Washburn's equation explained

In physics, Washburn's equation describes capillary flow in a bundle of parallel cylindrical tubes; it is extended with some issues also to imbibition into porous materials. The equation is named after Edward Wight Washburn;^[1] also known as Lucas–Washburn equation, considering that Richard Lucas^[2] wrote a similar paper three years earlier, or the Bell-Cameron-Lucas-Washburn equation, considering J.M. Bell and F.K. Cameron's discovery of the form of the equation in 1906.^[3]

Derivation

In its most general form the Lucas Washburn equation describes the penetration length (

L ) of a liquid into a capillary pore or tube with time t as L = (Dt) ^{(1)/(2)} , where D is a simplified diffusion coefficient.^[4] This relationship, which holds true for a variety of situations, captures the essence of Lucas and Washburn's equation and shows that capillary penetration and fluid transport through porous structures exhibit diffusive behaviour akin to that which occurs in numerous physical and chemical systems. The diffusion coefficient D is governed by the geometry of the capillary as well as the properties of the penetrating fluid. A liquid having a dynamic viscosity η and surface tension \gamma will penetrate a distance L into the capillary whose pore radius is r following the relationship:L = \sqrt{ (\gamma rt\cos(\phi))/(2η) }

Where

\phi is the contact angle between the penetrating liquid and the solid (tube wall).

Washburn's equation is also used commonly to determine the contact angle of a liquid to a powder using a force tensiometer.^[5]

In the case of porous materials, many issues have been raised both about the physical meaning of the calculated pore radius

r ^[6] and the real possibility to use this equation for the calculation of the contact angle of the solid.^[7] The equation is derived for capillary flow in a cylindrical tube in the absence of a gravitational field, but is sufficiently accurate in many cases when the capillary force is still significantly greater than the gravitational force.

In his paper from 1921 Washburn applies Poiseuille's Law for fluid motion in a circular tube. Inserting the expression for the differential volume in terms of the length

l of fluid in the tube dV = \pi r ^{2} dl , one obtains(\delta l)/(= \delta t) (\sum P)/(8 r ^{2} η l) (r ^{4} +4 \epsilon r ^{3)}

where

\sum P is the sum over the participating pressures, such as the atmospheric pressure P _{A} , the hydrostatic pressure P _{h} and the equivalent pressure due to capillary forces P _{c} . η is the viscosity of the liquid, and \epsilon is the coefficient of slip, which is assumed to be 0 for wetting materials. r is the radius of the capillary. The pressures in turn can be written asP _{h=h} g \rho - l g \rho\sin\psi P _{c = (2 \gamma)/(r)} \cos\phi

where

\rho is the density of the liquid and \gamma its surface tension. \psi is the angle of the tube with respect to the horizontal axis. \phi is the contact angle of the liquid on the capillary material. Substituting these expressions leads to the first-order differential equation forthe distance the fluid penetrates into the tube l :(\delta l)/(= \delta t) ([P _{A + g} \rho (h - l\sin\psi) + (2 \gamma)/(r) \cos\phi](r ^{4} +4 \epsilon r ^{3)})/(8 r ^{2} η l)

Washburn's constant

The Washburn constant may be included in Washburn's equation.

It is calculated as follows:

(10 ^{4} \left[ (\mum)/(cm) \right] \left[ (N)/(m ^{2}) \right])/(68947 . 6 \left[ (dynes)/(cm ^{2}) \right]) = 0 . 1450 ( 38 ) ^{[8] [9]}

Fluid inertia

In the derivation of Washburn's equation, the inertia of the liquid is ignored as negligible. This is apparent in the dependence of length

L to the square root of time, L \propto \sqrt{t} , which gives an arbitrarily large velocity dL/dt for small values of t. An improved version of Washburn's equation, called Bosanquet equation, takes the inertia of the liquid into account.^[10]

Applications

Inkjet printing

The penetration of a liquid into the substrate flowing under its own capillary pressure can be calculated using a simplified version of Washburn's equation:^{[11] [12]}

l = \left[ (r\cos\theta)/(2) \right] ^{(1)/(2)} \left[ (\gamma)/(η) \right] ^{(1)/(2)} t ^{(1)/(2)} where the surface tension-to-viscosity ratio \left[ \tfrac{\gamma}{ η } \right] ^{(1)/(2)} represents the speed of ink penetration into the substrate. In reality, the evaporation of solvents limits the extent of liquid penetration in a porous layer and thus, for the meaningful modelling of inkjet printing physics it is appropriate to utilise models which account for evaporation effects in limited capillary penetration.

Food

According to physicist and Ig Nobel prize winner Len Fisher, the Washburn equation can be extremely accurate for more complex materials including biscuits.^{[13] [14]} Following an informal celebration called national biscuit dunking day, some newspaper articles quoted the equation as Fisher's equation.^[15]

Novel capillary pump

The flow behaviour in traditional capillary follows the Washburn's equation. Recently, novel capillary pumps with a constant pumping flow rate independent of the liquid viscosity ^{[16] [17] [18] [19]} were developed, which have a significant advantage over the traditional capillary pump (of which the flow behaviour is Washburn behaviour, namely the flow rate is not constant). These new concepts of capillary pump are of great potential to improve the performance of lateral flow test.

