Out of equilibrium anomalous elastic response of a water nano-meniscus - Archive ouverte HAL
Article Dans Une Revue Applied Physics Letters Année : 2015

Out of equilibrium anomalous elastic response of a water nano-meniscus

Résumé

We report the observation of a transition in the dynamical properties of water nano-menicus which dramatically change when probed at different time scales. Using a AFM mode that we name Force Feedback Microscopy, we observe this change in the simultaneous measurements, at different frequencies, of the stiffness G'(N/m), the dissipative coefficient G"(kg/sec) together with the static force. At low frequency we observe a negative stiffness as expected for capillary forces. As the measuring time approaches the microsecond, the dynamic response exhibits a transition toward a very large positive stiffness. When evaporation and condensation gradually lose efficiency, the contact line progressively becomes immobile. This transition is essentially controlled by variations of Laplace pressure. Visco-elastic properties of water nanobridges[1] at very different time scales, have never been investigated despite ubiquitous presence of capillarity. Associated forces are among the most intense at nanoscales with important consequences in soils and granular media. Interest in dynamical properties is immediately raised if one considers interacting surfaces with roughness scales down to nanometer. Even at moderate speeds, such as v=1m/s, characteristic times of surface interaction down to mi-crosecond appear in these conditions. Our measurements approaching these time scales, further strengthen the relevance of the dynamical properties to describe how real surfaces interact and are certainly of crucial importance in numerous AFM experiments [2]. We here report measurements of dynamical properties of a water nanobridge for a continuous range of the surface gap and a frequency bandwidth up to 0.1 MHz. We identify two regimes: one is the thermodynamical equilibrium; the second is out of equilibrium. Evaporation and condensation of water molecules between the liquid and the gas phase ensures that the nano-meniscus curvature is the one at thermo-dynamical equilibrium(2H=-1/r k) where 2H is the water bridge curvature and r k is the Kelvin radius. At time short enough, molecule exchanges between the liquid and the gas phase are no longer efficient and the water nanobridge is led to acquire a constant volume. The liquid bridge relaxation time is the time needed for the bridge to adapt its shape as required by thermodynam-ical equilibrium, when its length h is abruptly changed by δh. This is controlled by molecular transport through diffusion mechanisms in gas phase. This relaxation time τ can be estimated as in SFA context, see Ref.[3]. τ = 2γρr 2 ln(R/ρ)/P sat r k 2 D (1)
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Dates et versions

hal-01876143 , version 1 (18-09-2018)

Identifiants

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Simon Carpentier, Mario Rodrigues, Miguel V Vitorino, Luca Costa, Elisabeth Charlaix, et al.. Out of equilibrium anomalous elastic response of a water nano-meniscus. Applied Physics Letters, 2015, 107 (20), pp.204101. ⟨10.1063/1.4935836⟩. ⟨hal-01876143⟩
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