Wetting of superfluid 4He by liquid 3He

Owing to its high resolution, interferometry is a powerful tool for studying wetting problems at small contact angles. Using interferometry in 3He/4He mixtures, we have discovered that in an experimental cell containing a thin layer of the phase-separated mixture, the upper 3He-rich phase is not able to wet the surface of the 4He-rich liquid below in the presence of a small, continuous feed of 4He atoms to the vapor phase [PRL73, 1388 (1994)]. This finding is quite surprising since the surface of the 4He-rich phase is covered by an adsorbed layer of 3He atoms which should favor spreading of the 3He-rich liquid. 

FIGURE  Interferogram of phase separated 15% mixture film at T = 0.32 K. The stripe-like feature in the middle of the picture is identified as a floating pool of 3He-rich phase on top of the lower 4He-rich superfluid. The lower frame displays a reconstructed surface profile from left to right over the stripe; the angle formed between the rising 3He surface with respect to the flat 4He-rich phase level is marked. Both bright and dark fringes were employed in the analysis.

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