We have studied the dynamics of phase separation in a binary liquid mixture contained between a pair of closely spaced quartz plates. The system behaves very difFerently from a bulk mixture undergoing spinodal decomposition. For example, the domains do not increase in size as a power of time, and in fact coarsening happens much faster than within a bulk mixture. This is because the process is dominated by the wetting of the plates by one of the two phases. One of the emerging phases, the one that wets the plates, forms an initially thick wetting layer on each quartz plate.Both layers are connected with a few isolated channels. As the time progresses, these layers thin, causing the channel radii to increase. We studied the growth of the channels in detail and found that there was a smallest radius r;", such that channels with radius r (r;"dissolved, while those with r & rm;n grew.PACS number(s): 64.60.@b When spinodal decomposition takes place in a metallic alloy, the domains, of characteristic size E, grow as E~ta, with a = 1/3 [1]. In fluid mixtures, on the other hand, domain growth is strongly influenced by flow, which arises as the domains seek to minimize the curva ture of the interfaces. As a result, a 1 in the late stage of phase separation [1,2]. This flow adds considerably to the complexity of the problem, since one must now deal

Dedicated to K. Dransfeld on the occasion of his 60th birthdayWe have studied the growth of atomically rough bcc and hcp4He crystals from the superfluid phase for temperatures T > 0.9 K. The growth coefficient displays a temperature dependence which can be represented by m4KaeAElkflT. The parameter AE is found to be in close agreement with the energy gap of rotons, suggesting that theqe thermal excitations dominate the growth kinetics. Besides, the absolute value of the growth coefficient depends on crystal orientation, with an anisotropy for the hcp phase of about a factor of 2.5 between the (10iO) and (0001) planes.

PACS. 67.M -Mixed systems; liquid helium 3 4 m i x t w a .PACS. 64.70 -Phase equilibria, p* transition%, and critical points.PACS. 64.75 -Solubility, xcpgmhon, and mixing Xbatraet. -We have investl~ated the rate of homogenroua nucleation in 11quid aHe.4He mlxturea ns a funct~on of the sped at which the system 16 quenched Into the mjscibillty flp below Ihc tncritical point. Rmsing the quench speed drldt (the denvativ? of the reduced temperature r wth respecl to t~me) leads to a pronounced increase of the numkr dens~ty N of thc droplets uhirh develop durmg the quench. ' r h~ data sugwr;t a power law dependenceThe phaqe separation of binary mixtuws IS a phenomenon which, due to its impor& ance for material properties and technical applications, has been investigatpd for many decades [IJ. Yet t h~ dynamics of the phase separation process i8 still incompletely understood [21. In the past the aim of most experiments was to locate the cloud Bne, 1.a. the border of metahtabilig in the miscibility gap of a supersaturated liquid mixture, where nucleation sets in on a laborably t~m e scale. Experimental information about details of the nucleation p m e s s itself, however, is rather sparse. This holds in particular for the dependence of the nucleation rate on the supersaturation of the &turn, which we address in this letter.One possible method to investigate thk problem has been u~e d by Siebert and Knw bler 1.31: the system is quenched 4nstantaneously~ to a certain point in the miscibility gap, where it is kept for a &en amount of time, and the droplets that emerge dudng that period sre counted &envards. Similar experiments were carried out by Kampmann and Wagner [dl, who invest~paterl the early stages o f decomposition of solid alloys by small-angle neutron scattering (SANS).An alternative method, which also appears to be of practical importance, is to vary the speed at which t h~ system is quenched into the miscibility gap, and to measure the density o f the n u c l~a k i n~ rlroplets as a function of the quench speed. To our knowlpdge this technique i s applipd here to simple liquids for the first time.Our systpms are liqrjid 'He-'He mixtures, wliich dread? have been the subject of nucleation experiment-. F, ti]. B~lrt\v the tricrilicai point (temperature T, = 0.867 K and 'He concentratton X, = 0.G71 ut saturated vapaur pressure) these mixtures exhibit a miscibility gap.is ~hifted to lower t~rnper,~tures as the PI-es-pswiee is raised 171. This behaviour i s

