on a particle is expressed by a dimensionless Weber number,
We:
sext
ð 1 = 2 ÞpL¼
sextd
2 gð 11 : 5 ÞWe has to be of order unity for breakup to occur. Assumingd¼ 10 ^6 mand
g¼ 0 :01 N?m^1 ,sextshould be about 2? 104 Pa. Since this implies a pressure
difference of that magnitude over a distanced, the local pressure gradient
would have to be of order 10^10 Pa?m^1 , a very high value, difficult to
achieve by agitation of a liquid.
Consequently, special machines are needed to obtain the small
emulsion droplets often desired. Numerous types are in use, and we will
just mention those that are most common.Stirrersare often used, but the
intensity of agitation is often too small. Higher intensities can be achieved
with rotor–stator type stirrers, as depicted in Figure 11.6a.Colloid millsare
also rotor–stator machines (Fig. 11.6b), but they have larger dimensions,
and the slit between rotor and stator is far more narrow. They are especially
used for highly viscous liquids.High-pressure homogenizersare pumps that
force the liquid through a very narrow slit in the homogenizer valve,
depicted in Figure 11.6c; the valve block is pressed onto its seat by a spring.
The pressure drop over the valve is, for instance, 20 MPa, and the liquid
velocity in the valve is very high. Because of the very small size of the valve
slit, a homogenizer cannot be used to make emulsions; it is especially
suitable to break up the drops of a coarse emulsion into very small ones.
FIGURE11.6 Active part of some emulsifying machines. (a) Rotor–stator type
stirrer (‘‘ultra-turrax’’). (b) Colloid mill. (c) Valve of a high-pressure homogenizer.
The slit width in (b) and (c) is greatly exaggerated.