Molecular dynamics (MD) simulations provide a powerful tool for studying fluid behavior under nanoconfinement, where the spatial constraints imposed by structures like carbon nanotubes or porous materials lead to phenomena that deviate significantly from macroscopic fluid dynamics. These simulations capture atomic-scale interactions, enabling the investigation of anomalous transport properties, interfacial effects, and dynamic processes that cannot be described by continuum models.

One of the most striking phenomena observed in nanoconfined fluids is the alteration of viscosity. In bulk fluids, viscosity is a well-defined property governed by momentum transfer between molecules. Under confinement, however, viscosity becomes spatially dependent due to the influence of solid boundaries. MD simulations reveal that near-wall regions exhibit higher effective viscosity due to molecular layering, while the central region may display reduced viscosity because of restricted molecular motion. In carbon nanotubes with diameters below 10 nm, water viscosity can decrease by up to an order of magnitude compared to bulk water, primarily due to the smoothness of graphene-like walls and the resulting slip effects.

Slip boundary conditions are another critical aspect of nanoconfined fluid dynamics. Unlike in macroscopic systems, where the no-slip condition is often assumed, MD simulations show that fluids can exhibit significant slip at solid interfaces. The slip length, which quantifies the extrapolated distance where the flow velocity would vanish, depends on the surface wettability, roughness, and fluid-solid interaction strength. Hydrophobic surfaces, such as those of carbon nanotubes, promote slip lengths ranging from tens to hundreds of nanometers, while hydrophilic surfaces may reduce slip or even enhance stickiness. This slip behavior directly impacts flow rates in nanochannels, leading to permeabilities that exceed continuum predictions.

Capillary filling dynamics in nanopores also deviate from classical Washburn theory, which assumes a continuum description of fluid flow. MD simulations demonstrate that the filling process is influenced by precursor films, where fluid molecules advance ahead of the main meniscus due to strong van der Waals interactions with the pore walls. In sub-10 nm pores, the filling speed is no longer proportional to the square root of time but instead exhibits a linear or even accelerated regime due to the dominance of surface forces over viscous dissipation. Additionally, the meniscus shape becomes highly distorted at the nanoscale, with pronounced layering of fluid molecules near the walls.

Electroosmotic flow (EOF) in nanoconfined systems presents further complexities. When an electric field is applied along a charged nanochannel, the resulting ion motion drags the surrounding fluid via viscous coupling. MD simulations reveal that the classical Helmholtz-Smoluchowski theory overpredicts EOF velocities in nanopores because it neglects ion crowding, steric effects, and dielectric heterogeneity. In carbon nanotubes, for instance, the high surface charge density leads to strong counterion accumulation, which screens the electric field and reduces flow rates. Moreover, the slip boundary conditions further modulate EOF, as the enhanced mobility near the walls amplifies the overall flow.

Ionic current modulation is another area where MD provides insights beyond continuum models. In nanopores, ion transport is governed by a combination of electrostatic interactions, hydration effects, and entropic barriers. For example, in sub-2 nm carbon nanotubes, potassium ions exhibit higher mobility than sodium ions due to their weaker hydration shells, leading to selective conduction. The ionic current also shows nonlinear dependence on voltage, as crowding effects become significant at high ion concentrations. MD simulations capture these phenomena by explicitly modeling ion-water and ion-wall interactions, revealing mechanisms like ion hopping and dehydration that are absent in continuum descriptions.

Contrasting nanoconfined fluidics with continuum fluidics highlights several atomic-scale phenomena. Layering effects, where fluid molecules form discrete shells parallel to the confining walls, are a hallmark of nanoconfinement. These layers exhibit distinct densities and orientations, influencing transport properties. In nanopores, the first adsorbed layer often behaves like a solid, while subsequent layers display progressively bulk-like behavior. Such structuring leads to oscillatory density profiles and solvation forces that are absent in continuum models.

Another key difference is the breakdown of the Navier-Stokes equations at the nanoscale. Continuum models assume local thermodynamic equilibrium and a linear stress-strain relationship, but under extreme confinement, these assumptions fail. MD simulations show that velocity profiles can become non-parabolic, and stress correlations extend over longer ranges due to molecular ordering. Additionally, thermal fluctuations play a more significant role in nanoflows, leading to stochastic variations in transport properties that are smoothed out in continuum approaches.

In summary, MD simulations of nanoconfined fluids uncover a rich array of phenomena that challenge classical fluid dynamics. Anomalous viscosity, slip boundaries, and nonlinear capillary filling arise from the interplay of molecular interactions and geometric constraints. Electroosmotic flow and ionic current modulation are further complicated by atomic-scale effects like ion crowding and hydration dynamics. These insights not only advance fundamental understanding but also inform the design of nanofluidic devices for applications in energy storage, desalination, and biomedicine. The contrast with continuum fluidics underscores the necessity of atomistic modeling for accurately predicting nanoscale transport behavior.