Daniel Y. Kwok

Lattice Boltzmann Simulation of Electroosmotic Flows in Micro- and Nanochannels

A Lattice Boltzmann Model (LBM) with the Poisson-Boltzmann equation for charge distribution is presented for the simulation of electroosmotic transport in straight rectangular micro- and nanochannels. Effects of the channel height, electrolyte concentration, surface potential, electric double layer thickness and externally applied electric field on the velocity profile of 50 to 800 nm channels were studied by means of a LBM. Our results are in excellent agreement with the corresponding analytical solution and the Lattice Boltzmann Model can be used to simulate electrokinetic transport phenomena in microchannels in the presence of an externally applied electric field.

Electrowetting on Dielectric: A Low Voltage Study on Self-Assembled Monolayers and Its Wetting Kinetics

A systematic study of electrowetting using alkanethiolate self-assembled monolayers (SAMs) as a dielectric layer is presented. SAMs surfaces are characterized by means of ellipsometry, and contact angle measurement. It is demonstrated that droplet spreading can be induced by a much lower voltage (4 volts), although SAMs appear to be irreversible at this operating potential. Experimental results show that the kinetics of contact angle of a NaCl solution droplet significantly depend on SAMs functional group, but not the hydrocarbon chain length. The modified molecular kinetic model employed to produce the fitted curves is in good agreement with the experimental results. A linear relation between the viscous parameter of the model and the number of hydrocarbon chain length of the thiol layer can be found. It appears that the friction force is also significantly influenced by the functionality of SAMs.

Lattice Boltzmann Simulations of Bubble Dynamics in Microchannels

The moving contact line problem of liquid-vapor interfaces on solid surfaces was studied using a mean-field free-energy lattice Boltzmann method recently proposed [Zhang et al., Phys. Rev. E, 69, 032602, 2004]. We have examined the static and dynamic interfacial behaviors by means of the bubble and capillary wave tests and found that both the Laplace equation of capillarity and the dispersion relation were satisfied. Dynamic contact angles followed the general trend of contact line velocity observed experimentally and can be described by Blakes theory. The velocity fields near the interface were also obtained and are in good agreement with fluid mechanics and molecular dynamics studies. Our simulations demonstrated that incorporating interfacial effects into the lattice Boltzmann model can be a valuable and powerful alternative in interfacial studies.

Lattice Boltzmann Simulation of Microfluidics with Non-uniform Zeta Potentials: Requirements of Flow Rate and Current Continuities

The characteristics of electrokinetic flow in a microchannel depend on both the nature of the surface potential, i.e. whether it is uniform or nonuniform, and the electric potential distribution along the channel. In this paper, the nonlinear Poisson-Boltzmann equation is used to model the electrical double layer and the Lattice Boltzmann Model coupled with the constraint of current continuity is used to simulate the flow field in a rectangular microchannel with step change surface potential. The results show that step change ion distribution caused by step change surface potential will influence significantly the electric potential distribution along the channel, the velocity profiles and the volumetric flow rate. This may indicate that if disturbance on the ion distribution due to nonuniform surface potential and velocity distortion is large and not negligible, the electric potential distribution along the channel in both pressure-driven and electroosmotic flow may not be linear.

Non-Equilibrium Injection Flow in a Nanometer Capillary Channel

Non-equilibrium molecular dynamics is applied to simulate liquid injection flow in a nanometer circular capillary. A full 12-6 Lennard-Jones potential, truncated at 2.2o, was used to govern the liquid-liquid and liquid-solid interactions. Our results show that there exists an oscillatory distribution of liquid density in the normal wall direction. The solid-liquid energy scale has a strong influence on molecular motion in the first liquid layer near the solid substrate. Under the injection condition, liquid transport in a nanometer capillary was found to behave in an unexpected wavelike motion.

