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|Title: ||New Electrokinetic Techniques for Material Manipulation on the Microscale|
|Authors: ||Chang, Suk Tai|
|Advisors: ||Glenn Walker, Committee Member|
Saad Khan, Committee Member
Peter Kilpatrick, Committee Member
Orlin D. Velev, Committee Chair
Lab on a chip
|Issue Date: ||25-Nov-2008|
|Discipline: ||Chemical Engineering|
|Abstract: ||We report the results of series of investigations how electrically induced forces and interfacial phenomena could be used to manipulate particles and fluids on the microscale. Particle microseparations in a droplet floating on a dielectrophoretic liquid-chip system were investigated. Particle-localized electroosmotic flow was used for designing autonomously moving microdevices and locally distributed micropumps/mixers. Microfluidics was adapted for enhancing mechanical properties of materials with embedded microchannel networks. Ionic current rectification in charged aqueous gels was used for constructing new types of "soft matter" diodes. Detailed analytical and numerical modeling was performed for each system. The results of this work can apply to new fields of microfluidics, self-propelling microdevices, and aqueous gel-based electronic components.
In the first part of this work we explored unusual phenomena of colloidal particle transport and separation inside microdroplets floating in fluorinated oil on electrically controlled chips. Microspheres suspended in a drying droplet on liquid-liquid chips were rapidly separated in the dropletâ€™s top region due to water evaporation. During the evaporation process, a surface tension gradient emerged as a result of a non-uniform temperature distribution within the droplet. This interfacial gradient generated a Marangoni flow inside the evaporating droplet. The suspended colloidal particles driven by the convective flow were collected at the top of the droplets by the hydrodynamic flux compensating for the evaporation. The flow pattern and temperature distribution within the evaporating droplet were simulated using finite element calculation. The internal flow pattern calculated by the simulation was consistent with the experiments using tracer particles. The levitated microdroplets were used as templates for colloidal assembly and containers for microbioassays based on particle agglutination inside droplets.
An alternative mechanism of self-propulsion based on electroosmotic force and the extension of this propulsion force to innovative microfluidic pumps/mixers were developed in the second part of this study. Various types of miniature diodes floating in water acted as self-propelling particles when powered by an alternating (AC) electric field. Direct (DC) electric field induced across the diodes as a result of rectification of the external AC field led to particle-localized electroosmotic flow. The resulting reactive force pushed the diodes in the direction opposite to the electroosmotic flux. The microelements began to move parallel to the electric field in the direction of either the cathode or the anode, depending on their surface charge. In effect, the semiconductor microelements harvest electric energy from external AC field and convert it into mechanical propulsion on the microscale. The particle-localized propulsion force was used in diode-actuated electroosmotic motors and actuators. Diodes embedded in microfluidic channel walls could serve as locally distributed pumps or mixers powered by a global AC external field. The experimental results were interpreted by numerical simulations of the electrohydrodynamic flows
The viscoelastic properties of fluids inside microchannels were used in the development of novel microfluidic materials in the form of flexible sheets that can be solidified on demand to yield preprogrammed shapes. These materials were based on microfluidic channel networks in polydimethylsiloxane (PDMS) filled with photocurable polymers. When the elastic sheets with embedded microchannel networks were shaped and exposed by UV light, the photoresist inside the channels was solidified and acted as endoskeleton within the PDMS layer, acquiring the pre-arranged shape. Bending and stretching moduli of the materials with solidified endoskeleton increased drastically and once the external force was removed, the memorized shapes were recovered. The permanent preservation of the shape of solidified microfluidic sheets could be used in making instant packages and supports on demand.
Finally, unidirectional ionic current flow across a fixed junction between two aqueous agarose gel phases containing oppositely charged polyelectrolytes was discovered. The non-linear current response of the interface between the cationic and anionic gels originated directly from anisotropy in the mobile charges within the system. The current densities in the forward bias and current rectifying ratios in the gel diodes were higher or comparable to those using ionic carries and junctions built from conductive polymers. The promising feature of this new type of rectifying junction is that it is operates on the basis of water-borne ions. The devices are extremely simple, inexpensive and possess good long-term stability in DC or AC conduction mode.|
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