Aerosols are useful for a variety of purposes such as applying deodorant, spraying pesticides and air conditioning. By carefully controlling the droplet size, the aerosol can be efficiently delivered for the particular application.
Instead of the aerosol droplet being a solid sphere it would be interesting to make hollow spheres - bubbles.
This could be achieved by running liquid down the outside of a hollow needle and gas through the inside of the needle. As the droplet forms at the tip of the needle, gas is pulsed into it. This could be done with an array of needles to make a large quantity of mist. The liquid could be encouraged to run down the needle by any force such as gravity, centrifugal, electrostatic etc.
The microbubbles would be useful to lower the average density of the droplets.
If the microbubbles were filled with a light gas (eg helium)the mist would float eerily. One application of this would be for cooling outdoor areas. Obviously for this application it would be preferable to choose a material you would want to be coated in such as a water/ethanol mix.
The material would also need to have physical properties (viscosity, surface tension etc.) conducive to microbubble formation.-- xaviergisz, Dec 01 2004 microbubbles in windtunnels http://www.newscien...of-future-cars.htmlThe 3-millimetre bubbles swirl around cars in a wind tunnel. Engineers use 12 cameras to track the bubbles, and so capture air flows in unprecedented detail. [xaviergisz, Sep 30 2010] Laplace pressure http://en.wikipedia...ki/Laplace_pressure [xaviergisz, Nov 15 2011] (?) Air pressure in bubbles http://www.soapbubb...les/airpressure.php [xaviergisz, Nov 16 2011] Raindrops hitting the ground form microbubble aerosols https://gfycat.com/...nderousIrishterrier [xaviergisz, Oct 03 2017] I agree that, in concept, this seems to work well. Aside from being able to float in the air for longer periods of time, would these "bubble aerosols" have any other uses in comparison to regular aerosols? Maybe they could be used to move sprays onto areas too high to other wise be reached, like spray painting a ceiling without a ladder?-- Waser8, Sep 30 2010 Small bubbles (of neutral density) are being used in windtunnels to help illustrate air flow (see link).
I like your idea of using microbubbles for spray painting ceilings.
I often wonder whether microbubbles are common in nature. For example, morning mist eerily floating above lakes could be microbubbles.-- xaviergisz, Sep 30 2010 //need to have physical properties// That phrase is doing a lot of the heavy lifting in this idea, but [+]. Vague memory: as radius of curvature decreases, pressure increases. If true, scaling bubbles down to micro size would raise internal pressure -- you'd need either very *low* surface tension (so that the pressure wasn't so high it popped the bubble) or very *high* surface tension (so that the bubble could contain high pressure without popping). Are bubbles possible with very low surface tension? If internal pressure is high, you also need very low permeability to whatever gas is contained. And to prevent evaporation, either very low vapor pressure, or maintain the system in a vapor-saturated atmosphere.
Perhaps if you had a flame at the needle, you could form hollow, gas-filled fullerenes?-- mouseposture, Sep 30 2010 Tiny bubbles make me feel happy. I got so tired of having my regular bubbles burst all the time, I'm reduced to microbubbles. They're harder to hit, so harder to burst. [+]-- Boomershine, Sep 30 2010 I thought of another solution to the smaller bubble = higher pressure problem. Somewhere around here, I've got a reprint of of Overholtzer's seminal paper "Nonmonotonic surface tension in thiotimoline-doped solutions of dilithium in molten gleepsite." I'll send you a copy: just forward me 700 Euros, care of [jutta] to cover duplication costs.
"Shorty" Overholtzer's career was tragically cut short: he died in a lab accident 6 months before writing that paper. That sort of thing used to have been common in thiotimoline research.-- mouseposture, Oct 01 2010 One of my business activities is to build and supply fogging systems for dust control in mines, on conveyor belt drop points and so on. One of the things required to build a successful fogging system is to match the droplet size to the dust particle. If they're mismatched they tend to bounce off one another and not acheive the desired bonding required to drop the dust particle from the air.
Perhaps this idea of [xaviergisz]'s would allow larger droplets but with the same mass. The larger surface area might create a higher probability of droplets colliding with dust particles but not bounce away from each other. Surfactants are often used so soapy droplets would be fine.-- AusCan531, Nov 15 2011 [mouseposture] //you'd need either very *low* surface tension (so that the pressure wasn't so high it popped the bubble) or very *high* surface tension (so that the bubble could contain high pressure without popping)//
... or very *ordinary* surface tension so internal pressure and surface tension exactly balance each other, as they must, by definition. Honestly, that's like saying boats must be very *small* (so they don't push down on the water so much that they sink), or very *big* (so that the buoyancy force is strong enough to counteract their enormous weight).
[xaviergisz] //The liquid could be encouraged to run down the needle by any force such as gravity, centrifugal, electrostatic etc.// I'm pretty sure that both the liquid and the gas would need to be pressurised, as they will both be at high pressure in the bubble. You may need something more like two coaxial needles.-- spidermother, Nov 15 2011 [spidermother] you sound like you know what you're talking about. <Goes off and works through the math. Returns:> OK, right, a spherical bubble, of some radius, will be stable for given values of surface tension and quantity of gas.
What prevents the bubble from popping if perturbed from perfect sphericity (inevitable, with real bubbles)? Does this require a source of wall tension other than surface tension?-- mouseposture, Nov 15 2011 Spherical is the most stable shape. The force perpendicular to the surface is proportional to the compound curvature at a given point, the upshot of which is that a sticky out bit is pulled back in, and a sticky in bit is pushed back out, restoring sphericality. The surface tension _is_ the required wall tension, in a sense.-- spidermother, Nov 15 2011 If these were truly microbubbles (ie, diameters on the order of microns), could they ever be made lighter than air? As pointed out, the internal pressure will be very high (inverse square or cube of radius kind of thing), which means that even hydrogen may give a gas density greater than that of air. Plus at small radii, the weight of the liquid wall becomes significant (especially if it has to be thick enough not to evaporate almost immediately.
I don't know the answer.-- MaxwellBuchanan, Nov 15 2011 Pressure is simply proportional to the inverse of radius; that still means the curve is hyperbolic, of course. The pressure doesn't get huge until the sphere is nanometer sized, though; for a 1 micrometre water drop, it's only 1.438 atm. above ambient pressure (Wikipedia). Of course, a soap bubble will be at lower pressure (because the surface tension is lower).-- spidermother, Nov 15 2011 Good link, [xaviergisz]. The only thing I find fault with is:
"The pressure in an infinitely small bubble will, in principle, be infinitely large. Therefore there are limit to how small a bubble that can be produced."
The lower limit to bubble size is due not to pressure, but to the molecular scale; for very small bubble sizes, and there are simply not enough molecules to form a stable bubble.-- spidermother, Nov 16 2011 //Pressure is simply proportional to the inverse of radius//
Ah - thanks. I knew it was inversely proportional to something.-- MaxwellBuchanan, Nov 16 2011 It's inversely proportional to budget, actually.-- mouseposture, Nov 16 2011 random, halfbakery