Molecular alignment in molecular fluids induced by coupling between density and thermal gradients

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Physics

Background:

• Understanding molecular fluid behavior in confined spaces is crucial for nanotechnology and materials design.
• External stimuli like thermal gradients and forces significantly influence molecular orientation.
• Existing models often struggle to fully decouple the effects of density gradients and thermal gradients on molecular orientation.

Purpose of the Study:

• To investigate the orientation response of molecular fluids confined in slit pores subjected to thermal gradients and/or applied forces.
• To elucidate the distinct mechanisms driving molecular orientation under these conditions.
• To quantify the relationship between molecular orientation, density gradients, and applied force magnitude.

Main Methods:

• Non-equilibrium molecular dynamics (NEMD) simulations were employed to model fluid behavior.
• Theoretical analysis was used in conjunction with simulations to interpret results.
• Simulations involved applying thermal gradients and/or forces mimicking gravitational effects.

Main Results:

• An applied force induces an inhomogeneous density and molecular orientation similar to thermal gradients.
• Molecular orientation is directly proportional to the fluid's density gradient and increases with applied force magnitude.
• Concurrent application of force and thermal gradient allowed disentanglement of orientation mechanisms.

Conclusions:

• Molecular orientation in confined fluids arises from both density variations and direct thermal gradient effects.
• A density-gradient-driven mechanism and a thermal-gradient-only mechanism (especially for asymmetric molecules) were identified.
• This study provides a fundamental understanding of molecular fluid response to external stimuli in confinement.