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Electroosmotic flow

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From Molecupedia, the encyclopedia of very small things
Motion of a liquid along a charged surface under an electric field

Electroosmotic flow, also written electro-osmosis, is the motion of a liquid past a charged surface when an electric field is applied along the surface. The surface gathers counterions into its electric double layer, the field pulls those ions toward one electrode, and they drag the liquid with them through its viscosity. In a channel of glass or silicon nitride at neutral pH the walls are negative, the mobile ions are positive and the liquid moves toward the cathode. In a nanopore the flow can carry a molecule along with the field or against it, and it competes with electrophoresis, which moves the molecule through the liquid.

Ferdinand Friedrich Reuss discovered the effect in Moscow in 1807, when he made water flow through a plug of clay with a voltage, and the name comes, indirectly, from the work of Robert Porrett Jr.[1][2]

Mechanism

The liquid touching the wall does not move, and the speed rises through the double layer to a constant value outside it. A few Debye lengths from the wall the whole liquid in a channel therefore slides at one speed, a plug, where a flow driven by pressure has a parabolic profile that is fastest in the middle. When the double layer is thin compared with the channel, the speed outside it is

u=−εrε0ζηE,

the equation of Helmholtz and Smoluchowski, in which ζ is the zeta potential of the wall, η the viscosity and E the field.[3] The mobility εrε0ζ/η contains no length belonging to the channel. For water at 25 °C a wall at −50 mV gives 3.9 × 10−8 m2/(V s), about the free-solution mobility of DNA, which is 3.75 × 10−8 m2/(V s) in a common buffer.[4] When the double layer is as thick as the radius of the channel, as it is in a narrow pore in dilute salt, the plug profile is lost and the speed depends on that radius.[5]

The zeta potential grows more negative as the pH of a silica or glass wall rises and shrinks in magnitude as salt is added, so the flow does the same.[3]

Uses

In capillary electrophoresis the flow is the pump. Jorgenson and Lukacs separated species in open glass capillaries in 1981.[6] In a capillary 50 cm long at 25 kV, a wall at −50 mV would move the liquid at about 2 mm/s and clear the capillary in four minutes. Pumps built on the effect have no moving parts and deliver a flow without pulsation.[7]

Soil engineers have used it to divert pore water away from excavations. L. Casagrande measured the flow through soils in 1949 and found a coefficient of 5 × 10−5 cm/s for a field of 1 V/cm that he took to be constant for all soils, which the equation allows because it has no term for the size of the pores.[8]

In nanopores

The voltage across a pore falls over a few tens of nanometers, so the field inside is about 107 V/m, two hundred times that of the capillary run above. A negative wall pushes the liquid toward the cathode, and DNA, which is negative, moves toward the anode. For avidin crossing silicon nitride pores, Firnkes and colleagues found that electroosmosis could enhance, dominate or reverse the electrophoretic motion, and that the direction followed the difference between the zeta potentials of protein and pore. Where the two cancelled, diffusion carried the protein across at a significant rate.[9]

Because the flow acts on any molecule in the liquid, charged or not, it can be built into a pore as a tool. A sphere of DNA origami docked on a solid-state pore created a flow that trapped nearby particles whatever their charge, and it held single proteins for hours.[10] Charges placed about 1 nm apart in the lumen of the biological pore CytK produced a flow that moved unfolded polypeptides against a strong electrophoretic force. In simulation the flow pushed with about 20 pN at −100 mV, similar to the force of the field on single-stranded DNA.[11] The flow also drew peptide and protein biomarkers into FraC pores.[12] A 2022 study proposed that electroosmosis and polarization in the pore, and not counterions, produce the conductive pulses that net-neutral proteins give at low salt.[13]

The outside of a pore has a flow of its own. Around a conical glass pore the flow runs in the direction that theory predicts above 100 mM salt, and it reverses below 1 mM, because the walls inside and outside the tip drive flows in opposite directions.[14] See Glass nanopipette.

