coefficient, especially for water-2017, and the gas phase vibrational frequencies with the aim to improve the vibrational properties of liquid water.

The drift motion experienced by colloids immersed in a fluid with an intrinsic temperature gradient is referred to as thermophoresis. An anisotropic mass distribution inside colloidal particles imparts a net orientation with respect to the applied thermal field, and in turn alters the thermophoretic response of the colloids. This rectification of the rotational Brownian motion is called thermal orientation. To explore the sensitivity of the thermal orientation effect with the internal composition of colloids, we investigate the thermophoretic response of rod-like colloids in the dilute regime, targeting different internal mass distributions. We derive phenomenological equations to model the dependence of the Soret coefficient with degree of mass anisotropy and test these equations using non-equilibrium molecular dynamics simulations. Using both theory and simulation, we show that the average orientation and the Soret coefficients of the colloids can depend significantly on the internal mass distribution. This observation suggests an approach to identify internal colloidal compositions using thermal gradients as sensing probes.