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Electrokinetic Flow in pH-Regulated Solid-State/Soft Micro-Nanochannels

Sadeghi, Morteza | 2020

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 53556 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Saeedi, Mohammad Hassan; Mousavi, Ali; Sadeghi, Arman
  7. Abstract:
  8. In recent years, advances have been made in the field of miniaturization of the devices (in order to increase their efficiency, lessen the materials needed for the constructions and experiments, reduce costs and energy usage) resulting in the dramatic increase of surface to volume ratio in these devices. This progress has led to the dominance of surface forces which can be used to control very important interface phenomena having a special application such as electrokinetic. Using today’s manufacturing technology; it is possible to construct micro-nanochannels made from different materials such as silicon, glass, quartz, polydimethylsiloxane (PDMS), polymethyl methacrylate (PMMA) whose surface charges can be adjusted either by the pH of the solution or the proton concentration near their surface. As a result, by adjusting the pH of the solution, it is possible to regulate the surface charge and conductivity of the channels, and thus the flow of the ions passing through them. In addition, ion transfer in micro-nanochannels coated with pH-regulated polyelectrolytes, known as soft channels, is used in a variety of applications, including synthetic ionic gates. Because of the longer range of electrostatic effects, the surface modified with polyelectrolytes possesses unique features in comparison with other surfaces and the desired properties of the interfaces can be created through grafting them to the surface. In the light of the mentioned points, the present dissertation first investigates the electrokinetic flow inside a pH-regulated rectangular micro-nanochannel to better simulate the real applications. Because the sectional geometry of the channels plays an important role in determining the desired parameters of the system as well as due to the limitation of manufacturing technology in micro and nano dimensions, the flow study was extended to micro- nanocanals with arbitrary cross sections. After studying the hydrodynamics of the flow, the effect of the pH-regulation and polyelectrolyte layer on the unsteady mass transfer was evaluated. Then, by examining the effect of pH-regulated polyelectrolyte brusehs grafted inside the nanopores, their application as a synthetic valve was investigated and the parameters affecting their performance were discussed in detail. Relevant simulations have been performed using analytical, semi-analytical, finite element and molecular theory methods. The results show that the parameters of the system are strongly dependent on the channel aspect ratios. Such findings, in addition to demonstrating the inefficiency of the slit micro-nanochannels in simulating the practical problems, lead us to a new mechanism for flow control. It has been shown that with increasing salt concentration and discrepancy of pH of the solution from the isoelectric point, the surface charge density and ionic strength inside the channel increases. Total electric charge and convective ionic conductance are greater for channels with larger peripher-to-area ratios, while the opposite is true for average surface charge density and velocity magnitude. The study of mass transfer provides valuable results including the transport of the center of mass of the analyte and the temporal changes of the injected analyte band. It was shown that by changing the surface properties, such as grafting the polyelectrolyte brushes to the inner surface of the channel, it is possible to control the mass transfer of the analyte for different applications such as separation and trsnsport of samples. Finally, another application of nanopores modified with pH-sensitive polyelectrolyte brushes as syntethic ionic gates was investigated using molecular theory. The effect of different parameters on controlling the ionic transport, the flow rate and also the selectivity of these nanopores were discussed in detail. It was shown that by controlling the pH of the solution, not only can the valve be changed from off to on state, but also in the on state, the ionic conductance and flow rate can be adjusted by controlling this parameter as well as the polyelectrolyte layer parameters and background salt concentration
  9. Keywords:
  10. Polyelectrolyte ; Mass Transfer ; Poly Methylmethacrylate ; Polydimethylsiloxane ; Soft Microchannal ; Syntetic Ionic Gate ; pH-Regulated Channel ; Electrokinetic Process ; Molecular Dynamics

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