Cryoma Labs

How do dissolved ionic compounds affect the viscosity of water?

Nov 18, 2007 by cas13091 | Posted in Chemistry

How does the charge magnitude, concentration, and ionic radius of the dissolved salts affect the viscosity of water, if at all? And why does this happen?

Dissolved ionic compounds increase the density of water, and therefore its viscosity. That's why an egg will float on salt water and sink in fresh: the egg doesn't change in density, but the water does; the salt water is more dense than the egg and the fresh water is less dense, so the egg floats on the salt water.

Ionic compounds accomplish this feat by organizing water molecules around the ions using ion-dipole interactions (water is the dipole, the dissolved ions are the ions). Organized things do not flow as fast! (look at glass; it is pretty highly organized compared to water and so flows really slowly)

Reasonably enough, highly charged things organize more water molecules than less charged things. This means highly charged things increase the density (and thus the viscosity) of the water.

Also, more ions dissolved in the water will organize more water molecules, logically enough. Same sort of thing as above: more organized water molecules, more dense solution, greater viscosity.

Likewise, a small, concentrated ion will organize water molecules more strongly than a large, diffuse ion. This is similar to highly charged versus less-charged ions above.

Take F- and I-, for example. The overall charge is the same on each ion, but for I- the charge is not felt as strongly by water molecules because it is spread over a larger, "floppier" electron cloud. Therefore I- will not organize water molecules as well as the more concentrated charge in F-.

You might say "but what about the fact that you could fit more water molecules around the bigger I- ion?" A water molecule is rather large, so you can't fit that many more of them around the I- ion. Also, the more concentrated charge on F- can organize a shell of water molecules 2 and 3 deep, and so still carries the day.

What equation do I need to find the kinematic viscosity of water given the temp and pressure?

Oct 01, 2008 by blueorion3 | Posted in Engineering

I am given the temp and pressure (10 deg C and 35564 kPa) and need to find the kinematic viscosity of water. I know that the kinematic viscosity is dynamic viscosity/density but how do I take into account the temperature and pressure? I don't want an answer, just an equation or explanation. Thanks.

I really doubt that pressure will be an issue with either viscosity or density. You may need to consult the Temp./ viscosity tables and the Temp./ density tables and just plug those values into the equation you alteady have. The viscosity will vary with temperature, the density, not much. To be practical, if you can find one , a (rho) at around 20C , just use that one.

What is a formula for the dynamic viscosity of water at lower temperatures?

Feb 09, 2011 by Brian S | Posted in Engineering

I am doing a mini group project and we need to estimate how many reverse osmosis machines we would need to hit a desired gph and ppm. What I need to know is a formula for the viscosity of water, in cP, at a temperature range of 2-28 degrees Celsius as a function of temperature only. The only ones I could find involved pressure or their accuracy range bottomed out at 30 degrees.

Amazing! may be useful as a personal reference http://giftchoice.info/291056/reverse-os mosis

The Coanda Effect demonstrating the viscosity of water with a spoon.

If a stream of water flows along a surface that is curved, the water will follow the surface, This is because of the viscosity of the fluid, and ...

Mixing fluids efficiently in confined spaces: Let the fingers do ...

Getting two fluids to mix in small or confined spaces is a big problem in many industries where, for instance, the introduction of one fluid can help extract another — like water pumped underground can release oil trapped in porous rock — or where the mixing of liquids is the essential point of the process. A key example of the latter is microfluidics technology, which allows for the controlled manipulation of fluids in miniscule channels often only a few hundred nanometers wide.

Microfluidic devices were first introduced in the 1980s and for many years were best known for their use in ink-jet printers, but have since been introduced in other fields, including the chemical analysis of blood or other sera in lab-on-a-chip technologies. These devices — usually not much larger than a stick of chewing gum — sometimes rely on nano-sized moving components, the geometry of the grooved channels or pulsed injections to induce a mixing of the fluids. But researchers in MIT's Department of Civil and Environmental Engineering suggest that a simpler method might be equally, if not more, effective.

This video shows the alternating injection into a microfluidic channel of blue-dyed water and a colorless mixture of water and glycerol that is 50 times more viscous than the water. The viscous fingers of the blue water quickly spread throughout the heavier fluid to create a homogenous mixture. Credit: Video / Michael Szulczewski of the Juanes Lab, MIT "Getting two fluids to mix in a very tight space is difficult because there's not much room for a disorderly flow," said Professor Ruben Juanes, the ARCO Associate Professor in Energy Studies and principal investigator on the research. "But with two fluids of highly contrasting viscosity, the thinner fluid naturally creates disorder, which proves to be a marvelously efficient means of mixing."

In an analysis published online May 12 in (PRL), the researchers show that the injection of a thin or low-viscosity fluid into a much more viscous fluid (think of water spurting into molasses) will cause the two fluids to mix very quickly via a physical process known as viscous fingering. The thinner liquid, say the researchers, will form fingers as it enters the thicker liquid, and those fingers will form other fingers, and so on until the two liquids have mixed uniformly.

Read more...