The preparation of dilute aqueous silicone oil emulsions has been investigated with particular attention
to the effect of oil viscosity (0.49–350mPa s), impeller selection (equal diameter Sawtooth and pitched
blade turbines) and the method of addition of the oil. Emulsification was found to be sensitive to how the
oilwas added to the vessel with narrower drop size distributions and smaller Sauter mean diameters, d32,
obtained when the oil was injected into the impeller region. The equilibrium values were also attained
in a shorter time with the equilibrium d32 ∝We−0.6. For addition of the oil to the surface the relationship
was weaker with equilibrium d32 ∝We−0.4. The viscosity group was particularly useful in describing the
behaviour of equilibrium particle sizes for different viscosity oils and also for viscosity changes arising
from different process temperatures.
An unexpected result is that the Sawtooth impellor proved to be more energetically efficient at drop
break-up producing smaller droplets than the Pitched Bade Turbine. This result is particularly interesting
since the power number for the latter is larger and therefore for equivalent operating conditions should
produce smaller drop sizes. We suggest that one possible reason is that the local shear rates for the
Sawtooth impellor are larger. Another possible reason is that the Sawtooth geometry provides more points
where the local shear rates are high.
© 2008 Elsevier B.V. All rights reserved.
It is well-accepted that local shear, elongation and necking are
very important aspects of drop formation as are the physical properties
of the fluids involved. Hence a successful design depends
on developing amechanistic understanding of how the equipment
selection, process strategy and material properties interact to affect
the resulting microstructure (e.g. particle size) and hence the performance
of the products. Typically two approaches are adopted:
• Scale-up at geometric similarity and constant tip speed.
• Scale-up at equal specific power input.
Scale-up on the basis of geometric similarity and constant tip
speed assumes that the relevant shear that produces the limiting
drop size occurs in the agitator region where the velocity gradients
are the steepest. These are assumed to scale with the peripheral
velocity of the impeller and the approach generally works