Tribology as a Tool to Study Emulsion Behaviour in the Mouth - Food Colloids: Self-Assembly and Material Science - page 456

The wetting characteristic of the surface is another aspect that can influence
the capacity of an emulsion to act as a lubricant and thus consequently its
capacity to reduce the friction. We have found 19 that a water droplet spreads
fast on a slightly moist pig's tongue which is not covered by a mucous layer.
Drying the tongue tissue with pressurized air results in a 'dry' surface. On such
a surface a water droplet scarcely spreads at all, indicating that the tissue is then
more hydrophobic, and hence comparable to PDMS. 19 In the human mouth
the oral mucosa is covered by a gel-like mucous layer, containing mainly
mucins (glycoproteins) and water. Our results show that the oral mucosa is
intrinsically hydrophobic, but it is covered by a mucous layer which gives it the
characteristics of a hydrophilic surface.
In summary, then, we can say that a pig's tongue not covered by a mucous
layer has a similar hydrophobicity to PDMS. In the mouth, however, a mucous
layer covers the surface; so PDMS fails to mimic the tongue in terms of
its wetting characteristics. Considering the elastic modulus and the rough-
ness, PDMS differs greatly from pig's tongue; the synthetic polymer surface is
much smoother and it has an elastic modulus that is two orders of magnitude
larger.
To what extent does the behaviour of an emulsion upon shearing between
artificial surfaces deviate from in-mouth emulsion behaviour? To identify these
differences we used a home-built experimental arrangement called the optical
tribological configuration (OTC). 19 In this arrangement (Figure 3) a sample of
emulsion was sheared between a pair of surfaces, either PDMS/glass or tongue/
glass, under a certain load F z . During each experiment the lower plate oscillated
for a number of cycles at a certain speed. The friction force F x was measured,
Figure 3 The OTC. Emulsion sample A is confined between an upper surface B (e.g., pig's
tongue or PDMS) and a glass surface C. A normal force F z is applied and the
friction force F x is measured while surface C is oscillating. The sample is
observed by CLSM