Cross-Flow Emulsification (XME)

An easy way to create an emulsion is to whiz up a mixture of oil and water in a household blender. Provided the starting mixture contains a suitable surfactant, the result of just a few seconds of blending is a shear-induced emulsion. However, the droplets thus created vary enormously in size. A typical alkane in water emulsion might contain droplets with diameters ranging from 0.2 to 4 microns.

The IFR's XME rig

Tracking droplets is much simpler if all the droplets are the same size. To create such (quasi) monodisperse emulsions, we have installed a cross-flow emulsification (XME) rig. The basic idea is to squeeze oil through lots of tiny holes in a membrane into a passing stream of water. Oil droplets are formed as the oil leaves the holes, and are carried off by the passing water.

The tall silvery cylinder left of centre in the picture (right) is the continuous phase reservoir. Since the continuous phase is cycled round the rig and through the membrane element many times, this reservoir gradually fills with emulsion. The half-clear container in the centre of the picture is the oil reservoir. The large black object beneath is a powerful vane pump which drives water, and the emulsion as it's created, round the circuit. The most important component is the thin vertical tube to the right of centre, which houses the membrane. The oil phase is forced through the membrane via air or gas pressure.

Schematic representation of the XME rig

The membrane itself is made from a ceramic material. The image below shows an exploded view of the membrane on the left, and a micrograph of the membrane surface on the right.

(Image courtesy of Emulsion Systems Ltd)

In the micrograph, the objects that look like boulders are just the membrane substrate. The small 'pebbles' to the right constitute a fine-grain microporous surface through which the oil is squeezed from left to right, emerging as quasi-monodisperse droplets into the passing continuous phase.

(Image courtesy of Emulsion Systems Ltd)

The droplet size is controlled by pressures and flow rates, in addition to the structure of the membrane.

Other techniques exist in the literature for the creation of monodisperse emulsions. The XME route is relatively simple, efficient, continuous rather than batch, and could be scaled up to industrial scale machines.

To learn more about the XME check out our supplier on www.emulsionsystems.com