structure of such materials varies from spherical, interconnected pores, to a bicontinuous type arrangement of silica, to an assembly of spherical particle aggregates loosely bonded together. We are currently investigating quantitatively the evaporation kinetics of both solvents alongside the temporal changes in structure during the process of formation of these porous solids. ExperimentalMaterials: Water was passed through a reverse osmosis unit and then a Milli-Q reagent water system. Toluene (Fisher, > 99.9 %), hexane (Sigma, > 99 %) and decane (Sigma > 99 %) were passed twice through a chromatographic alumina column before use. The fumed silica powders were from Wacker-Chemie (Munich), with primary particle diameters of between 10 and 30 nm, and surface areas of 200±250 m 2 g ±1 . The silanol content of the different particles is 100 % (N20), 76 % (SLM 079) and 50 % (H30), with the coating reagent being dichlorodimethylsilane.Methods: Dispersions of hydrophilic silica-in-water, or of hydrophobic silicain-oil, were prepared by dispersing a known mass of powder into the liquid using a high intensity ultrasonic vibracell processor (Sonics & Materials) of tip diameter 0.3 cm, operating at 20 kHz and up to 10 W for 2 min. In some experiments, both hydrophilic and hydrophobic particles were dispersed simultaneously in oil initially. Emulsions were made in glass vessels by mixing the appropriate particle dispersion with the second liquid phase using a Janke & Kunkel Ultra Turrax homogenizer (rotor-stator), with a 1.8 cm head operating at 13 500 rpm for 2 min. Their type was assessed using conductivity and droptest measurements. Drop-size distributions were determined using a Malvern MasterSizer MS20 particle sizer and checked with optical microscopy (Nikon Labophot). Emulsions of different particle concentrations, oil/water volume ratios, and oil and particle hydrophobicities were prepared. They were left in air at room temperature to allow evaporation of oil and water. Such systems dry first to a gel and subsequently to the solid phase. In many cases, a tablet-like material forms that retains the shape of the container. In other cases, small angular solid fragments result. After drying to constant weight, the solid samples were mounted on aluminum studs using epoxy resin, and gently scraped with paper to expose a fresh fracture surface. This was coated with a thin (20 nm) layer of carbon and examined using a Cambridge Instruments S360 scanning electron microscope. The details of the freeze fracture field emission SEM method of liquid emulsion samples were given previously [16].

By dc transport measurements, current-voltage characteristics of Tl 2 Ba 2 CaCu 2 O 8 thin films were determined in magnetic fields up to 5 T. For high magnetic fields (3, 5 T), it is found that all data can be interpreted in terms of the 2D vortex-glass theory: (1) All E-j isotherms can be scaled onto one common branch applying the scaling suggested for a 2D vortex system; (2) the linear resistivity r lin ͑T ͒ as well as the nonlinear current density j nl are found to follow the predicted relations r lin ͑T ͒~exp͓2͑T 0 ͞T͒ p ͔ and j nl ͑T ͒~T 11n2D , respectively. Both scaling of E-j curves and fitting j nl ͑T͒ consistently lead to value n 2D 2 as predicted by the 2D vortex-glass theory. Also, the values found for T 0 ͑200 6 30 K͒ and p ͑ഠ1.58͒ are either experimentally reasonable or within the limit expected by theory ͑p $ 1͒. [S0031-9007(98)05840-2] O 7 superlattices, it is expected that flux dynamics can be changed from 3D to 2D by increasing magnetic fields [1][2][3]. In the presence of a high density of weak pinning centers, for a 3D vortex system a second-order phase transition is predicted to occur at a finite temperature T g [4], with a new phase, the vortex glass (VG), existing below T g . The VG theory has triggered enormous experimental efforts [5][6][7][8], and most results show a reasonable agreement between theory and the experimental data. For 2D vortex systems, however, theory predicts that the VG transition will occur only at zero K. Experimentally, Dekker et al.[9] measured E-j curves on one-unit-cell thick YBa 2 Cu 2 O 7 thin films and found that the data can be fitted to the 2D VG theory. In a previous paper, Wen et al. [10] have shown that the m values for parametrizing the activation energy U͑ j, T ͒ U c ͑T͒ m ͓͑ j c ͑T͒ j ͒ m 2 1͔ are always negative for Tl 2 Ba 2 CaCu 2 O 8 , when the magnetic field is higher than about 0.7 T. Thus, for these higher fields the curvature of log͑E͒ vs log͑ j͒ is always positive, leading to a finite linear resistivity r lin E͞jj j!0 fi 0. The negative m values were attributed to a field induced vanishing of T g . In [10] emphasis is put on the resulting vortex phase diagram rather than on the 2D vortex dynamics. To the best of our knowledge, a direct experimental evidence for a genuine 2D VG is still lacking for highly anisotropic high-T c superconductors. In this Letter, we unambiguously show that all the data measured for Tl 2 Ba 2 CaCu 2 O 8 thin films at relatively high magnetic fields (3, 5 T) can be well described by 2D VG theory.Before presenting the experimental data, we briefly recall the major predictions for a 2D VG [2,11]. According to 2D VG theory, when approaching T g 0 K, a finite VG correlation length j 2D develops and divergeswith n 2D the 2D VG exponent and a 0 the spacing of the vortex lattice;´0d h 2 r s d͞m, the core energy of a vortex segment of length d, where r s and m are the density and mass of Cooper pairs, respectively. Dissipation in the vortex system is caused by vortex excitations. Assuming an excitation of size L, the energy b...

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