The Mayonnaise Effect
Liquids have a property called viscosity, which is a number that indicates how resistant a fluid is to flowing. For example, water is not very viscous as it flows easily, while oil is very viscous. It is well known that if you add something like a salt or sugar to a liquid such as water, that the viscosity increases with concentration. Already in 1906, Einstein came up with an equation predicting that the viscosity should increase linearly with the amount of stuff added. Einstein’s equation was improved upon a bit by Jones and Dole in 1929 but still has the linear dependence. This equation is used to this day.
The only problem is that nature doesn’t actually work that way: when you add more and more stuff to a liquid, the viscosity ramps up faster and faster and faster, completely against Einstein’s prediction. So why does the viscosity ramp up so quickly? I found that this can be explained by the presence of a so-called jamming transition. The idea is exactly the same as that which leads to a traffic jam on the road. As the density of cars on the road increases, traffic slows down. When the car-density reaches a critical point, traffic comes to a complete stop as it jams. It turns out that the same holds for molecules in solution. As you add more and more solute molecules, they start seeing each other more and more, so increasing the viscosity. It can be shown that there is a critical concentration where jamming occurs.
This idea then leads to a different equation for the viscosity that was already proposed by Australian chemist Austen Angell in 1972 based on the theory of glasses. I have shown that this equation turns out to be consistent with practically all known viscosity data of solutions over a huge range of viscosities. Most importantly, the critical concentration turns out to be that concentration where the solute molecules (plus one solvation shell) would jam into each other. So, even though this critical concentration cannot be achieved in practice, its virtual presence explains the nonlinear viscosities.
This effect is much more general than this. In any situation where a liquid is structured for some reason, the jamming effect will cause the viscosity to change. This could be structuring around the ions of a salt. It could be the intrinsic structuring in a room-temperature ionic liquid. It could be the concentration fluctuations near a critical point. It could even be the mixing of two liquids leading to droplets, as is the case in mayonnaise. This is why I called it the mayonnaise effect.
The paper, titled “The Mayonnaise Effect” is published in the Journal of Physical Chemistry Letters at http://dx.doi.org/10.1021/acs.jpclett.7b03207. The research was supported by funding from the Engineering and Physical Sciences Research Council (EPSRC).
Local copy of The Mayonnaise Effect (JPC Lett. 8, 6189−6192 (2017)).
- 21 December 2017: "The Mayonnaise Effect" is the 10th most read paper of JPC Lett. of the month. It was also spotlighted by the journal (see doi:10.1021/acs.jpclett.7b03289)
- 23 December 2017: 5th most read.
- 2 January 2018: "The Mayonnaise Effect" is now the #1 most read article in JPCLett of the past month