Faraday Instability in a Surface-Frozen Liquid

Abstract

Faraday surface instability measurements of the critical acceleration, ac, and wavenumber, kc, for standing surface waves on a tetracosanol (C24H50) melt exhibit abrupt changes at Ts = 54 • C, ∼4 • C above the bulk freezing temperature. The measured variations of ac and kc vs. temperature and driving frequency are accounted for quantitatively by a hydrodynamic model, revealing a change from a free-slip surface flow, generic for a free liquid surface (T > Ts), to a surface-pinned, no-slip flow, characteristic of a flow near a wetted solid wall (T < Ts). The change at Ts is traced to the onset of surface freezing, where the steep velocity gradient in the surface-pinned flow significantly increases the viscous dissipation near the surface.PACS numbers: 47.20.Ma, 61.25.Em, 64.70.Dv Spatial confinement of a liquid often changes its properties markedly. For example, superheating above the equilibrium melting temperature [1] and order quenching upon freezing [2] were observed under confinement only. In particular, flow under confinement is important for processes ranging from tribology to protein folding to transport through ion channels in cell membranes [3]. A transition from a liquid-like to a granular-solid-like shear response was observed at nano-scale confinements [4]. The surface freezing (SF) effect [5], where a solid monolayer forms at the surface of a pure normal-alkane (C n H 2n+2 ) melt, provides a unique system for studying semi-confined flow at a solid-liquid interface. The abrupt onset of SF at T s allows one to switch on (and off) the solid phase by a small temperature variation. Understanding such interfaces would also elucidate the role of flow in nucleation and growth processes of crystals from melts, which are dominated by such interfaces [6].SF occurs in melts of several chain molecules (alkenes, alcohols, semi-fluorinated alkanes, diols, C i E j ) [7], and at both liquid/liquid [8] and liquid/solid [9] interfaces. Related surface ordering effects were observed in melts of polymers comprising alkyl chains in the backbone or as side chains [10], in liquid alloys [11], and in several liquid crystals [12]. While the structural and thermodynamic aspects of SF have been studied in great detail [13], the influence of surface ordering on macroscopic near-surface flows has received little attention to date in this sizable, technologically-important class of materials.To study this issue, we employ the Faraday instability, which forms standing wave patterns (SWP) at the free surface of a vertically-vibrated liquid [14]. By virtue of its simplicity, this instability is outstanding among pattern-forming systems and a detailed theoretical description has been achieved. The SWP formed depend sensitively on, and allow a detailed study of, the changes in the surface hydrodynamics upon SF. As this study of SF demonstrates, the instability can therefore be employed to explore physical processes which are difficult to access by other, classical means. It also exemplifies the more general class of parametric inst...

The Faraday instability in wormlike micelle solutions

“…Only recently have vertically vibrated complex fluids been the subject of experimental [10][11][12][13][14] and theoretical research [15,16]. It was shown that the coupling between the microstructure of complex fluids and the Faraday instability either only slightly modifies the instability (e.g.…”

“…by affecting the critical acceleration [10,11] or by delaying the transition to disordered states [14]) or presents non-intuitive behaviours such as stabilized "persistent holes" in shear-thickening fluids [12]. With the notable exception of concentrated colloidal and granular suspensions [12,17], most of these works were performed on fluids whose viscoelasticity is only a small perturbation [10,13,14] to Newtonian behaviour. The case of a complex fluid with significant viscoelasticity was addressed in Ref.…”

Onset of Faraday Waves in a Liquid Layer Covered with a Surfactant with Elastic and Viscous Properties

“…It is interesting to note that the inextensible behavior of the free surface when Ma . 1 has been exploited by Huber et al 22 to analyze the formation of Faraday waves on the free surface of normal alkane under surface freezing conditions.…”

Phase Transition effect on the Parametric Instability of Liquid Crystals

“…We note from Figure 5 that (a c , k c ) display the same discontinuous behavior as the viscous and elastic parameters do across the thermal phase transition temperature. However, a different mechanism for discontinuity of these critical parameters was measured experimentally by Huber et al [57] during the surface freezing of a polymer monolayer made of tretraconazole melt. They found that a decrease of temperature leads to the formation of a monolayer at the interface of polymer-air that changed the surface tension but without an observable quantitative change of bulk viscosity.…”

“…A comprehensive experimental determination of the bulk Leslie viscosities of the most known liquid crystals in an ample interval of temperatures near their critical phase transition is scarce [53]. Their values can lead to the prediction of enhanced macroscopic hydrodynamic response on the parametric surface dynamics of liquid crystals [39,40], or polymer melts [57]. A similar experimental program may be set up for studying lyotropic liquid crystals to determine the viscosities and elastic parameters as a function of temperature.…”