Monday 15 June 2020

fluid dynamics - Viscosity of water in the presence of solutes


Some physical properties of water change in the presence of solutes: vapor pressure, boiling point, freezing point and osmotic pressure. In particular, these four properties are called colligative properties because they depend only on the ratio of concentrations of solute and solvent, and not on the nature of the solute.


I am interested in the viscosity of water. I have two questions:





  1. Does the viscosity of water change in the presence of solutes?




  2. Does the change depend on the nature of the solute? On the concentration of the solute? How?




Of course, if the answer to the first question is in the negative, the second question is nullified.



Answer





  1. Yes.

  2. Yes. Yes. See below.


The Falkenhagen relation (NB: paywall, but (a) it's on the first page of the "Look Inside" option and (b) your University's library might have a copy) suggests that $$\frac{\eta_s}{\eta_0}=1+A\sqrt{c}$$ where $\eta_s$ is the solution viscosity, $\eta_0$ the solvent viscosity, $A$ a constant that depends on the electrostatic forces on the ions, and $c$ the concentration of the solute.


There are other approximations, e.g. ones that go to higher order $c$, that account for larger concentrations, so the above may not be exactly what you need for whatever purposes you have.


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