We have studied the adsorption of 4 He on cesium in the temperature range from 1.3 to 4.1 K. The 4 He film undergoes a first-order phase transition known as prewetting. The prewetting line intersects the coexistence curve at the wetting temperature, 7^** 1.95 K, and terminates at 7^-2.5. K, the prewetting critical point. There are dramatic hysteresis effects when the temperature of the system is varied at coexistence in the vicinity of T w . This is the first experimental realization of a complete prewetting phase diagram.PACS numbers: 67.70.+n, 68.45.Gd On weak binding substrates, the interaction of an adatom with the surface can become comparable to the interaction of the adatoms with each other, and subtle free energy tradeoffs have large qualitative effects on the growth [1-5] and properties [6,7] of an adsorbed film. If, for example, a coexisting liquid and vapor are placed in a container at low temperature with sufficiently weakly binding walls, the walls will typically be covered with a fluid film that is only a few monolayers thick, i.e., they will be nonwet. As the temperature is raised, the energetic cost of forming a liquid-vapor interface (the liquidvapor surface tension) decreases, while the entropy of the film increases. Calculations [8][9][10] show that for many model systems the free energy of the film becomes equal to that of the bulk liquid at a characteristic temperature 7" w , the wetting temperature. For T >: 7 W , a macroscopically thick film wets the walls at coexistence. The phase transition between nonwet and wet walls can be either continuous or first order. If the transition is first order, with a discontinuous jump in the film thickness on the coexistence curve, thermodynamics requires that a phase boundary extend smoothly into the region of unsaturated films with n < /icoex-The phase boundary which separates regions of thick and thin films is known as the prewetting line, and is expected to terminate at a prewetting critical point. Although the prewetting line is a generic feature of a first-order wetting transition and its existence was first predicted 15 years ago [11], it has not been previously observed.Cheng, Cole, Saam, and Treiner (CCST) [1] have recently shown that the interaction of helium with substrates made of alkali metals is more than an order of magnitude weaker than with conventional substrates such as graphite. They also suggested that the phenomena of nonwetting and prewetting might be observable in these systems. Their calculations stimulated a number of experimental investigations [2-5] which have shown that adsorption on these substrates is highly anomalous. In this paper we present measurements of the complete wetting phase diagram for helium on cesium. We show that a first-order wetting transition occurs on the coexistence curve at 7^ = 1.95 K. Isotherms above T w show an abrupt step in the film thickness at pressures below the saturated vapor pressure characteristic of prewetting. Below T Wi there are large hysteresis effects and the film thickness of coexisten...

We have measured 4 He adsorption isotherms on a cesium substrate above and below 7\ using a quartz microbalance technique. The results show that the cesium surface remains dry until the pressure almost reaches the saturated vapor pressure. At liquid-vapor coexistence, however, the wetting film continuously thickens to more than 30 layers. Below 7\ the film at coexistence is superfluid. Above 7\ we resolve a feature in the isotherm just below saturated vapor pressure which is characteristic of a prewetting transition.

The breakup of a drop of inviscid fluid into two smaller drops is determined by a competition between surface and inertial forces. This process forms a thin filament of fluid with a connecting neck that shrinks to zero diameter at a finite time singularity. We present measurements of the electrical resistance of a liquid bridge of mercury as it undergoes pinch off. The electrical measurements allow us to probe the region of the singularity down to nanosecond times and nanometer lengths. Near pinch off, the resistance of the liquid bridge diverges as t(-2/3), as expected for inviscid flow.

Photographs are presented of isolated superfluid helium-4 droplets prepared on a cesium surface, the only material known that is not wetted by superfluid helium. Although thermodynamic measurements show that the cesium surface is highly uniform, the contact angle of the droplets is extremely hysteretic and depends on whether the contact line is advancing or receding. Superfluid helium-4 droplets on an inclined surface do not flow downhill but rather are strongly pinned to the surface.

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