No abstract

Nanoparticle surface charge density plays an important role in many applications, such as drug delivery and cellular uptake. In this study, surface charge properties of silica nanoparticles with different sizes are studied using a multi-ion surface charge-regulation model. In contrast to most previous studies utilizing constant surface charge, protonation and deprotonation surface reactions are used to obtain the local surface charge, which depends on the particle size and electrolyte solution properties, including the salt concentration and pH. For a fixed particle size, the magnitude of the surface charge typically increases with an increase in pH or background salt concentration. For fixed background salt concentration and pH, the magnitude of surface charge decreases with an increase in the particle size and reaches a constant when the particle size exceeds a critical value. Size dependent surface charge is further characterized by the ratio of electrical double layer thickness to the particle diameter, and the surface charge varies significantly when this dimensionless ratio is above 0.2.

Analytical results for the velocity distribution, mass flow rate, pressure gradient, wall shear stress, and vorticity in mixed electroosmotic/pressure driven flows are presented for two-dimensional straight channel geometry. We particularly analyze the electric double-layer (EDL) region near the walls and define three new concepts based on the electroosmotic potential distribution. These are the effective EDL thickness, the EDL displacement thickness, and the EDL vorticity thickness. We show that imposing Helmholtz-Smoluchowski velocity at the edge of the EDL as the velocity matching condition between the EDL and the bulk flow region is incomplete under spatial bulk flow variations across the finite EDL. However, the Helmholtz-Smoluchowski velocity can be used as the appropriate slip velocity on the wall. We discuss the limitations of this approach in satisfying the global conservation laws.

Gas microflows are encountered in many applications of Micro-Electro-Mechanical Systems (MEMS). Computational modeling and simulation can provide an effective predictive capability for heat and momentum transfer in microscales as well as means of evaluating the performance of a new microdevice before hardware fabrication. In this article, we present models and a computational methodology for simulating gas microflows in the slip-flow regime for which the Knudsen number is less than 0.3. The formulation is based on the classical Maxwell/Smoluchowski boundary conditions that allow partial slip at the wall. We first modify a high-order slip boundary condition we developed in previous work so that it can be easily implemented to provide enhanced numerical stability. We also extend a previous formulation for incompressible flows to include compressibility effects which are primarily responsible for the nonlinear pressure distribution in micro-channel flows. The focus of the paper is on the competing effects of compressibility and rarefaction in internal flows in long channels. Several simulation results are presented and comparisons are provided with available experimental data. A specific set of benchmark experiments is proposed to systematically study compressibility, rarefaction and viscous heating in microscales in order to provide validation to the numerical models and the slip-flow theory in general as well as to establish absolute standards in this relatively young field of fluid mechanics.