Emulsions: Principles and preparation - Making emulsions using membranes and microchannels - Membrane emulsification

12 important questions on Emulsions: Principles and preparation - Making emulsions using membranes and microchannels - Membrane emulsification

What mechanism is used for cross-flow membrane emulsification?

A Shirazu Porous Glass (SDG) or ceramic membranes. Each droplet is individually on the mouth of the pore. The oil is pushed through the pores of this membrane, the oil emerges as very small droplets in the aqueous phase at the permeate (other) side.

What is the drag force?

In cross-flow membrane emulsification, the oil droplet emerges through the pore and will form a small droplet on top of the pore mouth. The cross-flowing continuous phase exerts a drag force on the droplet.

Which 2 forces interact on a droplet during cross-flow membrane emulsification?

Drag force --> away from the pore mouth
Interfacial tension --> keep the droplet at pore mouth

Explain what happens to the Laplace pressures with cross-flow emulsification at the SPG membrane

The oil emerges from a pore as a droplet. At a certain size, the the Laplace pressure in the emerging droplet (gets lower when larger droplet), will be lower than the Laplace pressure inside the oil phase in the pore. Oil phase flows from pore into the droplet. At a certain moment, the cylinder of oil in the pore snaps off, and the droplet is release in the continuous phase.

The cross-flow does not have much influence on the process, it is driven by the interfacial tension.

At what kind of flow rate does the spontaneous snap off at cross-flow membrane emulsification take place?

At a low flow rate. A too high flow rate, will not give enough time for the continuous phase to replace the oil in the droplet.

When does spontaneous snap off take place?

When the droplet radius is twice as large as the pore radius.

What mechanism is used for dead-end (premix) membrane emulsification?

Started with a coarse pre-emulsion, featuring large droplets. Pushed this coarse emulsion through a porous membrane. Fine emulsion on the other side. High throughput.

How are the pre-emulsion and the droplet size distribution related?

Coarse pre-emulsion --> small droplets plus some very large

Every pass, the number of small droplets increases and large droplets decreases, until the emulsion consists only of small droplets.

What are 3 break-up mechanisms of dead-end (premix) membrane emulsification?

  • Snap-off due to localised shear
  • Break-up due to interfacial tension effects
  • Break-up due to steric hindrance

What is the typical droplet size in dead-end (premix) membrane emulsification? And what is it dependent on?

Droplet size is a bit larger than the average pore size of the membrane.

The droplet size is hardly dependent on the flux through the membrane or the volume fraction of the dispersed phase.
However, the flux is dependent on the volume fraction of the dispersed phase.

What is a disadvantage of dead-end (premix) membrane emulsification?

Its sensitivity to internal fouling, especially when proteins are used as emulsion stabilisers. The dispersed phase, the continuous phase, and all other components needs to flow through the membrane. Any particulate matter that would be present in the pre-emulsion, and that is small enough to be able to enter the pores, will be captures somewhere in the pore structure.

What is the mechanism of phase inversion in premix membrane emulsification?

To go from an O/W to an W/O emulsion, a hydrophobic membrane can be used (wetted by the dispersed phase). Vice versa with a hydrophilic membrane.

The same (O/W or W/O) will be obtained when the membrane is wetted with by the continuous phase, like in cross-flow and dead-end membrane emulsification.

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