A soap bubble filled with helium floats in air without rising or falling. For la
ID: 2077628 • Letter: A
Question
A soap bubble filled with helium floats in air without rising or falling. For large bubbles, find the ratio of the mass of helium to the mass of the soap solution. [You may assume, with justification, that the bubble walls are very thin.] Next, by considering the surface tension of the bubble wall, find the radius of a neutrally-buoyant bubble for which the mass of helium equals the mass of soap solution. [The mean molecular mass of air is 29; the surface tension of soap solution is 0.03 N m^-1]Explanation / Answer
A soap bubble has two surface, one the inner surface and the other outer surface. Thus there is extra work done increasing
the two surface as compared to liquid drop. Excess pressure of soap bubble is found to be two times the pressure inside the
liquid drop.
Thus P = 4S / R
helium-filled soap bubbles (HFSB) for low-speed aerodynamics is studied. The main interest of using HFSB in relation to micron-size droplets is the large amount of scattered light, enabling larger-scale three-dimensional experiments by tomographic PIV. The assessment of aerodynamic behaviour closely follows the method proposed in the early work of Kerho and Bragg (Exp Fluids 50:929-948, 1994) who evaluated the tracer trajectories around the stagnation region at the leading edge of an airfoil.
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