Speaker
Description
A liquid can sustain tensile stress due to intermolecular attractions, but only up to a critical value beyond which it breaks through the spontaneous formation of a vapor bubble. This process, known as cavitation, is observed for instance in the wake of ship propellers or during sap ascent in trees. Cavitation also occurs during the drying of porous materials, when liquid-filled cavities are connected to an external gas reservoir through narrow constrictions. In this so-called ink-bottle geometry, the liquid inside the cavity is driven into a deeply metastable state by lowering the vapor pressure in the reservoir. In this work, we use independent ink-bottle pores to study cavitation in a controlled and quasi-static manner.
Previous results have shown that Classical Nucleation Theory (CNT) [1–2] accurately describes cavitation in fluids such as nitrogen, provided that surface tension is corrected for nanometric bubbles and that the critical bubble remains small [3] compared to the pore size. In contrast, cavitation in helium is still debated at low
temperature, in the superfluid phase where quantized vortices may act as preferential nucleation sites: all previous experiments which have relied on focused ultrasonic waves to drive the liquid in a metastable state leads to inconsistent values for the cavitation pressure threshold.
To investigate cavitation in the bulk limit for this fluid, we use two model mesoporous systems. The first consists of porous alumina membranes fabricated by anodization of aluminum disks[1]. The second is based on newly designed porous silicon structures produced using nanolithography techniques. The latter system allows for finer control of the ink-bottle geometric parameters, such as the cavity radius, the constriction radius, and the constriction thickness. In both cases, cavitation evaporation can be reach only by reducing the pore apertures down to a few nanometers. This is obtained by atomic layer deposition (ALD).
The samples are subjected to condensation–evaporation cycles using helium at various temperatures while the state of the confined fluid is monitored using a capacitive measurement technique. We present the first helium measurements of the pressure dependence of the cavitation energy barrier and discuss the observed deviations from the predictions of classical nucleation theory (CNT).
| References | [1] Doebele et al, Direct observation of homogeneous cavitation in nanopores, PRL 2020 [2] Bossert et al, Evaporation process in porous silicon: cavitation vs pore-blocking, Langmuir 2021 [3] Bossert et al, Surface tension of cavitation bubbles, PNAS 2023 [4] Djadaojee et al, Brillouin Spectroscopy of Metastable Superfluid Helium-4, PRL 2022 |
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| Country | France |
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