Interdisciplinary Applied Mathematics

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FIGURE 10.1. A sketch of a nanochannel filled with a simple fluid. The filled circles denote the channel wall atoms, and the open circles denote the fluid atoms. The fluid atoms interact with each other by a Lennard-Jones potential Vlj,i, and the fluid atoms interact with the wall atoms by a Lennard-Jones potential Vlj,2.

theory for hydrophobic surfaces.

10.1 Atomistic Simulation of Simple Fluids

Atomistic simulation of nanofluids has gained considerable attention over the last two decades. Much of the existing literature has been devoted to understanding “simple fluids” in nanochanels. Though there is no consensus on the precise definition of a simple fluid in the literature, in most cases, it is simply taken as a collection of atoms that interact via the Lennard-Jones potential and the dynamics of which follow the classical mechanics described by Newton’s law. In practice, some noble gases (e.g., argon) can be modeled fairly accurately as a simple fluid. Figure 10.1 shows a schematic of a nanochannel filled with a simple fluid.

The investigation of simple fluids, in contrast to the study of complex fluids such as water (discussed in Chapter 11) or electrolytes (discussed in Chapter 12), has many advantages. First, the computational cost of atomistic simulation involving simple fluids is much lower compared to that of complex fluids, since it is much cheaper to evaluate the Lennard-Jones potential describing simple fluids compared to the evaluation of the electrostatic interactions that are required in the study of most complex fluids. Second, despite its simplicity, the investigation of simple fluids can provide deep insight into the physics of fluid transport in nanochannels, and such insight can guide the study of more complex fluids. For example, the study of simple fluids indicated that the classical Navier-Stokes equations breakdown in a channel as narrow as 4 fluid atomic diameters (Travis et al., 1997), and later, a similar finding was reported for electroosmotic transport in a silicon nanochannel that is also about 4 water diameters wide (Qiao and Aluru, 2003b). Third, the investigation of simple fluid transport provides data for the validation of theories describing fluid transport in nanochannels. Due to the complicated interactions involved in complex fluids, most of the nanofluid transport theories that have been developed so far are limited to simple fluids.

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