Question 1
Can you think of methods to decrease convection currents?
Answer
Convection currents in a quiescent liquid arise because temperature fluctuations
generally lead to variations in mass density of the liquid and thereby to unbalanced
buoyancy forces. To prevent this one can
Apply a very accurate and stable thermostat.
Store at about 4 8 C, since at that temperature water has dr/dT¼0, implying
that small temperature fluctuations do not lead to density fluctuations.
Increase viscosity; in first approximation, the value ofCwill be proportional
to 1/Z.
Perform the experiments in free space, where gravity is virtually zero (a rather
expensive method).
Question 2
In a research laboratory, fractal aggregation of emulsion droplets is studied. An O–
W emulsion is made with a buffer containingb-lactoglobulin as the continuous
phase. To prevent sedimentation of the droplets, part of the oil is brominated to
match the density of the oil phase and the aqueous phase. The droplet diameter
obtained is 2mm,j¼0.1. To prevent convection currents, the emulsion is stored at
precisely 20 8 C in a thermostat. The emulsion then forms a gel after 2.5 hours. By use
of Eq. (13.20), assumingRcr¼RgandD¼2, it is calculated thatW&10. In order to
check the assumptions, it is decided to dilute the emulsion with an equal amount of
water; it is expected that the gel time will then be 2^3 times as long, i.e., 20 h. However,
this proves incorrect: after a month, the emulsion is still liquid. What can be the
explanation?
Answer
The surfactantb-lactoglobulin is a small protein molecule that does not strongly
unfold upon adsorption at the O–W interface (Section 10.3.2). Consequently,
stability against aggregation will primarily be due to electrostatic repulsion. The
dilution with water will lower the ionic strength by about a factor of 2. It can be seen
in Figure 13.4 that this will cause a considerable increase inW, e.g., by a factor of 50.
Moreover, the largerWvalue will cause an increase in the value ofD, which will also
increase the gel time.