DOI 10.1140/epje/i2007-10333-x Eur. Phys. J. E 26, 345–353 (2008) T HE EUROPEAN P HYSICAL JOURNAL E Near-critical fluid boiling: Overheating and wetting films J. Hegseth 1, a , A. Oprisan 2 , Y. Garrabos 3 , C. Lecoutre-Chabot 3 , V.S. Nikolayev 4 , and D. Beysens 4 1 Department of Physics, University of New Orleans, New Orleans, LA 70148, USA 2 Physics & Astronomy, College of Charleston, Charleston, SC 29424, USA 3 ESEME, Institut de Chimie de la Mati` ere Condens´ ee de Bordeaux, CNRS, Universit´ e de Bordeaux I, Avenue du Dr. Schweitzer, F-33608 Pessac Cedex, France 4 Laboratoire de Physique et M´ ecanique des Milieux H´ et´ erog` enes, ´ Ecole Sup´ erieure de Physique et de Chimie Industrielles de la ville de Paris, 10, rue Vauquelin, 75231 Paris Cedex 05, France Received 3 September 2007 and Received in final form 16 January 2008 Published online: 26 June 2008 – c  EDP Sciences / Societ`a Italiana di Fisica / Springer-Verlag 2008 Abstract. The heating of coexisting gas and liquid phases of pure fluid through its critical point makes the fluid extremely compressible, expandable, slows the diffusive transport, and decreases the contact angle to zero (perfect wetting by the liquid phase). We have performed experiments on near-critical fluids in a variable volume cell in the weightlessness of an orbiting space vehicle, to suppress buoyancy-driven flows and gravitational constraints on the liquid-gas interface. The high compressibility, high thermal expansion, and low thermal diffusivity lead to a pronounced adiabatic heating called the piston effect. We have directly visualized the near-critical fluid’s boundary layer response to a volume quench when the external temperature is held constant. We have found that when the system’s temperature T is increased at a constant rate past the critical temperature T c, the interior of the fluid gains a higher temperature than the hot wall (overheating). This extends previous results in temperature quenching experiments in a similarly prepared system when the gas is clearly isolated from the wall. Large elliptical wetting film distortions are also seen during these ramps. By ray tracing through the elliptically shaped wetting film, we find very thick wetting film on the walls. This wetting film is at least one order of magnitude thicker than films that form in the Earth’s gravity. The thick wetting film isolates the gas bubble from the wall allowing gas overheating to occur due to the difference in the piston effect response between gas and liquid. Remarkably, this overheating continues and actually increases when the fluid is ramped into the single-phase supercritical phase. PACS. 05.70.Np Interface and surface thermodynamics – 44.35.+c Heat flow in multiphase systems – 68.03.-g Gas-liquid and vacuum-liquid interfaces 1 Introduction 1.1 Critical fluids Near the liquid-gas critical point, material and thermal properties, which play an important role in the boiling process such as surface tension, liquid-gas density differ- ence, and thermal diffusivity, vary considerably with tem- perature [1]. These properties vary according to the well- known universal power laws that either converge or di- verge as the critical temperature T c is approached (e.g., the surface tension goes to zero while the compressibil- ity and thermal expansion diverge). These properties lead to perfect wetting by the liquid phase (zero contact an- gle) near T c [2]. When heat is applied to a wall of a con- stant volume system, the low diffusivity (long relaxation a Current address: Department of Radiation Oncology, Uni- versity of Louisville, 529 S. Jackson Street, Louisville, KY 40202, USA; e-mail: jjhegseth@yahoo.com time) of a near-critical fluid keeps the heat near the wall to form a long-lasting hot boundary layer (HBL). The high thermal expansion makes this HBL into a “piston” that compresses the bulk. The high compressibility of the bulk implies an almost instantaneous increase in the tem- perature of the bulk. In a two-phase system a surprising response called overheating has been observed [3]. A gas bubble isolated from the heating wall by liquid responds to a temperature quench with the gas temperature becom- ing larger than the liquid temperature and the heating surface temperature. This is a transient effect that soon equilibrates so as to not violate the second law. 1.2 Previous results We previously observed a boiling process where heat was applied to a fluid near the critical point, pushing the system slightly out of equilibrium [4]. The perfectly wet- ted walls dried from evaporation resulting in spreading