# Standing wave interference patterns on liquid metals

1. Jan 28, 2012

### Skruples

Hi, first post, so take it easy on me,

I've had an idea for producing metallic objects with high surface area for several years, but lack the necessary physics knowledge to act on it or, really, inclination (and money) to go through years of school to gain such.

The idea is as follows:
A pan containing liquid metal is oscillated in the x and y dimensions using one or more piezoelectric transducers, such that a standing wave interference pattern is induced on the surface of the contained liquid. Because the pan itself acts as the free surface and not the liquid, it is my understanding the the resulting interference pattern would be stationary relative to the surrounding environment, whereas the pan would be moving relative to both the environment and a given volume in the contained liquid. The contained liquid could then either be cooled to solidify that arrangement, or left to cool over time.

I've spoken with several physics professors, and have been generally informed of multiple problems.

1. achieving a high enough rate of oscillation to produce the necessary effect:
again, its my understanding that the propagation of a wave through a liquid medium is determined by the speed of sound through same. In metal, a highly dense liquid, the speed of sound would be very fast and dependent on the particular density of whatever metal you happened to have melted. To overcome this, as even high rate piezoelectric transducers may lack the necessary speed to produce a noticeable or desirable effect, one could attach multiple transducers per side and either use a secondary timing device to activate them in sequence, or attach them in series and use the lengths of the conductive material between each consecutive transducer as a delay between activation, thus introducing spacing between each motivation of the pan.

2. Difference in rates of phase change between the surface of the liquid and the subsurface leads to disruption of the desired characteristics:
I've gotten conflicting answers regarding this, but in general the problem was described as being that the surface of the liquid and the sub-surface do not cool at the same rate, and the still molten sub-surface would disrupt or destroy the interference pattern on the surface during the cooling process, thus defeating the purpose. It was my suggestion that cooling from both sides, I.E, from the underside of the liquid by cooling the bottom of the pan and allowing the heat transfer to 'seep' up to the underside while simultaneously cooling from the top, would theoretically allow for both the surface and the immediate sub-surface to become solid at roughly the same time.

Also possible would be to produce many 'stand-alone' lumps of liquid material by having a small enough volume of liquid that the oscillations in the pan produce low pressure areas that extend all the way to the bottom of the container, thus creating, at least I think, areas of high pressure that contain little sine wave shaped lumps of the liquid. These could then be frozen, since, without the molten subsurface trying to 'shift' things and instead being contained on all sides, they wouldn't necessarily disrupt the curvature of the outside. With some clever math, the distance between 'lumps' could be small enough that, for all intents and purposes, the floor of the container would act as if it had itself been formed by the procedure and of the same material.

3. High surface tension of metals defeats sharp curvature in the liquid:
Liquids want to maintain the shape with the least surface area, because of complicated stuff that I don't really want to go into. However, as far as I know, all liquids share this characteristic and it is eminently possible to induce standing wave interference patterns in water, either by using the water as the free surface (as in streams or rivers that display stationary standing waves), or by using the container of the water as the free surface. In my mind, it becomes a matter of using force to overcome force; if the surface tension of liquid metal is higher than that of water, it simply takes a correspondingly greater amount of energy to overcome that boundary and produce the desired curvature. Thus, using transducers that can achieve a higher amplitude per cycle, or perhaps linking multiple transducers together 'back to back' in order to compound their force, might be able to transfer a correspondingly greater amount of energy into the system and defeat the surface tension problem.

Alternatively, or perhaps in addition, and I'm really getting out of my depth with this suggestion, a mixture of two or more liquid metals may disrupt the surface tension producing qualities of any one of them. Because the objective is to produce something with high surface area, not necessarily to produce it by the exact procedure I'm describing, it should be possible to produce a mold comprised of multiple metals which could then be used to shape other, lower temperature metals, which doesn't necessarily produce a useful outcome but is at least one step beyond the current problem.

4. Why am I doing this?:
Perhaps the biggest obstacle for getting any kind of conversation out of the physics profs. I've approached. High surface area seems to be useful in some industrial applications, as there are other methods for producing it. MIT uses nanotubules to produce surface area inside of potential batteries, allowing for greater energy storage and faster charge/discharge times. Powdered or sintered metals can produce surface area, though at this point I'm pretty far out of my area of expertise (psychology/neuroscience) and I probably don't know what I'm talking about. Suffice it to say, however, that other people who presumably DO know what they're talking about seem to think that having large surface area is useful, sometimes, and I have picked up on that.

So, Having lost, I'm sure, 90% of the internet audience at this point with my massive wall of semi-informed text, I would like the opinions of those that remain as to feasibility and usefulness of what I just described. Please keep unexplained math equations to a minimum, since trying to get wikipedia to tell me what [random Greek character] or [funny looking capital B] means is pretty difficult, and further trying to parse the meaning of complex equations with my mediocre math background is nigh impossible.

Thanks

2. Jan 28, 2012

### AlephZero

It seems to me there is a more fundamental reason why this won't work, than the objections you listed.

In a "standing wave" pattern, the liquid is moving. It it wasn't moving, the surface would be flat. You may not be able to see the movement, because it is too fast for your eyes to follow, but that doesn't mean it's not happening.

So, what you are hoping to do is instantaneously "freeze" all of the moving liquid into a solid. If you don't freeze it all very quickly, the moving liquid will destroy the shape of the frozen surface before it is strong enough to support itself.

Unfortunately, to freeze a liquid at X degrees of temperature into a solid at exactly the same temperature, you have to take out an amount of heat, called the "latent heat".

The latent heat given off when a liquid solidifies can be very big compared with the heat given off when the liquid (or the solid) cools down. For example, the latent heat when a quantity of molten aluminum solidifies is equivalent to cooling the same quantity of metal (solid or liquid) by about 400 degrees C, or 720 degrees F.

Incidentally this is why being scalded by steam is much worse than being scalded by hot water. The steam gives off a LOT of heat when it turns into water at the same temperature.

Unless you can think of a way to "fix" that problem, I don't think this can possibly work.