293305 Manipulation of Nanoparticles Using Electro-Kinetics Generated by Nano-Needles

Tuesday, October 30, 2012
Hall B (Convention Center )
Kurtis Heacox1, Nicholas R. Wood1, Amanda I. Wolsiefer1, Robert W. Cohn2 and Stuart J. Williams3, (1)Department of Mechanical Engineering, University of Louisville, Louisville, KY, (2)University of Louisville, Louisville, KY, (3)Mechanical Engineering, University of Louisville, Louisville, KY

Electro-kinetics is the motion of a fluid or a suspended particle due to the effects of an electric field on the fluid and particle. There are three types of electro-kinetic forces generated by a nano-needle that can manipulate nearby particles.  These are dielectrophoresis, AC electroosmosis, and electrothermal fluid flow.

Dielectrophoresis is the movement of a neutral particle through an electric field gradient due to an imbalance in induced charge forces on the particle. Dielectrophoresis is considered positive when the particles are attracted to the location of the highest electric field gradient, and it is considered negative when the particles are repelled.  The magnitude of the dielectrophoretic force is directly proportional to both the electric field gradient, and the volume of the particle. The electric field gradient needed to capture or repel particles is inversely proportional to the size of the particle because other forces begin to dominate particle motion as particle size decreases. AC electroosmosis (ACEO) is the electro-kinetic pumping of ionic fluids caused by an AC electric field.  Charges inside the double layer are acted upon by the tangential component of the AC electric field. The region influenced by ACEO is close to the electrode surface, and is dependent upon the dielectric properties of the surrounding fluid. ACEO typically dominates when the AC field has a low frequency and when the fluid has low conductivity.  Electrothermal fluid flow occurs when an electric field acts upon fluid dielectric gradients caused by a non-uniform temperature field. Electrothermal fluid flow typically dominates when the AC field has a high frequency and when the fluid has high conductivity.

When an AC signal is applied using a nano-needle and a planar surface as electrodes, sharp electric field gradients form in the surrounding fluid at the tip of the needle. These gradients cause the above electro-kinetic forces to act on the fluid and any nearby particles.  The dominating electrokinetic mechanism is a function of the conductivity of the fluid, the frequency of the AC field, the location of the particle, and the diameter of the particle.

The nano-needles used were made from siver gallium (Ag2Ga) and were manufactured at room temperature on the end of a probe pulled from a glass capillary tube. The nano-needles had a diameter less than approximately 500 nm and were up to 25 µm long. The nano-needles were typically positioned less than 40 μm above the planar electrode. Using these electrodes and supported by numerical simulation, these electrokinetic mechanisms were demonstrated in low conductivity solutions using particles of various sizes and materials.

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