thereby reconstituting milk. Most dried soups contain particles that become
dispersed and other particles that also dissolve.
Figure 10.27a depicts a heap or lump of powder particles on water.
Water should penetrate into it. This is usually aided by stirring, whereby
water can penetrate from all sides. If the water does not penetrate fast
enough, fairly firm lumps are formed, in which the outside is a gluey layer of
partly dissolved or swollen powder particles, whereas the inside is still dry. It
is quite difficult to disperse (and dissolve) such lumps.
Following are important phenomena in the dispersion process:
- Thecontact angleair–water–powder particle should be acute. It
can be measured by making a solid and smooth tablet of the powder by
applying high pressure, and then let a drop of water fall on it and
immediately measurey; this is an advancing contact angle. It should be
smaller than about 30 8. As illustrated in Figure 10.27b, even for a fairly
small value of y, the meniscus of the water tends to become flat with
increasing penetration between two particles, thereby stopping further
penetration. - Thesize of the poresbetween the particles should be large to allow
fast penetration. Although Eq. (10.16) only holds for cylindrical pores, the
trends predicted apply to the present case. The pores are smaller for smaller
powder particles and for a larger spread in particle size. A sample that is a
mixture of two powders, one fairly coarse and the other quite fine, is
notorious for slow penetration, since the small particles fill up the holes
between the large ones. - In many powders, the void fraction between particles is large, over
0.5. This means that most of the pores are relatively wide. However, wetting
FIGURE10.27 Dispersion of a powder in water; A¼air, W¼water, P¼powder
(particle). (a) Heap of powder on water into which water is penetrating. (b)
Penetration of water between two particles at various stages (1–3) at constant contact
angle (about 45 8 ). (c) Pulling action (arrows) of the surface tension of watergAWon
powder particles. (d) Situation after a large water drop has fallen on a layer of
powder in a cup.