See also

• Bosanquet equation
• Mercury intrusion porosimetry (MIP)

External links

• Powder wettability measurement with the Washburn method

Notes and References

1. Edward W. Washburn . The Dynamics of Capillary Flow. Physical Review. 1921. 17. 273. 3. 1921PhRv...17..273W . 10.1103/PhysRev.17.273 .
2. Lucas, R. . Ueber das Zeitgesetz des Kapillaren Aufstiegs von Flussigkeiten. Kolloid Z.. 1918. 23. 15. 10.1007/bf01461107. 97596580.
3. Bell, J.M. . Cameron, F.K. . amp . The flow of liquids through capillary spaces . J. Phys. Chem.. 1906. 10. 8 . 658–674 . 10.1021/j150080a005.
4. Liu. M. . etal . Evaporation limited radial capillary penetration in porous media . Langmuir . 2016 . 32 . 38 . 9899–9904. 10.1021/acs.langmuir.6b02404 . 27583455.
5. Alghunaim. Abdullah. Kirdponpattara. Suchata. Newby. Bi-min Zhang. 2016. Techniques for determining contact angle and wettability of powders. Powder Technology. en. 287. 201–215. 10.1016/j.powtec.2015.10.002.
6. Book: Dullien, F. A. L. . Porous Media: Fluid Transport and Pore Structure.. 1979. New York: Academic Press. 978-0-12-223650-1.
7. Book: Marco. Brugnara. Claudio. Della Volpe. Stefano . Siboni. Wettability of porous materials. II. Can we obtain the contact angle from the Washburn equation?. Contact Angle, Wettability and Adhesion. 2006. Mass. VSP. Mittal, K. L. . https://www.researchgate.net/publication/285774466.
8. Micromeritics, "Autopore IV User Manual", September (2000). Section B, Appendix D: Data Reduction, page D-1. (Note that the addition of 1N/m2 is not given in this reference, merely implied)
9. Micromeritics, 1970 . A new method of interpolation and smooth curve fitting based on local procedures . Journal of the ACM . 17 . 4. 589–602 . 10.1145/321607.321609 . Akima . Hiroshi. 33862277 .
10. 10.3183/npprj-2000-15-05-p422-430. Influence of inertia on liquid absorption into paper coating structures. Nordic Pulp & Paper Research Journal. 15. 5. 422–430. 2000. Schoelkopf. Joachim. Matthews. G. Peter. 36690492 .
11. Book: Oliver. J. F.. Colloids and Surfaces in Reprographic Technology. 200. 1982. 435–453. 1947-5918. 10.1021/bk-1982-0200.ch022. ACS Symposium Series. 978-0-8412-0737-0. Wetting and Penetration of Paper Surfaces.
12. Leelajariyakul. S.. Noguchi. H.. Kiatkamjornwong. S.. Surface-modified and micro-encapsulated pigmented inks for ink jet printing on textile fabrics. Progress in Organic Coatings. 62. 2. 2008. 145–161. 0300-9440. 10.1016/j.porgcoat.2007.10.005.
13. Web site: The 1999 Ig Nobel Prize Ceremony . . improbable.com . Improbable Research . 2015-10-07 . Len Fisher, discoverer of the optimal way to dunk a biscuit..
14. Web site: No more flunking on dunking . Barb . Natalie . 25 November 1998 . bbc.co.uk . BBC News . 2015-10-07 .
15. Physics takes the biscuit . Fisher . Len . 11 February 1999 . Nature . Washburn will be turning in his grave to learn that the media have renamed his work the "Fisher equation". . 10.1038/17203. 397 . 6719 . 469. 1999Natur.397..469F . 4404966 . free .
16. Weijin Guo . Jonas Hansson . Wouter van der Wijngaart . 2016 . Viscosity Independent Paper Microfluidic Imbibition . MicroTAS 2016, Dublin, Ireland.
17. Weijin Guo . Jonas Hansson . Wouter van der Wijngaart . 2016 . Capillary Pumping Independent of Liquid Sample Viscosity . Langmuir . 32 . 48 . 12650–12655 . 10.1021/acs.langmuir.6b03488. 27798835 . 24662688 .
18. Book: Weijin Guo . Jonas Hansson . Wouter van der Wijngaart . 2017 IEEE 30th International Conference on Micro Electro Mechanical Systems (MEMS) . Capillary pumping with a constant flow rate independent of the liquid sample viscosity and surface energy . 2017 . IEEE MEMS 2017, Las Vegas, USA. 339–341 . 10.1109/MEMSYS.2017.7863410. 978-1-5090-5078-9 . 13219735 .
19. Weijin Guo . Jonas Hansson . Wouter van der Wijngaart . 2018 . Capillary pumping independent of the liquid surface energy and viscosity . Microsystems & Nanoengineering. 4 . 1 . 2 . 10.1038/s41378-018-0002-9. 31057892 . 6220164 . 2018MicNa...4....2G.