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How Does Surface Tension Relate to Viscosity?

 

 You may recall that I promised to offer my interpretation as to how Surface Tension is related to Viscosity.

To begin with, liquid surface tension and viscosity share a common trait: they both involve properties of fluids. After that, things start to get murky.

Let us start with surface tension. This relates Surface Tension - FloatingPaper Cilpto the property of a liquid’s surface that resists force; it serves as a barrier to foreign materials as well as keeping the liquid together. This ever-present property is caused by unbalanced forces on surface molecules that pull toward the main part of the liquid.

Viscosity, on the other hand, is related toviscosity shear rate a liquid’s resistance to being deformed or moved. This is caused by the friction between molecules.

Compared to viscosity, surface tension is a simpler phenomenon. It is basically stable, changed mostly by temperature and chemicals that modify the bonding characteristics of the molecules. As temperature decreases, surface tension increases. The effects of adding an unrelated substance is illustrated by the example of putting soap (a surfactant) in water to reduce the surface tension, which allows the dirt on your hands to more easily mix with the water.

Regarding viscosity, knowing the type of liquid is essential. For example, there are Newtonian fluids that react to forces (sometimes called shear rate) that move the liquid (sometimes called shear stress) in a straight-forward, linear manner.

However, non-Newtonian fluids follow different sets of rules. Shear-thinning fluids decrease in viscosity as the Ketchuppressure or force increases. Thixotropic fluids change viscosity over time -- Example gels and colloids, and yes ketchup are stable at rest, but become fluid when agitated.

Thus we see that finding the true value of viscosity [which some of us may think of as simply thickness] is a complex process. Viscosity, unlikeViscosity Thick to Thin surface tension [which tends to be a static phenomenon], is all about movement. All that should concern us in regard to measuring surface tension is whether to use a Wilhelmy Plate or a duNouy Ring. (That is enough to keep me entertained.)

The last question, which perhaps should have been the first, is about the correlation between surface tension and viscosity. You would think that thick fluids would translate to a high surface tension and that thin fluids would produce lower surface tension. Not true. In fact, my research has shown that there is no conclusive correlation.

This got into a lot more theoretical entanglement than I expected when I first considered taking on what seemed like a international Nosimple comparison. The answer, however, is clear: no correlation. The reasons are not so simple. I guess a good summary would be that surface tension is about steady state and viscosity is about movement.

Until next time, I remain as confused as ever.

Art

 

Newtonian vs. Non\u002DNewtonian Fluids

 

P.S. Did you know that you can subscribe to these exposés, rants, raves and ramblings? All you have to do is click on the RSS Feed symbol at the upper left and you will get a notice when a new one is published. Or, if you prefer, you can also subscribe for e-mail notice by jotting your address in the box just to the right of the title.

By Art Gatenby

Comments

How would you define the difference between Hard water and Soft water? Is this surface tension and or does the viscosity have a role in this scenario? 
 
Thanks, 
Mark 
Posted @ Thursday, January 20, 2011 9:09 PM by Mark Levi
soap ladder in hard water but reverse is the case for soft water
Posted @ Monday, February 07, 2011 3:47 AM by gbolahan
"...there are Newtonian fluids that react to forces (sometimes called shear rate) that move the liquid (sometimes called shear stress)..." 
 
That´s wrong: Forces are connected with (shear) stresses and velocities (or movements) are connected with shear rates.
Posted @ Tuesday, March 13, 2012 3:17 AM by Lutz
Thank you for the input. We do not want this to mislead anyone. 
 
Thanks again, 
 
Art
Posted @ Tuesday, March 13, 2012 10:59 AM by Arthur Gatenby
Does anyone know who makes equipment that measures viscosity of the surfactants?
Posted @ Thursday, July 05, 2012 9:17 AM by MikeSB
Hello.l want to know what is the relation between the liquid and the surface tension
Posted @ Saturday, December 22, 2012 12:47 AM by Duha
Do you know any equipment that could measure both viscosity and surface tension of a liquid? Thanks a lot!
Posted @ Wednesday, March 20, 2013 12:46 PM by Wenshou Wang
Is there a correlation between density and interfacial-tension?
Posted @ Thursday, July 04, 2013 9:12 PM by Rob W
I get by experimentally that the value of surface tension of distill water is greater than ethenol but the value of viscosity of water is lower than ethenol on account of this i can say that both are opposite characterstics of a liquid.
Posted @ Thursday, October 17, 2013 4:57 AM by rahul roy
This paper gives an exponential relationship between surface tension and viscosity. (Equation (2) in the paper) 
 
"Surface Tension-Viscosity Relationship for Liquids." by Harold Schonhorn, J. Chem. Eng. Data, 1967, 12 (4), pp 524–525 
 
http://pubs.acs.org/doi/abs/10.1021/je60035a016 
Posted @ Monday, March 24, 2014 8:24 PM by Gordon
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