Negative Thermophoretic Force in the Strong Coupling Regime

A Mode-Coupling Model of Colloid Thermophoresis in Aqueous Systems: Temperature and Size Dependencies of the Soret Coefficient

Thermophoresis allows for the manipulation of colloids in systems containing a temperature gradient. A deep understanding of the phenomena at the molecular level allows for increased control and manipulation strategies. We developed a microscopic model revealing different coupling mechanisms for colloid thermophoresis under local thermodynamic equilibrium conditions. The model has been verified through comparison with a variety of previously published experimental data and shows good agreement across significantly different systems. We found five different temperature-dependent contributions to the Soret coefficient, two from bulk properties and three from interfacial interactions between the fluid medium and the colloid. Our analysis shows that the Soret coefficient for nanosized particles is governed by the competition between the electrostatic and hydration interfacial interactions, while bulk contributions become more pronounced for protein systems. This theory can be used as a guide to design thermophoretic transport, which is relevant for sensing, focusing, and separation at the microscale.

Tunable Self-Thermophoretic Nanomotors with Polymeric Coating

Thermophoretic micro/nanomotors (MNMs) generate self-propulsion without a chemical reaction. Intrinsically, this promises excellent biocompatibility and is thus suitable for biomedical applications. However, their propulsion efficiency is severely limited due to the poor understanding of the thermophoretic process, which dominates the conversion from thermal energy into mechanical movement. We here developed a series of self-thermophoresis light-powered MNMs with variable surface coatings and discovered obvious self-thermophoresis propulsion enhancement of the polymeric layer. An intrinsically negative self-thermophoretic movement is also observed for the first time in the MNM system. We propose that enthalpic contributions from polymer-solvent interactions should play a fundamental role in the self-thermophoretic MNMs. Quantitative microcalorimetry and molecular dynamics simulations are performed to support our hypothesis. The polymer solvation enthalpy and coating thickness influences on self-thermophoresis are investigated, further highlighting the essential enthalpy contributions to thermophoresis. Our work indicates that surface grafting would be important in designing high-efficiency thermally driven nanorobotic systems for biomedical applications.

Modeling thermodiffusion in aqueous sodium chloride solutions-Which water model is best?

Thermodiffusion is the migration of a species due to a temperature gradient and is the driving phenomenon in many applications ranging from early cancer detection to uranium enrichment. Molecular dynamics (MD) simulations can be a useful tool for exploring the rather complex thermodiffusive behavior of species, such as proteins and ions. However, current MD models of thermodiffusion in aqueous ionic solutions struggle to quantitatively predict the Soret coefficient, which indicates the magnitude and direction of species migration under a temperature gradient. In this work, we aim to improve the accuracy of MD thermodiffusion models by assessing how well different water models can recreate thermodiffusion in a benchmark aqueous NaCl solution. We tested four of the best available rigid non-polarizable water models (TIP3P-FB, TIP4P-FB, OPC3, and OPC) and the commonly used TIP3P and SPC/E water models for their ability to predict the inversion temperature and Soret coefficient in 0.5, 2, and 4M aqueous NaCl solutions. Each water model predicted a noticeably different ion distribution yielding different inversion temperatures and magnitudes of the Soret coefficient. By comparing the modeled Soret coefficients to published experimental values, we determine TIP3P-FB to be the water model that best recreates thermodiffusion in aqueous NaCl solutions. Our findings can aid future works in selecting the most accurate rigid non-polarizable water model, including water and ion parameters for investigating thermodiffusion through MD simulations.

Statistical Mechanics at Strong Coupling: A Bridge between Landsberg's Energy Levels and Hill's Nanothermodynamics

We review and show the connection between three different theories proposed for the thermodynamic treatment of systems not obeying the additivity ansatz of classical thermodynamics. In the 1950s, Landsberg proposed that when a system comes into contact with a heat bath, its energy levels are redistributed. Based on this idea, he produced an extended thermostatistical framework that accounts for unknown interactions with the environment. A decade later, Hill devised his celebrated nanothermodynamics, where he introduced the concept of subdivision potential, a new thermodynamic variable that accounts for the vanishing additivity of increasingly smaller systems. More recently, a thermostatistical framework at strong coupling has been formulated to account for the presence of the environment through a Hamiltonian of mean force. We show that this modified Hamiltonian yields a temperature-dependent energy landscape as earlier suggested by Landsberg, and it provides a thermostatistical foundation for the subdivision potential, which is the cornerstone of Hill's nanothermodynamics.

Strong Coupling and Nonextensive Thermodynamics

We propose a Hamiltonian-based approach to the nonextensive thermodynamics of small systems, where small is a relative term comparing the size of the system to the size of the effective interaction region around it. We show that the effective Hamiltonian approach gives easy accessibility to the thermodynamic properties of systems strongly coupled to their surroundings. The theory does not rely on the classical concept of dividing surface to characterize the system’s interaction with the environment. Instead, it defines an effective interaction region over which a system exchanges extensive quantities with its surroundings, easily producing laws recently shown to be valid at the nanoscale.

Role of Interfacial Entropy in the Particle-Size Dependence of Thermophoretic Mobility

We show that changes in the surface tension of a particle due to the presence of nonionic surfactants and impurities, which alter the interfacial entropy, have an impact on the value of the thermophoretic mobility. We have found the existence of different behaviors of this quantity in terms of particle size which can be summarized through a power law. For particles that are small enough, the thermophoretic mobility is a constant, whereas for larger particles it is linear in the particle radius. These results show the important role of the interfacial entropic effects on the behavior of the thermophoretic mobility.