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Fluid dynamics

Impact on Everest

When a drop of liquid plummets onto a surface, the result is a splash — but not it seems if the process occurs at reduced atmospheric pressure. Here, perhaps, is a way to tune splash behaviour for practical ends.

A drop of liquid surrounded by air — is there anything left to discover in such a simple system, 200 years after Thomas Young and Pierre-Simon de Laplace laid the scientific foundations of capillary action? Writing in Physical Review Letters, Xu, Zhang and Nagel1 reveal that air, which has been viewed as a passive fluid in the story, plays an unexpectedly active role in creating the splash that occurs when the drop hits a solid surface.

The first drops of heavy rain hitting a pond or a puddle encapsulate the full complexity of liquid–liquid impact. Hemispherical ‘storm bubbles’ (‘frozen’ by the presence of surfactants that are always present in such open environments) are distinctive features produced by the ejection of thin sheets of liquid after the shock of impact. Pioneers of high-speed imaging, such as Harold Edgerton, confirmed that for any liquid or solid substrate, a corona forms upon a fast impact, which explodes into droplets to form what we usually call a splash2,3.

Xu and colleagues1 similarly use a high-speed camera (47,000 frames per second), in this case to view the impact of an ethanol drop hitting a dry glass surface at a velocity of about 3 m s−1 (Fig. 1). In ambient conditions, a thin sheet of liquid emerges around the impacting drop, and after 0.2 ms a corona forms, which precedes the usual splash. However, the behaviour is dramatically different in a reduced atmosphere: below 0.3 atm (a pressure still large enough to prevent the drop from evaporating), the thin film ejected after impact remains stuck to the glass. Neither corona nor splash occurs. Instead, the drop gently spreads — as if it had been placed, rather than dropped — on the glass, except the spreading is much quicker owing to the energy of the falling drop.

Figure 1: Impact zone.
figure1

As shown in these high-speed photographs taken by Xu et al.1, air pressure determines the outcome of the impact of an ethanol drop on dry glass. a, At ambient pressure there's a splash. b, In a rarefied atmosphere — 0.3 atm, equivalent to that found at the summit of Everest — a corona fails to form and there is no subsequent splash.

This is a spectacular finding because it unambiguously demonstrates the effect of air in the splashing story. And it might also have practical consequences. The pressure reduction required to suppress the splash is modest — the air is only as rarefied as that on top of Everest. Such a partial vacuum is easily generated, providing a simple and efficient way of suppressing the emission of droplets that limits the resolution in ink-jet printing, for example. Moreover, an understanding of the role of air might also help to enhance the generation of drops in situations where it is desired, as in combustion or the production of sprays.

Xu and colleagues1 have a surprising proposition to explain their results. As a liquid drop approaches a solid surface, the liquid can either displace or compress the air. Based on the rapid initial spreading of the liquid on the surface (the radius of the spreading disk increases as the square root of time), the authors suggest that the liquid front compresses the air. At a standard pressure, the gas resists compression, forcing the liquid edge to lift, after which this free liquid sheet cannot avoid breaking. The corresponding force (per unit area) is proportional to the gas density, speed of sound (resulting from air compression) and liquid velocity. Surface tension, which maintains the liquid cohesion, opposes this destabilizing force. Splashing occurs when the two forces are equal, and so is reduced as the gas density vanishes.

This subtle argument needs to be confirmed, because it assumes a speed on the order of the speed of sound, which is only true in the first fraction of a microsecond after impact (a much shorter timescale than observed experimentally). But it also allows us to understand why different gases behave differently; Xu et al. observed that splashing occurs more easily with a heavier gas (such as krypton or sulphur hexafluoride), showing an unexpected way to tune a splash.

The idea that air can be used to control the behaviour of a drop can be exploited for other purposes. A drop deposited on a pool of the same liquid will coalesce with that pool, but only after the film of air between the drop and the surface has disappeared. This typically takes a tenth of a second — a time hardly appreciable to the human eye. But if one finds a trick to renew this film, coalescence can be inhibited. Couder and colleagues4 have applied such a trick. Vibrating the pool at a sufficiently high frequency prevents coalescence ‘for ever’ (more than three days). The drop simply bounces on the surface of the pool, provided that the acceleration of the vibrating bath is at least the same as the acceleration due to gravity (9.8 m s−2). The impact of the liquid on the surface is much softer than in Xu and colleagues' experiment. This maintains the integrity of the floating drop, whose mobility is enhanced through the lubricating effect of the air, making it easy to manipulate. By ‘feeding’ the drop, Couder et al. can maintain floating globules of liquid several centimetres across, which persist long after the vibration has stopped, because of the slow ‘drainage’ of air at these large scales.

The two experiments1,4 demonstrate that air is not as passive as we thought, and that it can be used for tuning the behaviour of drops. Our understanding of such problems as air entrapment or jet impact5,6 should benefit from this amended vision.

References

  1. 1

    Xu, L., Zhang, W. W. & Nagel, S. R. Phys. Rev. Lett. 94, 184505 (2005).

  2. 2

    Thoroddsen, S. T. J. Fluid Mech. 451, 371–381 (2002).

  3. 3

    Thoroddsen, S. T. & Takehara, K. Phys. Fluids 12, 1265–1268 (2000).

  4. 4

    Couder, Y. et al. Phys. Rev. Lett. 94, 177801 (2005).

  5. 5

    Zhu, Y., Oguz, H. N. & Prosperetti, A. J. Fluid Mech. 404, 151–177 (2000).

  6. 6

    Eggers, J. Phys. Rev. Lett. 86, 4290–4293 (2001).

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