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ramé-hart instrument co. March 2014 Newsletter
 

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Surface Tension and Viscosity

Here's a question that I've been asked more than once: What's the relationship between surface tension and viscosity? Intuition tells us that a thick liquid with high viscosity will have a higher surface tension than a thin liquid with low viscosity. However, this may not be a safe conclusion.


Honey, with a high viscosity, is thick and slow but has a lower surface tension than water

Viscosity is the measurement of internal resistance to change in shape by the forces of shear or tensile stress in a fluid in motion. It explains the friction between molecules which can change based on pressure and velocity. Viscosity can get involved. For example, some fluids conform to Newton's law, such as water, and are considered Newtonian fluids. While other fluids, such as ketchup, are considered non-Newtonian because their viscosity is dependant on shear rate. Have you ever noticed that ketchup doesn't want to budge until you whack the bottle - and then once it does starts flowing, it doesn't want to stop? We don't really get involved in measuring viscosity but let us say that it's a dynamic measurement which is about capturing properties of fluids in motion.

Surface tension, by contrast, is actually a much simpler concept. It's characterized by what appears to be an elastic sheet on a liquid's surface. Surface tension is what allows a water strider to walk on water.


A Water strider relies on surface tension

The water molecules inside the liquid (shown in blue in the graphic below) are attracted to one another through cohesive forces which balance out to zero. The (green) molecules at the liquid's surface, however, do not have as many molecules to attract to and are thus pulled inward causing the surface of a liquid to behave like an elastic sheet. Surface tension is what causes drops to bead up on a surface. And it's the balance between the surface tension at the gas/liquid interface and the surface energy at the liquid/solid interface that drive the contact angle.


Cohesive forces describe surface tension

Surface tension is one thing that we are very good at measuring using ramé-hart instruments. However, surface tension, is generally a static measurement. We often measure it using a pendant drop. Viscosity, on the other hand, is about movement and velocities. So, while it's tempting to draw a correlation between surface tension and viscosity, we conclude that there is none. Viscosity has to do with the property of fluids in motion; surface tension describes the behavior of the outermost layer of molecules of a liquid at the liquid/gas interface. There is no correlation.

Take honey, for instance, which has a very high viscosity: its surface tension is actually lower than that of water which has much lower viscosity. Acetone, by contrast, has a lower viscosity than water but its surface tension is also lower. And so it is between any pair of liquids - there is no rule of thumb or correlation between surface tension and viscosity. Nonetheless, despite there there being no relationship between these two metrics, it is generally true that as temperature increases, both viscosity and surface tension decrease.
 

Viscosity and Dispensing
While we conclude that there is no correlation between viscosity and surface tension, we cannot ignore viscosity - especially when dispensing liquids. For that reason we offer a wide range of dispensing needles which can be used with our manual microsyringe, our Automated Dispensing System, and Elevated Temperature Syringe. For water we use a 22 gauge needle. For liquids with higher viscosity we use a lower gauge needle: 18 gauge for glycerol, 12 gauge for melted polymers (using our Elevated Temperature Syringe). For thin low viscosity liquids we use higher gauge needles - 26 gauge for hexane, 30 gauge for acetone, for example. We keep in stock dispensing needles from 8 gauge to 33 gauge although 12 gauge to 30 gauge are the most common with 22 gauge the standard.


Triaxial Needle made by ramé-hart

For a complete chart of our needles including inside and outside diameters, see this chart: http://www.ramehart.com/pdf/needles.pdf. To order straight dispensing needles, go to: http://www.ramehart.us/straight-304-stainless-steel-reusable-dispensing-needle/ and for inverted needles, go to: http://www.ramehart.us/inverted-304-ss-needle/. ramé-hart is also the world's number one fabricator of coaxial and triaxial needles; get started here: http://www.ramehart.com/coaxial_needles.htm

 

Regards,

Carl Clegg
Director of Sales
Phone 973-448-0305
www.ramehart.com
Contact us


 

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