Cornell University - Visit www.cornell.edu Kirby Research Group at Cornell: Microfluidics and Nanofluidics : - Home College of Engineering - visit www.engr.cornell.edu Cornell University - Visit www.cornell.edu
Cornell University, College of Engineering Search Cornell
News Contact Info Login

Donations keep this resource free! Give here:

Copyright Brian J. Kirby. With questions, contact Prof. Kirby here. This material may not be distributed without the author's consent. When linking to these pages, please use the URL http://www.kirbyresearch.com/textbook.

This web posting is a draft, abridged version of the Cambridge University Press text. Follow the links to buy at Cambridge or Amazon or Powell's or Barnes and Noble. Contact Prof. Kirby here. Click here for the most recent version of the errata for the print version.

[Return to Table of Contents]


Jump To: [Kinematics] [Couette/Poiseuille Flow] [Fluid Circuits] [Mixing] [Electrodynamics] [Electroosmosis] [Potential Flow] [Stokes Flow] [Debye Layer] [Zeta Potential] [Species Transport] [Separations] [Particle Electrophoresis] [DNA] [Nanofluidics] [Induced-Charge Effects] [DEP] [Solution Chemistry]

10.5 Modifying the zeta potential [zeta potential top]

Many techniques can be used to modify the electrokinetic potential in a microchannel system, including changing salt concentrations, adding surfactants, and chemically functionalizing the surface.

10.5.1 Indifferent electrolyte concentrations

As was shown earlier, increasing salt concentrations on most oxide and polymer surfaces tends to reduce the electrokinetic potential. For monovalent ions, this is simply increased shielding of and nonspecific adsorption onto a charged surface. For multivalent ions, this can be caused by specific adsorption or more complicated ion-ion correlation effects.

10.5.2 Surface-active agents

Surface-active agent (surfactant) is a catch-all term for molecules or ions that act at surfaces to change their properties. Many surfactants are known to change the electrokinetic potential on oxides and polymers, including SDS, cellulose ethers, and polyelectrolytes.

Ionic surfactants: SDS

Ionic surfactants are usually amphiphilic molecules (i.e., molecules with both hydrophobic and hydrophilic regions) that consist of a hydrophobic alkyl region and a hydrophilic charged group. The hydrophobic end of these molecules often adsorb to hydrophobic surfaces, extending the charged hydrophilic end of the molecule. The most commonly used ionic surfactant is sodium dodecyl sulfate (i.e., sodium lauryl sulfate or SDS), which consists of a C12H25 straight-chain alkyl group attached to a charged OSO4- group. The solid form of this surfactant is a salt with Na+. When used on hydrophobic surfaces, SDS often coats the surface, leading to a strongly negative surface charge. Figure 10.7 shows a negative wall potential generated by adhesion of sodium dodecyl sulfate to a hydrophobic surface.


microfluidics textbook nanofluidics textbook Brian Kirby CornellFigure 10.7: A negative wall potential generated by adhesion of sodium dodecyl sulfate to a hydrophobic surface.


Cellulose ethers

Cellulose ethers, e.g., hydroxypropylmethylcellulose, cellulose, and methylcellulose tend to reduce the observed electrokinetic potential of microfluidic substrates by at least an order of magnitude and are commonly used for this purpose. These molecules bind to the surface, limiting the mobility of ions in the double layer.

Polyelectrolytes

Polyelectrolytes, e.g., polystyrene sulfonate, are typically polyanions or polycations that adhere electrostatically to charged surfaces. These can be used to reverse the charge of a surface; however, their lifetime is finite. Figure 10.8 shows an example, in which a negative surface is rendered positive by incubation with a positive polyelectrolyte.


microfluidics textbook nanofluidics textbook Brian Kirby CornellFigure 10.8: Reversal of the charge of a negative surface by incubation with a positive polyelectrolyte such as poly(styrene sulfonate).


10.5.3 Chemical functionalizations

Surfaces can be chemically functionalized using a variety of techniques. Plasmas and UV sources are often used to oxidize surfaces, usually increasing the negative charges on surfaces. Chemicals with alkoxysilyl- or chlorosilyl- functional groups can be used to chemically bond to glass or silicon surfaces, releasing alcohols or hydrochloric acid, respectively. These chemicals covalently bond chemical groups to the wall using a siloxane bond. For example, trimethoxysilylpropylamine reacts to glass, bonding the propylamine group to the wall using a siloxane bond. Using these techniques, a wide variety of functional groups can be bonded to a glass or silicon surface. Often, polymers are grafted to surfaces by combining self-assembled monolayer processes (e.g., bonding of trimethoxysilylpropyl acrylate) with free radical chain polymerization or atom-transfer polymerization.

Related work on zeta potential from our research group can be found here.

[Return to Table of Contents]



Jump To: [Kinematics] [Couette/Poiseuille Flow] [Fluid Circuits] [Mixing] [Electrodynamics] [Electroosmosis] [Potential Flow] [Stokes Flow] [Debye Layer] [Zeta Potential] [Species Transport] [Separations] [Particle Electrophoresis] [DNA] [Nanofluidics] [Induced-Charge Effects] [DEP] [Solution Chemistry]

Copyright Brian J. Kirby. Please contact Prof. Kirby here with questions or corrections. This material may not be distributed without the author's consent. When linking to these pages, please use the URL http://www.kirbyresearch.com/textbook.

This web posting is a draft, abridged version of the Cambridge University Press text. Follow the links to buy at Cambridge or Amazon or Powell's or Barnes and Noble. Contact Prof. Kirby here. Click here for the most recent version of the errata for the print version.


Ad revenue from these pages is used to support student research. The presence of an advertisement on these pages does not constitute an endorsement by the Kirby Research Group or Cornell University.

Donations keep this resource free! Give here: