causescapillary contraction. As illustrated in Figure 10.27c, the surface
tension of the water tends to pull the particles toward each other at a certain
stage. The resulting contraction can be considerable. When a drop of water
is allowed to fall on a layer of powder, a situation as depicted in Figure
10.27d may occur; it is no exception if the total volume of the powderþ
water is reduced by a factor of 2. This then causes the pores between the
particles to become much narrower, slowing down penetration.
- If the powderparticles can swellon the uptake of water, they will
do so, given enough time. This will further decrease pore size, hence
penetration rate. - If (material from) the powder particle dissolves in water, this
causes an increase in viscosity of the penetrating liquid, thereby further
slowing down penetration.
A combination of phenomena 2, 3, and 4 or 5 will readily lead to the
penetration coming to a standstill and hence to the formation of lumps. To
give a powder instant properties, i.e., easy dispersability, the powder
particles are oftenagglomeratedinto fairly large units. Such a powder then
has large pores, that allow rapid penetration of water, and the agglomerates
become readily dispersed, after which they can either dissolve or swell,
according to powder type. If needed, thecontact angle can be effectively
decreasedby coating the particles with a thin layer of lecithin. (Lecithin is a
food grade surfactant that does not immediately dissolve upon contact with
water; moreover, most lecithin preparations readily give a thin layer on the
particles.) If a given powder cannot be readily dispersed it generally helps to
increase temperature (i.e., decrease viscosity) and to apply vigorous stirring.
10.7 INTERFACIAL TENSION GRADIENTS
Figure 10.28a depicts an interface between pure water and a pure oil, where
the water is caused to flow parallel to the interface. At the interface, there is
a velocity gradientC¼dvx/dy. There is thus atangential (shear) stressZWC
acting on the interface (ZWis the viscosity of water, about 1 mPa?s). The
interface cannot withstand a tangential stress, which implies that the liquid
velocity must be continuous across the interface: interface and oil also move.
The velocity gradient is not continuous, sinceZW6¼ZO, and the shear stress
must be continuous; in the picture,ZO& 5 ZW. If the upper fluid is air rather
than oil, the velocity gradient in air will be very much larger, sinceZW&
5500 ZA(see Table 9.2).
In Figure 10.28b the situation is the same except for the interface
containing a surfactant. For the moment we will assume that the surfactant
is not soluble in either phase. Now the flow will cause surfactant to be swept