Formation of stable aggregates by fluid-assembled solid bridges

<p>Earh's surface is covered with soil; particulate mixtures subject to cycles of wetting and drying. The role of this transient hydrodynamic forcing in creating and destroying aggregates is virtually unexplored. We examine this process at the grain scale. When a colloidal suspension is dried, capillary pressure may overwhelm repulsive electrostatic forces, assembling aggregates that are out of thermal equilibrium. This poorly understood process confers cohesive strength to many geological and industrial materials. Here we observe evaporation-driven aggregation of natural and synthesized particulates, and then probe their stability under rewetting using a microfluidics channel as a flume to determine the entrainment threshold. We also directly measure bonding strength of aggregates using an atomic force microscope. Cohesion arises at a common length scale (~5 microns), where interparticle attractive forces exceed particle weight. In polydisperse mixtures, smaller particles condense within shrinking capillary bridges to build stabilizing “solid bridges” among larger grains. This dynamic repeats across scales forming remarkably strong, hierarchical clusters, whose cohesion derives from grain size rather than mineralogy. Transient capillary pressures are even sufficiently large to sinter the smallest particles together. These results may help to understand the strength and erodibility of natural soils, and other polydisperse particulates that experience transient hydrodynamic forces.</p>

FABRICATION AND TESTING OF POLYDIMETHYLSILOXANE (PDMS) MICROCHANNEL FOR LAB-ON-CHIP (LOC) MAGNETICALLY-LABELLED BIOLOGICAL CELLS SEPARATION

Microfluidics channel of micron- to millimeter in dimension has been widely used for fluid handling in transporting, mixing and separating biological cells in Lab-on-Chip (LoC) applications. In this research, fabrication and testing of Polydimethylsiloxane (PDMS) microfluidic channel for Lab-on-chip magnetically-labelled biological cells separation is presented. The microchannel is designed with one inlet and outlet. A reservoir or chamber is proposed as an extra component of the microchannel design for ease of trapping the intended biological cells in LoC magnetic separator system. The PDMS microchannel of circular-shaped chamber geometry has been successfully fabricated using replica molding technique from SU-8 negative photoresist mold. An agglomerate-free microbeads flowing has been observed using the fabricated PDMS microchannel. Trapping of microbeads in the trapping chamber with 2.0 A current supply in the continuous microfluidics flow > 100 mL/min has also been demonstrated. In conclusion, a separation of biological cells labelled with magnetic microbeads is expected to be realized using the fabricated PDMS microchannel.

Electrodes Microfluidics System for Microbio Object Analysis

We have designed and fabricated an electrode microfluidics system for microbio object analysis. Two parallel-plate electrodes were fabricated using the soft lithography technique integrated with a polydimethylsiloxane (PDMS) microfluidics channel. Gold (Au) material was decomposed to fabricate the electrodes. The voltage response through the charging and discharging of the electrodes was observed using an oscilloscope. For a constant dc voltage of 5 V we obtained a time constant for the electrodes of 3.6 ms. On the other hand, it requires 850 ms to discharge completely without an external load. The capacitance of the electrodes in air (room environment) is 0.39 pF. Due to the higher dielectric constant of distilled water (80.1), in the water medium the electrode capacitance is 0.77 pF. We also measured the capacitance of the electrodes by changing the medium to microbio objects such as yeast cells (5 pF) and live bacteria cells (30 pF). The results showed that bacteria have a higher electrical capacitance than yeast.

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.