ORCID Profile
0000-0002-7178-8275
Current Organisations
Fundación IMDEA Nanociencia
,
National University of Singapore
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Publisher: Elsevier BV
Date: 09-2008
Publisher: IOP Publishing
Date: 28-04-2006
Publisher: Royal Society of Chemistry (RSC)
Date: 2011
DOI: 10.1039/C1LC20495E
Abstract: This paper studies the principles of fluid flow manipulation based on DC-biased AC-electrokinetics. This method makes use of planar parallel electrodes in a microfluidic channel in contact with an electrolyte solution, with a DC biased AC electrical signal applied to the electrode pair. Due to the application of DC bias, incipient Faradaic electrolytic reactions take place resulting in an increase of the ionic content of the bulk solution. The ionic content was found to be dissimilar at the cathodic and anodic sides of the channel and a conductivity difference of approximately 10% was measured for 2 V(DC). Fluid flow is generated by the action of the DC biased AC electric signal acting on the transverse conductivity gradient generated across the microchannel. The induced flow in the form of vortex was characterized experimentally and the results substantiated theoretically. The velocity of the induced flow vortex under the employed experimental conditions was ~600 to 700 μm s(-1) which is faster than those obtained in conventional AC-electroosmosis and AC-electrothermal types of flows.
Publisher: American Chemical Society (ACS)
Date: 28-02-2011
DOI: 10.1021/NN103191Q
Abstract: A gecko's superb ability to adhere to surfaces is widely credited to the large attachment area of the hierarchical and fibrillar structure on its feet. The combination of these two features provides the necessary compliance for the gecko toe-pad to effectively engage a high percentage of the spatulae at each step to any kind of surface topography. With the use of multi-tiered porous anodic alumina template and capillary force assisted nanoimprinting, we have successfully fabricated a gecko-inspired hierarchical topography of branched nanopillars on a stiff polymer. We also demonstrated that the hierarchical topography improved the shear adhesion force over a topography of linear structures by 150%. A systematic analysis to understand the phenomenon was performed. It was determined that the effective stiffness of the hierarchical branched structure was lower than that of the linear structure. The reduction in effective stiffness favored a more efficient bending of the branched topography and a better compliance to a test surface, hence resulting in a higher area of residual deformation. As the area of residual deformation increased, the shear adhesion force emulated. The branched pillar topography also showed a marked increase in hydrophobicity, which is an essential property in the practical applications of these structures for good self-cleaning in dry adhesion conditions.
Publisher: AIP Publishing
Date: 23-04-2009
DOI: 10.1063/1.3120273
Abstract: This paper investigates the phenomenon of Faradaic charging in ac electrokinetics. Faradaic reactions were suggested as a key effect responsible for the reversal of pumping direction in ac micropumps. However, this hypothesis has yet to be proven convincingly and directly. Here we present an ion detection strategy to determine the production of ions through Faradaic hydrolytic reactions originating from direct application of voltage to electrolytic solutions during ac electrokinetics. Experiments were performed with symmetrical planar electrodes aligned along a microfluidic channel. Fluorescein, a pH-dependent dye, was employed as the pH indicator for the detection of ion production. Images were captured for analysis at various voltage levels. From analyzing the fluorescence intensity and its distribution, it can be concluded that the production of ions from hydrolytic reactions takes place and increases with the ac voltage. The coefficient of deviation indicates a significant enhancement at ac voltage above 11 Vpp. Lastly, we demonstrate a strategy using dc-biased ac electrokinetics to achieve controllability in direction and magnitude of the net fluid flow in pumping application.
Publisher: American Chemical Society (ACS)
Date: 17-08-2007
DOI: 10.1021/AC070810U
Abstract: This study presents a new cell manipulation method using a moving dielectrophoretic force to transport or fractionate cells along a microfluidic channel. The proposed moving dielectrophoresis (mDEP) is generated by sequentially energizing a single electrode or an array of electrodes to form an electric field that moves cells continuously along the microchannel. Cell fractionation is controlled by the applied electrical frequency, and cell transportation is controlled by the interelectrode activation time. The applicability of this method was demonstrated to simultaneously fractionate and transport Saccharomyces cerevisiae yeast cells, both viable and nonviable, by operating at conditions under which the cells were subjected to positive and negative dielectrophoresis, respectively. Compared to the conventional dielectrophoresis (cDEP and traveling wave dielectrophoresis (twDEP), moving dielectrophoresis allows cells to be separated on the basis of the real part of the Clausius-Mossotti factor, as in cDEP, but yet allows the direct transportation of separated cells without using fluid flow, as in twDEP. This dielectrophoresis technique provides a new way to manipulate cells and can be readily implemented on programmable multielectrode devices.
Publisher: Wiley
Date: 17-07-2008
Publisher: American Chemical Society (ACS)
Date: 18-06-2008
DOI: 10.1021/AC800947E
Abstract: Moving dielectrophoresis has been recently developed by the authors as an alternative method to achieve simultaneous cell fractionation and transportation. With an array of independently excitable microelectrodes, this method generates a moving electric field to sequentially fractionate and transport cells across a microchannel. Due to the peculiarity of this method, the motion of the cells is unsteady and there are interesting and distinct differences between cells experiencing positive or negative dielectrophoresis. For a proper understanding and design of a microdevice utilizing this methodology, this study presents a model for the equation of motion for a polarized cell and its unsteady motion under moving dielectrophoresis. The model considers the basic module to generate a moving electric field, where there is a finite-width top electrode and an infinite-width bottom electrode, in a parallel-plate configuration. The forces considered include dielectrophoretic force, fluid drag, buoyancy, and gravitational force. These forces are modeled as equivalent point forces acting at the center of mass of the cell. A parallel-plate wall correction factor is employed to account for the effect of the large cell size to microchannel height ratio. Various parameters are examined including the initial position of the cell relative to the electrodes, cell's Clausius-Mossotti factor, cell size, applied voltage, electrode width, interelectrode gap, microchannel height, number of energized electrodes, and types of electrode configurations. Reasonable agreements were obtained between simulated and experimental results. As the solution of the unsteady motion is rather tedious, a MATLAB algorithm, with all the associated files, for the prediction of the cell trajectory, is available on request.
Publisher: Royal Society of Chemistry (RSC)
Date: 2009
DOI: 10.1039/B813639D
Abstract: This paper presents a novel approach of mixing two laminar flowing streams in microchannels. The mixer consists of a pair of electrodes disposed along a fluidic channel. By energizing the electrodes with a DC-biased (2.5 V) AC voltage (20 Vpp), an electrokinetic flow is induced with a flow profile perpendicular to that of the incoming laminar streams of liquids to be mixed. As a result, the flow lines of the incoming streams and the induced flow are forced to crossover and very efficient stirring and mixing at short mixing length can be achieved. The mixer can be operated from the AC-electroosmotic (ACEO) (sigma=1 mS/m, f=100 kHz) to the AC-electrothermal (ACET) (sigma=500 mS/m, f=500 kHz) flow regimes. The mixing efficiency in the ACEO regime was 92%, with a mixing length of 600 microm (Q=2 microL/min), an estimated mixing time of 69 ms and an induced ACEO flow velocity of approximately 725 microm/s. The mixing efficiency in the ACET regime was 65% for a mixing length of approximately 1200 microm. The mixer is efficient and suitable for mixing reagents in a fluid media from low to high conductivity as required in erse microfluidic applications.
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