The physics of dancing peanuts in beer

Abstract

In Argentina, some people add peanuts to their beer. Once immersed, the peanuts initially sink part way down into the beer before bubbles nucleate and grow on the peanut surfaces and remain attached. The peanuts move up and down within the beer glass in many repeating cycles. In this work, we propose a physical description of this dancing peanuts spectacle. We break down the problem into component physical phenomena, providing empirical constraint of each: (i) heterogeneous bubble nucleation occurs on peanut surfaces and this is energetically preferential to nucleation on the beer glass surfaces; (ii) peanuts enshrouded in attached bubbles are positively buoyant in beer above a critical attached gas volume; (iii) at the beer top surface, bubbles detach and pop, facilitated by peanut rotations and rearrangements; (iv) peanuts containing fewer bubbles are then negatively buoyant in beer and sink; and (v) the process repeats so long as the beer remains sufficiently supersaturated in the gas phase for continued nucleation. We used laboratory experiments and calculations to support this description, including constraint of the densities and wetting properties of the beer-gas-peanut system. We draw analogies between this peanut dance cyclicity and industrial and natural processes of wide interest, ultimately concluding that this bar-side phenomenon can be a vehicle for understanding more complex, applied systems of general interest and utility.

Keywords: Stokes settling; bubbles; floatation; multiphase system; nucleation.

Figure 1.

A scheme for the dancing peanut cyclicity in beer. (a) Peanuts are introduced into the beer, sink part way and work as nucleation sites for bubbles; (b) bubble–peanut aggregates rise due to positive buoyancy; (c) bubbles are released by bursting at the free surface; (d) peanuts rotate on the free surface allowing further outgassing; (e) bubble–peanut aggregates become negatively buoyant and sink.

Figure 2.

Example of a measurement of contact angle Ψ between (a) peanut–beer immersed in air and (b) glass–beer immersed in air. The average values of 10 measurements are Ψ_peanut = 46.48 ± 2.43° and Ψ_glass = 23.20 ± 2.80°.

Figure 3.

The geometric parameter α as a function of the contact angle ψ. The dashed curve is given by equation (3.2). Shown here are the three values determined herein for bubbles forming in beer: in the bulk liquid (homogeneous), on clean glass surfaces (ψ_glass) and on roasted-shelled peanut surfaces (ψ_peanut); the latter being the larger ψ and therefore the lower value of $\alpha .$ Inset is a photograph of a peanut immersed in beer, showing the high relative contact angle ψ for nucleation.

Figure 4.

Histogram of bubble radii for experimental data obtained at different time windows: 2–13 min, 60–64 min and 120–126 min after the peanut introduction into the beer. For each window, five peanuts coated with bubbles were analysed. The theoretical critical bubble radius for stable attachment is displayed in the plot as a grey dashed line. The mean and s.d. of bubble radii just before detachment, obtained from image analysis of the electronic supplementary material, are displayed in red.

Figure 5.

Bubble radius as a function of the number of bubbles per peanut to make the assembly neutrally buoyant (grey lines). The experimental data obtained at different time windows: 2–13 min, 60–64 min and 120–126 min after the peanut introduction into the beer are displayed in as circles (mean), the s.d. as black bars and the whole dataset as little coloured circles.

Figure 6.

Quantitative observations of the peanut dance. The upper plot displays the peanut vertical position as a function of time, and the bottom one, the peanut velocity as a function of time.