PNNL

The Science

Fluids flow differently in confined channels than in open spaces, requiring alternative approaches to modeling their behavior. As the channels become smaller, the effects of molecular-scale processes can increase. Researchers investigated where including molecular details is needed when modeling fluid flow in small, confined channels. Their study establishes a theoretical analysis to define two regions, one where nanoscale interfacial dynamics are critical and another where the flow is accurately modeled by standard continuum theory. By demonstrating the important role of chemistry and molecular-scale interactions on confined fluid flows, the results can help guide future studies on when to apply different modeling approaches.

The Impact

Confined fluid flows happen during material self-assembly and in biological nanochannels in a process critical for the transport of nanoparticles and ions. During nanoparticle self-assembly, flow contributes to the formation of superlattice structures. Understanding how chemistry at the interfaces of pores and transient channels affects fluid flow is important for accurately modeling the kinetics of processes such as self-assembly, ion transport, and flow reactions. These results provide guidance for when continuum descriptions of fluid flows are appropriate to use in models.

Summary

Nanoconfined flows occur during nanoparticle self-assembly, where the fluid between particles is expelled during aggregation, or in biological nanochannels such as lipid bilayer membranes. Traditional continuum theories often fail to accurately describe these systems because they lack the molecular-level resolution required to capture confinement effects and account for non-uniform properties and fluctuations across the fluid domain. To articulate the effect of molecular details on fluid flows under confinement, researchers combined equilibrium and non-equilibrium molecular dynamics simulations with detailed statistical mechanical analyses as a function of fluid–wall interactions and salt concentrations. The study demonstrates consistency between equilibrium and non-equilibrium approaches across flow types and confinement levels, pointing out the robust nature of linear response theory for understanding confined fluid flows. More importantly, the study reveals quantitative correlations between physicochemical parameters at the molecular scale and characteristics of fluid flows under confinement such as hydrodynamic wall and slip length. This consistency allows for rational applications of continuum descriptions of fluid flows, depending on chemistry. These findings can help enhance the effectiveness of molecular-based simulations for investigating complex confined systems in nanofluidics, biology, and colloidal science, offering a complementary molecular-scale perspective to traditional continuum approaches.

Contact

Greg Schenter, Pacific Northwest National Laboratory, greg.schenter@pnnl.gov 

Jaehun Chun, Pacific Northwest National Laboratory, jaehun.chun@pnnl.gov

Funding

This work is supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Chemical Physics and Interfacial Sciences Program, Grant No. FWP 16249.