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i d e~e r , F a c h b e r e~c h Ph ysi k . Johannes G u t e n b e r g -U n i v e r s r t s t . 6500 Marnz, W e s t G e r m a n y and D. Savignac. T e c h n~s c h e U n i v e r s i t a t Mijnchen. 8046 G a r c h i n g , W e s t G e r m a n y The I n t e r f a c e b e t w e e n 5 3 1 16 and s u p e r f l u~d '~e has revealed unusual dynarnkc properties. C h z r a c t e r i s t~c o f t h i s quantum systen: a r e t h e v e r y hlgh thermal c o n d u c l r v~t y of t h e sclperflurd phase. and an e x t r e m e l y s r n a l l h e a t o f fusron below t e m p e r a t u r e s of about 1 K. A s a r e s u t t , e q u~l~b r i u m a t t h e s o l i d -s u p e r f l u i d 4~e interface is establtshed qurte rapidly, w h l c h glves r i s e , e.g., t o melting -c r y s t a ll l z a t i o n waves r 1 . 2 J -s r m i l a r i n appearance t o s u r f a c e waves on a f r e e l~q u i d s u r f a c e -and t o anomalous transmrssron of sound [ 3 J .When a p e r t u r b a t~o n IS a c t i n g on t h e sotld-llqurd i n t e r f a c e . t h e response o f t h e s y s t e m is governed b y t w o coupled e q u a t~o n s which d e s c r~b e m a t e r l a l and t h e r m a l transport. The k i n e t i c c o e f f i c i e n t s c h a r a c t e r i z i n g these processes have been calculated f r o m the d e n s i t y o f t h e r m a l l y e x c l t e d phonons and rotons b y SOWLEY and E D W A R D S 141 . E x p e r r r n e n t a l l y the relevant p a r a m e t e r f o r t h e r r j~t i o n o f the i n t e r f a c e , t h e k l n e t r c g r o w t h c o e f f r c~e n t K , has been derived f r o m t h e damping o f m e l t~n g -f r e e z t n g ~a v e s by KESHJSHEV e t al. f o r T < 0.6 K [ 2 1 .A t hrgher t e m p e r a t u r e s t h e r e l a x a t i o n t!rnes f o r c r y s t a l g r o w t h were t o o long t o be measured w i t h this technique. H e r e w e descrlbe a method w h~c h I S a p p l i c a b l e atso in t h e range o f e x t r e m e l y long r e l a x a t r o n t i m e s . so t h a t t e m p e r a t u r e s above 1 K , where t h e density o f t h e r m a l e x c~t a t r o n s rs h~y h . become accessible, and I n a d d i t i o n t o t h e h c p phase of sol l d Me also t h e b c c phase can be studied. As a b y -p r o d u c t , also t h e rnterfac~al tensban of t h e s o l t d -i~q u l d boundary is obtarned.O u r method 1s based on m e l t i n g a tnirr l a y e r of t h e hellurn c r y s t a l b y the a p p i~c a t l o n o f a small e l e c t r o s t a t i c pressure, and then r e c o r d i n g t h e relaxatron of the i n t e r f a c e back t o i t s o r l g i n a l e q u i l i b r r u m posltion as t h e pressure IS r e m o v e d . F o r t h~s purpcse, t h e ~n t e r f a c e 1s charged w~t h negative ions, w h~c h are generated I n t h e s u p e r f l u~d b y a f~e l d emlssron t t p and d r a w n t o w a r d t h e s o l i d b y an e l e c t r i c f~e l d o f t h e o r d e r of 1 k V / c m . T h e ground s t a t e e n e r g y o f t h e ions i n t h e c r y s t a l IS about 18 rneV hlghes t h a n rn t h e llquld [ S ] , t h ...