Apparatus

Our apparatus creates the vertical flowing soap film between two fishing lines as described here.

There are a number of interesting hydrodynamic instabilities that can be visualized with the bubble film:

• A shock wave or hydraulic jump occurs when the fluid flow speed exceeds the wave speed of the turface-tension waves.
• The rate of divergence and convergence of the upper and lower portions of the sheet can cause interesting effects, similar to adverse pressure gradient effects in nozzles.
• At low flow rates, depending on the nozzle shape, one can induce a drip mode; most of our photos are of the thin, colorful, turbulence that occurs under these circumstances.

We found it helpful to use a well-crafted conical nozzle, and a slowly divergent top section (approx. 1cm width per 5cm drop), to yield more stable laminar flow conditions. Also, we found that thicker guide wires (22lb-test monofilament) is better than thinner wires. The thinner wires induce a Rayleigh-Plateau-like instability.

As mentioned in the literature, the flow is strongly damped by air friction.

Measurement techniques

Film thickness

This paper by Greffier, Amarouche, and Kellay, from 2002, describes the use of a Mach-Zehnder interferometer to measure the fluctuations in film thickness. Our local tech. suggested an easier approach might be based on a two-color interferometry from photographs.

We attempted this, but the homogeneous turbulence produced in the wake of a comb seems to require higher flow rate, and thicker films, than the drip-induced turbulence. The thicker films do not produce bright colorful reflections, so photographing the color fluctuations seems to be not possible with the soap solutions we currently use (based on blue Dawn dish soap).

Particle shadow velocimetry

Particle shadow velocimetry is described in this paper.

The central idea of most particle velocimetry techniques is to image the light scattered by passive particles embedded in the flow. Estevadeoral and Goss suggest a shadow-based technique in the above paper, which requires much lower light source intensity.

Currently we are working to determine the feasibility of this approach using a digital SLR, a Nikon D200, and a good macro lens. The key issues seem to be:

• focussing on the bubble film; the film is essentially invisible through the viewfinder;
• optimizing the exposure to maximize the contrast between direct illumination and shadows;
• optimizing and controlling the light source; and
• properly seeding the flow with particles which can be resolved.

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