Nanoscale fluid flow systems involve both microscopic and macroscopic parameters, which compete with each another and lead to different flow regimes. In this work, we investigate the interactions of four fundamental parameters, including the fluid-fluid, fluid-wall binding energies, temperature of the system, and driving force, and their effects on the flow motion in nanoscale Poiseuille flows. By illustrating the fluid flux as a function of a dimensionless number, which represents the effective surface effect on the fluid, we show that the fluid motion in nanochannels falls into different regimes, each of which is associated with a distinct mechanism. The mechanisms in different situations reveal the effects of the parameters on the fluid dynamics. The potential applications of nanofluidics in science and engineering have motivated intensive investigations of fluid flow in nanochannels, where the classical Navier-Stokes ͑NS͒ equations may not apply. Some microscopic parameters that are not considered in the classical treatments play important roles in nanoscale fluid flows, such as the molecular binding energies for the fluid-fluid and fluid-surface interactions ͓1,2͔. These microscopic parameters are directly related to some macroscopic variables, including the temperature and external driving force of the flow system. The temperature of the system measures the thermal vibration of the fluid molecules, which can break the fluid-surface binding if the temperature is high ͓1,3,4͔. Similarly, a large driving force can help fluid molecules overcome the attraction of the surface ͓5-8͔. On the other hand, the competition between fluid-fluid and fluid-surface binding energies becomes critical when the temperature is low and the driving force is small ͓2,7,9͔. The coupling of these parameters at different scales determines the flow regimes, where the mechanisms of how the parameters affect the fluid motion are different. For both theoretical analysis and practical applications, it is essential to probe the flow regimes and the parameter dependence in nanochannel flows.Over the past two decades, many attempts have been made to understand the behavior of confined fluids in the nanoscale, including static and dynamic properties as well as boundary conditions ͓10-21͔. Many attentions were focused on how surfaces affect the fluid motion through molecular interactions ͓4-8,16-21͔. It has been well known that strong fluid-wall binding energy fw favors fluid adsorption on the wall surface and stick boundary condition tends to be valid in this case ͓1,4,5,9,11,17͔. The temperature of the system T is another factor that affects fluid adsorption and the ratio fw / kT is usually used to predict the adsorption, where k is the Boltzmann constant. However, fw / kT may not be sufficient to describe the behavior of fluids at the surface. Many other parameters, such as the external driving force and fluid density, also affect the fluid properties at the interface ͓15,16,18,21,22͔. Furthermore, the molecular binding energy among fluid m...

In this work, we investigate the validity of the Navier-Stokes (NS) equations for nanoscale liquid flows through molecular dynamics simulations. We focus on the role of channel size by considering the fluid-wall interaction. Liquid flows between two planar parallel walls driven by an external force with channel size ranging from 2 to 80 nm are studied. The volumetric flux is computed and the dependence of the volumetric flux on the channel size is explained both qualitatively and quantitatively. It is found that the flow is sensitive to the fluid-wall binding energy and the classical fluid mechanics falls apart in small nanochannels. However, the wall effects become insignificant and the NS equations are valid when the channel size is larger than about 150 molecular diameters (∼ 50 nm)

In this Letter, we propose a composite nanochannel system, where half of the channel is of low surface energy, while the other half has a relatively high surface energy. Molecular dynamics simulations show that fluids in such channels can be continuously driven by a symmetric temperature gradient. In the low surface energy part, the fluid moves from high to low temperature, while the fluid migrates from low to high temperature in the high surface energy part. The mechanisms that govern the flow are explained and the conditions required to guarantee the flow and the possible applications are discussed. DOI: 10.1103/PhysRevLett.105.174501 PACS numbers: 47.61.Àk Liquid transport in micro-or nanochannels is of great importance in many applications, including biomolecule separation, energy conversion, and thermal management [1][2][3][4]. Mechanical, electrokinetic, and acoustic approaches can be used to drive the fluid through the channel [4][5][6][7][8].The fluid can also be circulated by heterogeneous forces caused by a surface tension, chemical, or temperature gradient [1,[9][10][11]. However, in certain applications, these gradients can be symmetric and the total net force on the fluid vanishes. In this case, the fluid cannot be constantly transported without external forces. Inspired by our recent work on the flow regimes in nanochannels [12], here we propose a composite nanochannel system, where half of the channel has low and the other half has relatively high surface energy, as will be explained later. Through molecular dynamics simulations, it is shown that liquids in such channels can be continuously pumped by a symmetric temperature gradient along the channel. One advantage of this system is the application for chip-level cooling, where the heat generated in the chip can be used to drive the liquid without using external pumps, which consume energy, occupy space, and therefore conflict with the miniaturization objectives of next generation electronic devices.The composite nanochannel system is illustrated in Fig. 1. The channel is formed by two parallel walls. A liquid (green particles), which is in connection with two reservoirs, is confined by the walls. For the convenience of numerical simulation, the same structure and potential are used for the walls. The composite channel is defined in terms of the surface energy or fluid-wall interaction, which is heterogeneous and realized by controlling the fluid-wall binding energy " fw . The binding energy between the fluid and left half of the wall (orange) " fwðLÞ is weak (low surface energy) and that between the fluid and right half of the wall (gray) " fwðRÞ is strong (high surface energy). The low or high surface energy represents different types of molecular interactions. For low surface energy, the fluid-wall interaction is weak and mainly repulsive, while both repulsive and attractive interactions are strong for high surface energy.The channel walls are constructed by truncating a rectangular portion from a face-centered cubic structure with a lattic...