# Liquid Metal Under Applied Voltage

1. Aug 22, 2014

### ApplePi314

Hi guys!

I have a question regarding liquid metals. Suppose you had a droplet of liquid metal in empty space. Take the droplet and connect it to a DC source. What happens if the voltage of the DC source exceeds the surface tension of the liquid metal droplet? Would the droplet split in two? Would it expand?

I'm really curious and appreciate anyone taking the time to answer.

Thanks!

2. Aug 22, 2014

### Staff: Mentor

You mean that you connect one side of the voltage source to one side of the droplet, and the other side of the voltage source to the to other side of the droplet? Liquid metal conducts (google for "mercury switch") so you've just created a short circuit. Whatever is the weakest link in the circuit will burn, explode, boil, or do something else exciting.

3. Aug 22, 2014

### ApplePi314

Hey Nugatory! thanks for the reply.

I guess a better example would be two droplets, one connected to the positive terminal of the DC source and the other connected to the negative terminal. I'm trying to figure out how the droplets would respond to the applied potential (Would they split into smaller droplets or expand until the total surface tension matched the voltage? Would they boil?).

Thanks again!

4. Aug 22, 2014

### Bobbywhy

Voltage is measured in volts obviously. How could volts "match" the dimensional units used for surface tension?

Surface tension measurement are often expressed as dynes-per-centimeter, is the same as surface energy, which is often expressed as ergs per square centimeter (erg/cm2 = dyne cm/cm2 = dyne/cm. A soap film may have a surface tension of 25 dynes/cm. Mercury would be typically in the region of 480 dynes/cm.

For an excellent review of exactly how liquid metals are measured see:
Surface Tension Measurements of Liquid. Metals by the
Quasi-Containerless Pendant Drop Method
Kin F. Man
Jet Propulsion Laboratory
California Institute of Technology
mN-m-1
http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/18292/1/99-1764.pdf [Broken]

Field-emission electric propulsion (FEEP) is an advanced electrostatic space propulsion concept, a form of ion thruster, that uses liquid metal (usually either caesium, indium or mercury) as a propellant.
http://en.wikipedia.org/wiki/Field-emission_electric_propulsion

Thank you in advance for making your search more clear and specific for members here on PF!

Bobbywhy

Last edited by a moderator: May 6, 2017
5. Aug 22, 2014

### ApplePi314

Hey Bobbywhy! Thanks for the reply.

I apologize if my question is unclear. I have a limited background in physics and am trying to figure out the best way to word it.

So if you have a droplet of liquid, there is a surface tension (a force) associated with that droplet. In introductory physics, I was taught the electronic-hydraulic analogy in which voltage was essentially "electric pressure". What I'm trying to figure out is if you had two droplets of liquid metal, one connected to the positive lead and the other connected to the negative lead of a voltage source (say a battery), how would the droplets respond? Because if you take a droplet of liquid and apply a physical pressure (say blowing air at it), it will respond by deforming, moving, or splitting into smaller droplets.

It may simply be a dumb question brought about by my very, very rudimentary understanding of physics, but I appreciate you taking the time to respond.

Thanks!

6. Aug 22, 2014

### Okefenokee

The two droplets would have a slight capacitance depending on how close together they are. Charge would build up on the sides closest to each other and create an attraction. The droplets would stretch out towards each other. How much they stretch and how much voltage it would take to split the droplets is going to be a difficult problem to model much less solve.

They may not even split. There will necessarily be an adhesive force holding them to your voltage probes, like a drop of water hanging from your finger. The attractive force may overcome the adhesion and simply pull the droplets off of the probes.

7. Aug 23, 2014

### ApplePi314

Thanks Okefenokee, your explanation is exactly what I was I was looking for.

Thanks!