Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session E6: Electrokinetics III |
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Chair: Ali Mani, Stanford University Room: 24B |
Sunday, November 18, 2012 4:45PM - 4:58PM |
E6.00001: Electroosmosis in a potassium chloride aqueous solution in a silica nanochannel with counter-charged surface patches Harvey Zambrano, Marie Pinti, A.T. Conlisk, Shaurya Prakash Controlling Electroosmotic flow (EOF) in nanochannel is important for several nano and bio-technology applications. In this work, the EOF is studied by conducting Non-Equilibrium MD Simulations (NEMDS) of an electrolyte confined in a silica nanochannel. having dimensions of 34.76 x 2.53 x 7.0 nm. We model a relatively long channel compared to other MD studies in order to investigate in detail the effect of the amorphous walls on the confined aqueous electrolyte. The system was studied as axial electric fields (AEF) were applied and as the surface charge (SC) was modified by implementing counter-charged patches (CP) on the channel walls. From the velocity profiles, a linear response of the system was observed. Smaller velocities were observed for the cases with increasing surface charge on the patches. Our velocities for the reference case with no patches (i.e. bare silica nanochannel) are in agreement with results from previous MD studies. We infer that ionic accumulation on the CP is responsible for the EOF velocity changes for systems with different CP and the same AEF applied. We show that by increasing the SC on a wall, the velocity field decreases monotonically. [Preview Abstract] |
Sunday, November 18, 2012 4:58PM - 5:11PM |
E6.00002: Effect of divalent ions on electroosmotic transport in a sodium chloride aqueous solution confined in an amorphous silica nanochannel A.T. Conlisk, Harvey Zambrano, Necmettin Cevheri, Minami Yoda A critical enabling technology for the next generation of nanoscale devices, such as nanoscale ``lab on a chip'' systems, is controlling electroosmotic flow (EOF) in nanochannels. In this work, we control EOF in an aqueous sodium chloride (NaCl) solution confined in a silica nanochannel by systematically adding different amounts of divalent ions. Multivalent ions have a different affinity for the silica surface and different hydration characteristics in comparison to monovalent ions. Therefore by adding Mg$^{++}$ and Ca$^{++}$ to the sodium chloride solution, the electroosmotic velocity and the structure of the electrical double layer will be modified. The effects of adding Mg$^{++}$ and Ca$^{++}$ will be compared using non-equilibrium molecular dynamics simulations of the EOF at different electric fields of a NaCl solution in a silica nanochannel with different fractions of Ca$^{++}$ and Mg$^{++}$ ions. In general, the wall zeta-potential magnitude, and hence the EOF velocity, decreases as the Ca$^{++}$ or Mg$^{++}$ concentration increases. The system responds linearly with electric field. We will compare the computational results with the experimental data of Cevheri and Yoda (2012). [Preview Abstract] |
Sunday, November 18, 2012 5:11PM - 5:24PM |
E6.00003: Electrowetting of electrolyte solution in a nanoslit with overlapped electric double layer: continuum approach In Seok Kang, Jung A. Lee In a nanotube or nanoslit of O(10nm) lengthscale, the electric double layer (EDL) is expected to be overlapped. For this lengthscale, the continuum approach is still valid. In the present work, electrowetting phenomenon in a nanoslit is analyzed by using the electromechanical method based on the continuum governing equation. From the analysis, we obtain the formula of the extra-pressure that is generated by the electrowetting effect in a nanoslit. We also obtain the deformed shape of the electrolyte-gas interface by using the first order perturbation method. In order to handle the problem analytically, two limiting situations are considered: (i) the low surface potential limit to have a linearized Poisson-Boltzmann equation (PBE), and (ii) the high surface potential limit for which it is assumed that only the counter ions are present inside the nanoslit. [Preview Abstract] |
Sunday, November 18, 2012 5:24PM - 5:37PM |
E6.00004: Nonlinear electrokinetic transport in networks of microscale and nanoscale pores Shima Alizadeh, Mathias B. Andersen, Ali Mani The objective of this study is to develop the understanding of nonlinear electrohydrodynamic effects in a wide range of systems including lab-on-a-chip systems, electroosmotic pumps, and, in general, porous media with random or fabricated pore morphology. We present a continuum model in which these systems are described as massive networks of long and thin pores. The thickness of the pores can vary from nanoscale to microscale, corresponding to the highly overlapped electric double layers (EDL) to the thin double layer limit. Within each pore the transport in the wall-normal direction is assumed to be in equilibrium leading to a reduced order model for the axial transport of species in the form of a transient one-dimensional partial differential equation (PDE). PDEs from different pores are coupled through boundary conditions at the pore intersections by proper implementation of the conservation laws. We show that this model can capture important nonlinear dynamics, which are typically ignored in homogenized models. Specifically, our model captures concentration polarization shocks and flow recirculation zones respectively formed when micropores and nanopores are connected in series and in parallel. We present a comparison between our model and recent experiments in microfluidics, and will discuss applications in porous media modeling for energy storage and water purification systems. [Preview Abstract] |
Sunday, November 18, 2012 5:37PM - 5:50PM |
E6.00005: Large Apparent Electric Size of Solid-State Nanopores Obtained by Focused Ion Beam Milling Remy Fulcrand, Choongyeop Lee, Laurent Joly, Alessandro Siria, Anne Laure Biance, Lyderic Bocquet Here, we report experimental results that show unexpectedly large ionic conduction in solid-state nano-pores, taking its origin in anomalous entrance effects [1]. The surface conductance inside the nano-pore is shown to perturb the three dimensional electric current streamlines far outside the nano-pore in order to meet charge conservation at the pore entrance. This supports the idea that ion transport is strongly perturbed outside the pore over a healing length given by the so-called Dukin length so as to meet ion current conservation at the entrance of the nano-pore. We developed a simplified analytical model for the conduction in nano-pores, which provides a very good agreement with experimental results. This unexpected effect can be interpreted in terms of apparent electrical size of the nano-pore much larger than its bare geometrical size. Our findings can have a major impact on the electrical detection of translocation events through nano-pores, as well as for ionic transport in biological nano-pores, which use electrical detection of translocation events [2].\\[4pt] [1] C. Lee, L. Joly, A. Siria, A.L. Biance, R. Fulcrand and L. Bocquet, \textit{NanoLetters} \textbf{2012}, DOI: 10.1021/nl301412b \\[0pt] [2] L. Song, M.R. Hobaugh, C. Shustak, S. Cheley, H. Goaux \textit{Science }\textbf{1996}, 274, 1859. [Preview Abstract] |
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