At Molecular Reality

The company's guide to its own instrument gives the effect one sentence: "Water and ions flow through the pore too, and that flow, called electroosmosis, can help push or hold a molecule in place."[15] The pore-size slider in its world-model essay lists "Electro-osmotic flow significant" among the effects that appear once a pore is narrower than a few micrometers, beside the surface charge and a larger role for the access resistance.[16]

See also

References

  1. ^ Biscombe, C. J. C. (2017). "The discovery of electrokinetic phenomena: setting the record straight". Angewandte Chemie International Edition 56 (30): 8338-8340. doi:10.1002/anie.201608536. PMID 27902877.
  2. ^ Kenndler, E.; Minárik, M. (2021). "Capillary electrophoresis and its basic principles in historical retrospect. Part 1. The early decades of the "long nineteenth century": the voltaic pile, and the discovery of electrolysis, electrophoresis and electroosmosis". Substantia 5 (1). doi:10.36253/Substantia-1018.
  3. ^ a b Kirby, B. J.; Hasselbrink, E. F. (2004). "Zeta potential of microfluidic substrates: 1. Theory, experimental techniques, and effects on separations". Electrophoresis 25 (2): 187-202. doi:10.1002/elps.200305754.
  4. ^ Stellwagen, N. C.; Gelfi, C.; Righetti, P. G. (1997). "The free solution mobility of DNA". Biopolymers 42 (6): 687-703. PMID 9358733.
  5. ^ Rice, C. L.; Whitehead, R. (1965). "Electrokinetic flow in a narrow cylindrical capillary". The Journal of Physical Chemistry 69 (11): 4017-4024. doi:10.1021/j100895a062.
  6. ^ Jorgenson, J. W.; Lukacs, K. D. (1981). "Zone electrophoresis in open-tubular glass capillaries". Analytical Chemistry 53 (8): 1298-1302. doi:10.1021/ac00231a037.
  7. ^ Wang, X.; Cheng, C.; Wang, S.; Liu, S. (2009). "Electroosmotic pumps and their applications in microfluidic systems". Microfluidics and Nanofluidics 6 (2): 145-162. doi:10.1007/s10404-008-0399-9.
  8. ^ Casagrande, L. (1949). "Electro-osmosis in soils". Géotechnique 1 (3): 159-177. doi:10.1680/geot.1949.1.3.159.
  9. ^ Firnkes, M.; Pedone, D.; Knezevic, J.; Döblinger, M.; Rant, U. (2010). "Electrically facilitated translocations of proteins through silicon nitride nanopores: conjoint and competitive action of diffusion, electrophoresis, and electroosmosis". Nano Letters 10 (6): 2162-2167. doi:10.1021/nl100861c.
  10. ^ Schmid, S.; Stömmer, P.; Dietz, H.; Dekker, C. (2021). "Nanopore electro-osmotic trap for the label-free study of single proteins and their conformations". Nature Nanotechnology 16 (11): 1244-1250. doi:10.1038/s41565-021-00958-5.
  11. ^ Sauciuc, A.; et al. (2024). "Translocation of linearized full-length proteins through an engineered nanopore under opposing electrophoretic force". Nature Biotechnology 42 (8): 1275-1281. doi:10.1038/s41587-023-01954-x.
  12. ^ Huang, G.; Willems, K.; Soskine, M.; Wloka, C.; Maglia, G. (2017). "Electro-osmotic capture and ionic discrimination of peptide and protein biomarkers with FraC nanopores". Nature Communications 8: 935. doi:10.1038/s41467-017-01006-4.
  13. ^ Lastra, L. S.; et al. (2022). "On the origins of conductive pulse sensing inside a nanopore". Nature Communications 13: 2186. doi:10.1038/s41467-022-29758-8.
  14. ^ Laohakunakorn, N.; Thacker, V. V.; Muthukumar, M.; Keyser, U. F. (2015). "Electroosmotic flow reversal outside glass nanopores". Nano Letters 15 (1): 695-702. doi:10.1021/nl504237k.
  15. ^ "Keltar's Field Guide to Molecular Reality". Molecular Reality Corporation. August 2026.
  16. ^ "Keltar's Critique v2: The Nanopore World Model (Hardened)". Molecular Reality Corporation. June 2026.
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