Molecular Dynamics Simulation of Injection Flow in a Nano-Meter Syringe by Consideration of Extra Pairwise Interactions for Positive and Negative Ions

The average concentration of ions in a liquid depends on the size of a channel if the charges on solid surface do not change. The relation between them is that the concentration of ions is inversely proportional to the channel size. When a channel decreases from a micro to a nano size, the concentration of ions will increase 1000 times. In this case, the ion’s distribution in liquid may not be considered as dilute if the charges on solid surface is large, and interactions among ions have to take into account. In this paper, molecular dynamics is applied to study the effect of extra-pairs of positive/negative ions on liquid transport properties in a nano syringe under a constant injection flow rate boundary condition. In simulations, the Coulomb’s law and 12-6 Lennard-Jones potential are used to govern the interaction between ion-ion, ion-liquid, ion-solid, liquid-liquid and liquid-solid molecules. Four different cases (no ions, counter-ions, and counter-ions combining with small and large extra-pairs of positive/negative ions in liquid) are carried out. The non-equilibrium molecular dynamics (NEMD) simulation results show that the concentration of extra-pairs of positive/negative ions has significant influence on liquid velocity profile and ion distributions. For liquid flow without ions, a quasi-parabolic velocity distribution was obtained. When the counter-ions and extra-pairs of positive/negative ions are considered, the flow approaches a plug flow as the number of extra-pair of ions increases. We also found that charges in liquid do not follow the Poisson-Boltzmann distribution, especially for the net charges which have a valley located at about 1.5 molecular sizes away from the solid surface.Copyright

Effect of Solid-Fluid Energy Parameters on Streaming Potential and Flow Rate in Pressure-Driven Flow in Microchannels

Electrokinetic phenomena play an important role in microfluidic transport behavior. Review of literature suggests that surface energetic can also be an important factor, but rarely explored. Typically, surface energetic is taken into account by consideration as an arbitrarily selected slip boundary condition in the modified Navier-Stokes equation. In this paper, instead of selecting this arbitrary slip condition, we examine how solid-liquid energy parameters influence the transport of microfluidics in terms of streaming potential. The simultaneous effects of surface energetics and electrokinetics will be conducted by means of a mean-field free energy lattice boltzmann approach recently proposed. Rather than using the conventional Navier-Stokes equation with a slip condition, the description solid-liquid energetic is manifested by the more physical energy parameters in the mean-field description of the method. As a result, the magnitude of liquid slip can be related directly to the solid-liquid interfacial slip. These results will be employed in conjunction with the description of electrokinetic transport phenomena for streaming potential. The variation of streaming potential as a function of the energy parameters (solid-liquid interaction) is clearly demonstrated. In pressure-driven liquid microfluidics, the flow rate may be decreased due to the counter-effect between the electrokinetic and slip.Copyright

Reversible Protein Detection Method Based on Self-Assembled Monolayers Using Ellipsometry

We propose a simple, and selective optical method for biological detection, using reflection or absorption of light upon transmission through a homogeneous/inhomogeneous thin film surface. In this experimental study, a wavelength band is sought as an optical signature to identify biological sample immobilized on an ultrathin film. We have used variable angle spectroscopic ellipsometry, a technique generally used in the semiconductor industry, to identify proteins. For immobilizing the proteins, 1-octadecanethiol (C-18) coated to gold (Au) substrate has been used because of its low surface energy. In order to characterize 1-octadecanethiol and evaluate its re-usability, fourier transform infrared spectroscopy (FTIR) was employed. Experimental results illustrate that proteins are sensitive to optical investigation in visible and near infrared region of the electromagnetic spectrum.

Climbing Liquid Drops on Chemically Patterned Surfaces: Potential Applications for On-Chip Microfluidics

We employ reactive-wetting drops to move fluid against gravitational force. Early experiments have shown that the surface energy gradient must achieve a critical value in order to induce a significant increase in the front dewetting. The velocity of a climbing-drop is constant, unlike the gradual decrease intuitively linked to object under gravitation. The drop velocity does decrease, however, for increased slope. The maximum inclination, or stopping, angle for our particular setup is 25.. A non-dimensional ratio is employed to reproduce the experimentally obtained trend, and its use may be of value in the future.

Liquid Wettability and Micro-droplet Self-Movements on Heterogeneous Solid Surfaces

In this paper, we studied the contact angle behaviors on heterogeneous surfaces by a mean-filed free-energy lattice Boltzmann method recently proposed [Zhang et al., Phys. Rev. E, 69, 032602, 2004]. Simulation results showed that the Cassie law was valid when the heterogeneous patterns were smaller than or of the same order to the liquid-vapor interface thickness. A droplet would move spontaneously if it experienced unbalanced Young forces on a heterogeneous surface. Such self-movements were similar to previous experimental observations. Our simulations also showed more potential of the lattice Boltzmann method in future interfacial studies.