Dewetting dynamics of helium films on cesiated surfaces
Dietmar Reinelt, Andreas Engel, Jiirgen Klier*
Fakullal fiir Physik Unioersilal Konsranz, 784.57 Konslanz, Germany
Abstract
The nonwetting of liquid 'He on Cs below
=
2 K is uow well established. However, therc exist discrepancies between the measured contact angles and hysteresis at the phase transition of various groups. These are attributed to the ii~fluence of the underlying C s surface. Little is known, however, about the morphology of this suruace. There is also a lack o l information conccrning the dewetting dynamics of metastable 4He films and the dewetting process of 4He drops on this weak binding surface Cs. We present results showing the influence o l surface properties on the dewetting behaviour of these metastable films and discuss the scenario o l dewetting of a macroscopic helium drop.0
2000 Elsevier Science B.V.All rights reserved.
Keywork: Cesium: "He film; Surface ~norphology; Wctting
Liquid 4He is extremely weakly bound to Cs and hence shows nonwetting below T, % 2 K [I] (see Ref. [2] Tor the first experimental evidence for nonwetting of 'He on Cs). At T<< T , only a microscopically thin 4He film exists on Cs forming a two-dimensional gas [3]. When cooling a Cs surface below T,, which is initially covered with a thick 4He film and in contact with a 'He reservoir, then in most cases this film does not dewet, but rather a meta- stable thick film remains. However, most experiments show strikingly different hysteresis in film thickness and contact angle [4-61. This hysteretic wettillg behaviour is attributed to the surlace properties of the underlying substrate, namely roughness and chemical impurities.
We have studied the dynamics of dewetting 4He films on several Cs substrates imaged by a surface plasinon microscope [4]. O u r C s substrates are 1 5 M L thick films evaporated at low T onto thin A g films. We ana- lysed the profile oTa contact line separating a metastable 4He film on Cs from a dry Cs area. This gives information about the stability of the film and its depcnder~ce on the surface morphology. We also studied the dynamics of isolated superfluid *He drops on an otherwise dry C s surface. To characterize the Cs surface due to chemical
* Corresponding author.
E-~noil nddress: juergen.klicr@uni-konstanz.dc (5. Klier)
irnpur~ties, we have done local measurements o l t h e work [unction, @(I.), i.e. measured the current of the photo- electroi~s from our Cs surface as a lunction of the cut-OK light wavelength. A detailed description of the setup is given in Re[. [7].
Fig. 1 shows the dewetting of an isolated 4He d r o p on Cs a t T < T,. The Cs surface is in thermodynamical equilibrium, i.e. there is no thick inetastable film on the surface. The dewetting is only possible via the gas phase and the thin nonwetting film surrounding it. IC this film is in its 2D-gas phase ihen mass transport is negligible and the main mechanism has to be evaporation from the drop. However, if the thin film is in a 2D-liquid state [8]
transport through it could play a role in the dynamics of the dcwetting process. Due to our limiting resolution for the thin film we must await lurther work to answer these questions.
In close vicinlty to this drop is a contact line, see Fig.
l a , separating the nonwet Cs from a Cs area which is permanently covered with a thick superfluid 4He film alter first deposition of a hclium d r o p on this area. When Curther drops are deposited on this part of [he Cs the dewetting process is much fasier than on the dry Cs part.
The dewetting time is always < 3 s, although these de- posited drops are much bigger in size than the isolated ones. The transport of helium out of these big drops is dominated through the already existing superfluid meta- stable film which is in contact to thc surrounding wetted 0921-4526/00/$-sec front matter
a
2000 Elsevier Scicnce B.V. All rights reservedPI[: SO92 1 - 4 5 2 6 ( 9 9 ) 0 2 2 0 7 - 3
First publ. in: Physica / B [Condensed Matter], Vol. 284-288 (2000), Part 1, pp. 149-151
Konstanzer Online-Publikations-System (KOPS) URL: http://www.ub.uni-konstanz.de/kops/volltexte/2007/2851/
URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-28517
I SO D. Reinell e/ ul / Physico B 2844288 (2000) 149- 151
Fig. I. Dewetting or 4He drops deposited on a horimn~al Cs suritlce (4 x 4 mm2) at 7' = 1.38 K, shown via a surface plasmon n>icroscope. The upper part of each picture (c-e) shows the dewetting of a n isolated drop on the dry Cs-part, deposited a1 r = 0 s rvich a dewetting lime or = S. I s. During this time several big drops were deposited on the lower part of the Cs (c and e), which is always covered with a metastable 'He film and separated from t h e dry part via a sharp fixed contact line. Picture I is identical to b, i.e. no chick
'1-le film is leCt on the upper part. The grey area indicates a thick metascable film, the black area the dry Cs. (b) L =
-
0.3 s: (c) L = 1.3 s;(d) t = 4.8 s; (e) L = 8.0 s; (f) I = l l . 8 s.
Ag area (not shown). The contact linc to the dry C s is fixed although during deposilion o l [he blg drops bulk material moves up to this line. The fraying of this line indicates a rough Cs surface with many pinning centres.
To check on the influence of chemical impurities on the wetting properties, we have scanned our Cs surface and measured the currenl of the photoelectrons (every 0.5 mm over an area of x 0.2 mm2). F o r pure Cs @ = 1.8 eV [9]. O u r Cs surface showed variations in @(r) between 1.7 and 1.9 eV over the whole area. However, there was no correlation between the observed wetting propertics and q r ) . So chemical impurities can be excluded as the cea- son of the metastable film and the sharp contact line.
A possible explanation is that the wetted parts are very rough which enhances wetting.
In conclusion w e observed high stability ofmetastablc 4 ~ e films on somc Cs surfaces. Other Cs surfaces, how- ever, show sponlaneous disappearance of this film below T , which influences the dynamics of dewetting helium drops. Our results show that the dynamics of 'He films on cesiated surfaces are strongly dependent on the film
quality. As a consequence these eHects should also have an influence on the interface rree energies oC this system, important for wetting parameters like the contact angle [lo]. Characterization o f the Cs surface is essential for further wetting studles on this otherwise ideal system. In addition, measuring the film thickness or the equilibrium 'thin film' seems essential to understand the wetting properties.
We thank P. Leiderer for uselul discussions and the DFG-Schwerpunkt 'Benckung' Tor support.
References
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