Fine-Tuning Electrokinetic Injections Considering Nonlinear Electrokinetic Effects in Insulator-Based Devices

The manner of sample injection is critical in microscale electrokinetic (EK) separations, as the resolution of a separation greatly depends on sample quality and how the sample is introduced into the system. There is a significant wealth of knowledge on the development of EK injection methodologies that range from simple and straightforward approaches to sophisticated schemes. The present study focused on the development of optimized EK sample injection schemes for direct current insulator-based EK (DC-iEK) systems. These are microchannels that contain arrays of insulating structures; the presence of these structures creates a nonuniform electric field distribution when a potential is applied, resulting in enhanced nonlinear EK effects. Recently, it was reported that the nonlinear EK effect of electrophoresis of the second kind plays a major role in particle migration in DC-iEK systems. This study presents a methodology for designing EK sample injection schemes that consider the nonlinear EK effects exerted on the particles being injected. Mathematical modeling with COMSOL Multiphysics was employed to identify proper voltages to be used during the EK injection process. Then, a T-microchannel with insulating posts was employed to experimentally perform EK injection and separate a sample containing two types of similar polystyrene particles. The quality of the EK injections was assessed by comparing the resolution (Rs) and number of plates (N) of the experimental particle separations. The findings of this study establish the importance of considering nonlinear EK effects when planning for successful EK injection schemes.

Improving the accuracy of simulated chaotic N-body orbits using smoothness

ABSTRACT Symplectic integrators are a foundation to the study of dynamical N-body phenomena, at scales ranging from planetary to cosmological. These integrators preserve the Poincaré invariants of Hamiltonian dynamics. The N-body Hamiltonian has another, perhaps overlooked, symmetry: it is smooth, or, in other words, it has infinite differentiability class order (DCO) for particle separations greater than 0. Popular symplectic integrators, such as hybrid methods or block adaptive stepping methods do not come from smooth Hamiltonians and it is perhaps unclear whether they should. We investigate the importance of this symmetry by considering hybrid integrators, whose DCO can be tuned easily. Hybrid methods are smooth, except at a finite number of phase space points. We study chaotic planetary orbits in a test considered by Wisdom. We find that increasing smoothness, at negligible extra computational cost in particular tests, improves the Jacobi constant error of the orbits by about 5 orders of magnitude in long-term simulations. The results from this work suggest that smoothness of the N-body Hamiltonian is a property worth preserving in simulations.

Optimal Design of Electrode Structure and Microfluidics Channel for Effective Particle Separations

This paper presents the optimal design of electrode structure and microfluidics channel for effective particle separations. The purpose of the microfluidics chip is to generate the DEP (dielectrophoresis) force within the micro channel to separate both negative DEP (nDEP) and positive DEP (pDEP) particles of same sizes. The particles will experience DEP force when passing through the electric field created by electrode arrays located in different positions within the channel. The channel contains several electrode arrays where the pDEP particles are moved away from the electrodes and the nDEP particles are attracted towards them. In some existing microfluidics chips, because of the high intensity of the electric field around the electrodes, which results in a very high DEP force near the electrodes, most of the positive DEP particles are captured in the space between electrodes without being guided to the target outlet. Moreover, the effective area of DEP force is limited to a small region near the corner of the electrodes, therefore only those particles very close to the electrodes will experience sufficient attractive forces to be guided towards the target locations. This will decrease the efficiency of the particle separation. In this study, we developed an optimization methodology for design of electrode configurations using numerical modeling. The optimized electrode structure can provide much more evenly distributed DEP field. The design of the channel, the number and position of the electrode arrays were optimized in order to improve the efficiency of the particle separation. Finally, the optimized electrode structure and microfluidics channel were modeled and tested using multiphysics simulation software and the results show that this optimized design of microfluidics channel can provide high throughput and more effectiveness for particle separation compared to many existing microfluidics devices.