Acoustic Crystallization and Heterogeneous Nucleation

Abstract

By focusing a high intensity acoustic wave in liquid helium, we have observed the nucleation of solid helium inside the wave above a certain threshold in amplitude. The nucleation is a stochastic phenomenon. Its probability increases continuously from 0 to 1 in a narrow pressure interval around Pm + 4.7 bars (Pm = 25.3 bars is the melting pressure where liquid and solid helium are in equilibrium). This overpressure is larger by two to three orders of magnitude than what had been previously observed. Our result strongly supports the recent suggestion by Balibar, Mizusaki and Sasaki [1] that, in all previous experiments, solid helium nucleated on impurities.67. 80.-s, 64.60.-i, 43.35.+d To our knowledge, no one has ever observed that a high intensity sound wave travelling in a liquid can crystallize this liquid. Several experiments have shown that the negative pressure swings of the wave can produce cavitation, in other words nucleate bubbles or trigger the liquid-gas transition [2]. By focusing an acoustic wave in liquid helium, we have found that the positive swings can also trigger the liquid-solid transition. It occurs if the wave amplitude reaches a certain threshold which we have measured. We could also estimate the size of the crystallites formed in the wave (15 µm) and their typical growth velocity (100 ms −1 , close to the speed of sound). We believe that it is the ability of helium crystals to grow at very high speed which allowed us to make this observation. The observed threshold corresponds to a pressure 4.7 bar higher than the equilibrium pressure. This overpressure is two to three orders of magnitude larger than what had been observed in all previous experiments [3][4][5][6][7]. Our result strongly supports the recent suggestion by Balibar, Mizusaki and Sasaki [1] that, in all these previous experiments, solid helium nucleated on favourable impurities.Let us first describe our experimental method. Inside a cryostat with optical access, we have focused an acoustic wave through liquid helium 4. The acoustic focus is located on a clean glass plate which allows us to measure the local instantaneous density of liquid helium [8] (see Fig.1). The acoustic wave is emitted by a hemispherical piezoelectric transducer similar to the one we used for the study of cavitation [2]. It resonates in a thickness mode at 1.019 MHz and we pulse it with bursts of six oscillations, consequently about 6 µs wide. The focusing is achieved by the transducer geometry. Given the characteristics of our RF amplifier, we can reach a maximum acoustic power of 5 kW.cm −2 (200 dB) at the focus. The transducer is gently pressed against the glass plate. At the melting pressure, the sound velocity is 366.3 ms −1 in the liquid phase [9] , so that the acoustic wavelength is 360 µm. A lens of focal length 2 cm is located inside the experimental cell and focuses an Ar + laser beam onto the acoustic focal region. The waist of the laser beam at the focus is 30 µm, less than one tenth of the acoustic wavelength. The light spot position is c...

Acoustics and metastable liquids

“…Could it be sufficiently strong to crystallize the liquid in the path of the wave? The answer is yes, and we observed this phenomenon for the first time in 2001 [5], The physics is similar but the nucleation of crystals is usually favored by walls. In all practical situations, crys tals nucleate on walls or have not enough time to grow during one sound period, and that's probably why acoustic crystallization had not yet been observed.…”

“…A first important step was the recent observation of crystalliza tion by acoustic waves on a clean glass plate [5]. This plate allowed a local an d in stantaneous m easurem ent o f the density at the acoustic focus, from the reflectivity of light at the glass/helium interface ( Figure 2).…”

“…Why is it so interesting to do this? It happens that, even at the beginning of the 21st century [5] shows a high amplitude wave propagating in liquid helium 4 (blue curve). With 11 Volt excitation, a pressure oscillation of ± 4.3 bar is obtained at the acoustic focus, which corresponds to a 2% oscillation in density around the static density in the cell (0,173 g/cm3).…”

“…For our first experiments with M.S. Pettersen in 1994 [3], we used rather long bursts but we now use short ones [4,5], typically with 1 to 6 oscillations only. As a consequence of the geometry, the sound waves converge at the center, far from any wall (see Fig.…”

“…The discoveries of the integer and fractional quantum Hall effect are key milestones in condensed matter sciences [4,5]. The search for the elusive electron-crystal state in modulation-doped quantum structures led to the unexpected discovery of fraction al quantization of the Hall effect of the two-dimensional electron gas in a perpendicular magnetic field [5], The transport signa tures of the fractional quantum Hall effects are seen as macroscopic manifestations of a quantum fluid in which the highly correlated electron liquid is represented by a quantum state with a single many-particle wave function [6].…”