Creating nanoemulsions by means of Electrospray

[Taylor]

Hi everyone,

I have recently been trying to make a nanoemulsion of an aqueous solution by means of Electrospray. The basic principle is supplying a fluid through a very thin capillary tube and applying an electric field. Usually, when a fluid is supplied through one of these capillary tubes, a spherical meniscus is formed. Under the presence of the apropriate electric field, the meniscus becomes conical (Taylor cone). In the theoretical model for this phenomenon developped by Taylor, along the surface of the cone the pressure due to the surface tension equals the electric pressure. Nonetheless, this model implies the existence of singularities at the tip of the cone (such as an infinite electric pressure). Actually, in order for the cone to be stable, a jet of fluid must be emitted from its tip, thus losing mass (creating the need for a constant fluid supply through the capillary).
Due to the Rayleigh isntability, this jet breaks down into very small droplets, its size depending just on the diameter of the jet, which in turn depends on the rate at which more fluid is supplied through the capillary tube.

So my set up is as follows: a fixed steel capillary, with a copper wire around it, and with a plastic tube (of an inner diameter of 0.35 mm) attached to it to supply a constant flux of fluid. The tip of the capillary is submerged in hexane. The hexane is contained within a glass tube of 5 mm diameter. The capillary is placed in the center of this tube. The capillary is only 3 cm long, but the glass tube is much longer. The glass tube is coated with an aluminium foil. Both the capillary tube and the glass tube are placed vertically. The aluminium coating doesn't cover the whole glass tube: it starts at a height 2 cm below the capillary tip. I have connected another copper wire to this aluminium foil.
So in order the create the electric field, I have used a high voltage DC power supply. I have connected one electrode to the copper wire around the steel capillary, and the other one (ground) to the wire around the aluminium foil.
I have managed to create Taylor cones, which are quite stable (the only problem is they slowly grow larger, and finally emite a big droplet, then reset their size. This can probably avoided by better adjusting the electric field by varying the applied voltage, or by slightly changing the fluid flux). However, even though this should create an emulsion, all I get is a lot of larger than expected droplets (I'm using different fluxes in the range of 1 μl/h or 0.1 μl/h, so the droplets shouldn't be visible) adhered to the walls of the glass tube.

I am a little bit puzzled by this result. Could it be due to the glass walls getting charged?
If so, is there any effective way to discharge them?

Thanks in advance.

 

[UltrafastPED]

This may be helpful: http://esdeposition.wikispaces.com/

 

[Taylor]

That is indeed quite simmilar to my setup, and it says multiple Taylor cones indicate too high a voltage. I will definitely try to reach the electric field that makes the Taylor cone stable by increasing the voltage little by little, without reaching the voltage necessary to create multiple Taylor cones. Maybe this will prevent the formation of large droplets.
However, there's an important difference between this setup and my setup, which is basically that my capillary tube is surrounded by a glass tube of 5 mm diameter. The glass is definitely getting charged, and this is attracting the droplets to its surface, so in the end I don't get a thin emulssion, but many big droplets attached to the glass surface. I think this would be probably prevented by discharging the surface. But since it's an insulating material (with a low conductivity), I don't really know to effectively discharge it.

 

[UltrafastPED]

But since it's an insulating material (with a low conductivity), I don't really know to effectively discharge it.

You can electroplate the glass with a very thin metallic coating ... or buy some conductive tubing.

 

[Taylor]

Hi there! Yeah, that sounded like a good idea. The problem is though, we cannot alter the properties of the material, since the tube itself is the container designed for further analysis (NMR).
We have now thought of changing the polarity of the circuit every 1 or 2 seconds. Thus, the problem is now reduced to a DC power supply, whose polarity we want to invert automatically every second or so. Is there any easy way to do so?