Tribology as a Tool to Study Emulsion Behaviour in the Mouth - Food Colloids: Self-Assembly and Material Science - page 457

Figure 4 CLSM images of an oil-in-water emulsion (1 wt%WPI, 40 wt% sunflower oil).
Sample sheared between pig's tongue and glass: (A) before applying a load; (B)
after applying a load and shearing, still under compression. Sample sheared
between PDMS and glass: (C) before applying a load; (D) after applying a
load and shearing, still under compression. Image size ¼ 144 108 m m 2
and simultaneously the emulsion droplet behaviour under shear was observed
by CLSM, making it necessary to shear the sample against an optically
transparent glass plate. Shearing an emulsion sample between PDMS and glass
results in a friction force that is much lower than the force measured upon
shearing the same emulsion between pig's tongue and glass. In fact, the friction
force is so low that it cannot be determined accurately with the set-up used. 19
Owing to the fact that PDMS is much smoother than pig's tongue, it seems
likely that a hydrodynamic layer is formed on the surface upon shearing,
resulting in a very low friction force.
The behaviour of the lubricating emulsion was observed by confocal mi-
croscopy during shearing in the OTC. The black spheres in Figure 4 are
emulsion droplets and the white structures in Figure 4(B) are tongue papillae.
Shearing with the pig's tongue [Figure 4(A) and 4(B)] resulted in coalescence of
the emulsion, whereas shearing with PDMS [Figure 4(C) and 4(D)] gave no
observable coalescence. A possible reason for this difference in coalescence
behaviour is the dissimilarity between PDMS and pig's tongue with respect to
deformability and roughness of the surfaces. The emulsion droplets are, in the
case of shearing with the pig's tongue, confined between the papillae. This can
cause the droplets to be subject to an extensive shear flow regime in the confined
space between the papillae hairs. Another explanation is that the droplets are
sheared against each other between the papillae surfaces. In the latter case the
micro-roughness of the tongue surfaces is of importance, 19 as well as the
deformability of the surface. In contrast to the pig's tongue, PDMS has a
completely smooth surface with a much higher elastic modulus. Therefore,
upon shearing with PDMS, no coalescence was observed.