Fluid Jets

Alright, this one isn't about amplifiers, but I recently got accused of having a "dead blog," so as I'm sitting around the lab waiting for our copper vapor laser (more on that later!) to cool, I thought I'd share some awesome images with the world of some micron-sized bubbles undergoing asymmetric oscillation and the creation of a fluid jet.

Excited yet? I figured not. We'll start at the beginning. Check out this propeller:


Looks pretty sweet, huh? Judging by the damage on the lower left, you might think someone had a little too much of Grandpa's cough syrup and run the boat upon the shore or something. Well, boating drunk can certainly damage a propeller, but this damage has a pretty characteristic look to the trained eye, called "cavitation" damage.

In order to keep this entry fun, I would like you now to turn back the clock to the 1800s. You're the captain of a mighty steamboat and you motor around the seven seas fearless. However, every few months your propellers need to be replaced because you see damage similar to that shown above. Why the hell is this propeller, which just spins in the water, always getting this same damage??? Now, please return to the present with the understanding that this unexplained damage was certainly a mystery for a period of time.

Fortunately, the process slowly began to be understood (first modeled by Lord Rayleigh). As a propeller turns, it creates pockets of negative pressure in the fluid. These negative pressure areas are strong enough to pull gas out of the liquid creating small bubbles. Since negative pressure was necessary to pull the gas out of solution, when the negative pressure is no longer present, the bubble will collapse. So you have this small bubble (let's say a few millimeters in diameter) and it collapses. You might, think "oh, big deal. a small bubble bursts underwater." HOWEVER, this little bubble collapses so fast it will make your head spin. In some situations the bubble can collapse so fast it generates light through a process called sonoluminescence (the generation of light through sound). Think about that for a second--when was the last time you saw something move in such a way that it forced light to be emitted from surrounding molecules? Yeah, not too often. Pretty cool stuff if you ask me.

Sonoluminescence does not occur in the situation with the propeller to my knowledge, though. However, the bubble does collapse at a very high velocity near the boundary. A typical growth and collapse cycle of a bubble near a boundary looks something like the following images (from Kodama et al.):

What you see above is a fluid jet that is formed as the bubble collapses asymmetrically. In this case, the jet penetrates a gel surface, and the bubble is on the order of 1 mm in diameter. This all occurs quite quickly and the above images have a frame rate of 100 kHz (100,000 frames per second). The speed of this so-called "fluid jet" is 66 meters / second. Consider this in terms of say .. your typical super soaker water gun. Imagine if you fired that thing from the end zone of a football field and 1 second later it had crossed the 50 yard line. When you think of it this way, you can see how these small jets can cause some major damage. Clearly, there are equations that describe this activity in terms of the momentum of the fluid, but c'mon .. this is a blog.

This brings me to the beautiful image I want to share, which is a micron-sized bubble undergoing the creation of a fluid jet near a gel boundary. Since this bubble is only a couple of microns in diameter it oscillates much faster than the one above. Not to mention it is smaller, so it's a little harder to image. Anyway, with a very good microscope and a 30 nsec laser for a flash, I was able to capture the following image, which is a fluid jet impinging through the bubble near a boundary.


These bubbles are being researched for applications in localized drug delivery, where they can be injected (since they are the size of blood cells). During appropriate insonation parameters, you can see how a small fluid jet is created, which would impinge on the wall of a living vessel, potentially increasing the permeability of that vessel so a drug could reach beyond the vasculature.

And there you have it: the reason I have not been building amplifiers and writing about it :)