obtained from settling alone. Bulk temperature measurements
after microwave heating indicate that the emulsion temperature
is of the order of 40–50 C, so the data in Fig. 9 represents a conservative
estimate of the minimum separation time that can be
achieved. The experimentally observed settling time is still much
lower than that predicted by the model, indicating that the average
droplet sizes must be larger than 345 lm.
The predictions shown in Figs. 8 and 9 show that the enhanced
separation of emulsions due to microwave heating cannot be solely
attributed to a viscosity reduction in the oil phase – significant coalescence
of the water droplets must also occur during the heating
and settling process. The localised temperature of the oil adjacent
to the water droplets is not known, therefore it is not possible to
completely decouple the effects of coalescence and viscosity from
the data presented in this study. However future work in this area
will aim to quantify the two effects using in situ droplet size measurement
techniques along with more sophisticated CFD modelling
approaches to assess thermal and viscosity gradients throughout
the emulsion.
The results of this work can be used to elucidate the mechanism
of the well-known phenomenon of microwave enhanced settling of
water-in-oil emulsions. While the complexity of the system has
prevented complete decoupling of the effects of viscosity and coalescence
on settling time, it is possible to explain both factors by
the thermal effects of microwave heating. The water phase is
heated selectively, so the temperature of the oil at the droplet
interface is much higher than the bulk oil temperature. A temperature
gradient exists from the bulk oil to the droplet interface,
which is unique to microwave heating. The high interface temperature
results in a lower oil viscosity, but also a reduced interfacial
tension [1,2]. Interfacial tension is reduced further due to the volumetric
expansion of the water droplet resulting from its selective
heating. Coalescence can be increased by increasing the collision
frequency between droplets, and by decreasing the resistance to
coalescence. A lower viscosity at the interface increases the collision
frequency as there is less resistance to droplet flow. A lower
interfacial tension means that resistance to coalescence is reduced
when droplets collide, therefore improving the probability of
coalescence.
The improvements in separation time observed with microwave
heating of saline-based emulsions compared with waterbased
emulsions can also be explained in terms of thermal phenomenon that arise from microwave heating. The evidence
presented in Figs. 4 and 7 leads to the conclusion that the predominant
effect of the addition of NaCl is to enhance the thermal effect
during microwave heating, rather than directly changing the interfacial
tension.