SCHLIEREN PHOTOGRAPHY IN LIQUID 4He

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SCHLIEREN PHOTOGRAPHY IN LIQUID 4He

A. Goulyaev

To cite this version:

A. Goulyaev. SCHLIEREN PHOTOGRAPHY IN LIQUID 4He. Journal de Physique Colloques, 1970, 31 (C3), pp.C3-105-C3-107. �10.1051/jphyscol:1970309�. �jpa-00213852�

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JOURNAL DE PHYSIQUE Colloque C 3, supplément au n° 10, Tome 31, Octobre 1970, page C 3 - 105

SCHLIEREN PHOTOGRAPHY IN LIQUID

⁴

He

A. I. GOULYAEV

Institute for Physical Problems, Moscow

Résumé. — La méthode optique de Foucault-Toepler a été employée pour observer visuelle- ment des certains phénomènes dans l'hélium liquide aux températures de 1,3-2,3° K, comme par exemple, l'existence d'une densité maximum dans l'He I a proximité de TX, la formation des films liquides « superfluides » sur la surface de corps chauffant dans l'He II, la propagation des ondes sonores et des ondes du « second son » dans l'He II, la transformation mutuelle de ces ondes sur la surface ouverte du liquide.

Abstract. — The Foucault-Toepler optical method has been applied for visual observation of certain phenomena in liquid helium at temperatures 1.3-2.3° K, such as the existence of a maximum density of the liquid near TX in He I, the formation of « superfluid » liquid films on the heated body surface in He II, the propagation of sound and « second sound » waves in He II and mutual transformation of these waves at the free interface of the liquid.

The schlieren method in combination with a short light flash permits to visualize instantaneous fields of refractive index gradients in a transparent medium and, thus, to observe two-dimensional pictures of unsteady disturbances connected with density gradients.

In Hell, owing to the high sensitivity of this method, one can see not only sound waves, but waves of the second sound too, obtaining pictures of their propagation and interaction with boundary surfaces. Wave packets which have been excited in the liquid as a result of a pulsed rise of a heater's temperature and which propagate without signs of dispersion and non- linear distortion, carry information concerning proces- ses in liquid He near the heated body surface. Besides that the schlieren method provides also independent and visual data about some other phenomena in liquid He such as the existence of a maximum density of the liquid in Hel near the temperature of the X- transition Tx, or the formation of a liquid film on the surface of a heater which is partly immersed into Hell.

A detailed description of the experimental technique and some results of this work have been published elsewhere [1].

Figure 1 is a schematic diagram of the experimental arrangement used now for schlieren photography in liquid He. The optical cryostat with glass plane-parallel windows 230 mm in diameter is placed between the collimator 1 and the observation apparatus 10 of the schlieren system (a mirror-meniscus system with the focal length of about 2 m). A flat layer of the liquid He under study (57 mm thick) fills the space between two plane-parallel glass disks 8 which are sealed by compression of indium O-rings against the stainless steel body of the test chamber 9. The outer surface of the discs 8 is protected from the infrared radiation by the glass discs 7 and 6 which are in ther-

Fio. 1.

mal contact with the 1. He bath 20 and the liquid nitro- gen bath 19 respectively, and by thermally insulated discs 17. All these parts of the cryostat are placed in the evacuated casing 4. The helium gas is pumped out of the test chamber through the pipe 18. The tempera- ture of the liquid T is controlled and measured by the saturated vapour pressure (T⁵⁸ scale). Vertical and horizontal flat walls devide the chamber 9 in two test sections having the shape of parallelepiped. The plane heater 12 placed far from the walls is made of vertical strips of a constantan ribbon 0.02 mm thick and ~ 0.5 mm wide. The strips are stretched along one plane with a pitch of ~ 1 mm across the entire test layer.

The collimating slit 2 of the schlieren system has 50 microns of width and 4 mm of length. The light source is a xenon flashlamp with the spark duration about of 1 [is. The beam of parallel light rays which

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1970309

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C 3 - 106 A. I. GOULYAEV has been passed through the cryostat's windows and

the test layer is collected in the focus of the observation apparatus where a Foucault knife partly closes the image of the slit, the knife edge being parallel to the slit. The image of the test layer is projected onto a 36 mm film by an additional lens array. Presented photographs have been obtained with the slit in vertical position parallel to the plane 12 (registered are, therefore, horizontal projections of the density gradient). The knife edge is so adjusted that the growing illumination in a region of the layer's image, compared to the zero gray tone, corresponds to the positive direc- tion (left to right) of the density gradient. The least density gradient which can be detected by this arrangement is (dpldx), z 1.5 x g/cm4, being temperature-independent in the range 1.3 < T < 2.3 OK.

The least detectable temperature gradient varies from

near TA to

(dT/dx), z 5 x gradlcmat T = 1.5 OK.

The upper photographs in figure 2 illustrate inten- sive boiling of He1 and He11 during evacuation of the vapour. When joulean heat is steadily generated in

upward along the heater when the liquid temperature is reduced (see Fig. 2 : 2.15 and 2.12 OK). It may be supposed that a part of the heated plane 12 (from the free liquid surface up to this boundary) is covered by a liquid film. The transfer rates of the film flow calculated from these pictures are in good agreement with the values obtained from other experiments.

Packets of sound and second sound waves have been initiated by supplying to the heater a single rectangular current pulse of controlled duration t,.

The electrical power N of the pulse (calculated per unit area of the metallic surface of the heater) dissipa- tes in both directions. The light flash is triggered after a time-delay t, counted off from the beginning of the pulse. Detectable sound wave packets are generated only when N 2= 1.5 W/cm2 and Nt, 2= 15 pJ/cm2, while visible fore fronts of the sound packets begin with compression waves, even in HeII, due to the formation of a thin vapour layer on the heating surface.

Figure 3 shows photographs of vapour and liquid with wave packets initiated by short current pulse (t, = 10 and 20 ps, N z 10 W/cm2) and propaga- ting symmetrically in both directions from the heater 12. In He11 (lower photograph), besides sound waves,

slow convective streams of liquid and vapour in the gravitational field are developed near the heater (lower photographs in figure 2 ; thin vertical lines are the projections of the plane heater 12 ; horizontal lines correspond to the liquid-vapour interface).

Streams of the liquid gravitating downward along the heated surface (Fig. 2,2.174 OK) point out that He1 has the maximum density. The observation of this picture may also be used as a sensitivity check of the whole arrangement since in this case dp < 5 x g/cm3.

At T < T, stationary disturbances in the liquid enti- rely disappear while heated layers of the vapour acquire a clear-cut, steady, lower boundary which moves

FIG. 3.

packets of second sound waves are also seen (nearer to the heater). Note that the lines of intersection of the heating plane with the walls and with the liquid- vapour interface are found to be sources of cylindrical waves.

Figure 4 illustrates changes in the structure of sound packets in the liquid (t, = 50 ps, N = 3.2 W/

cm2) when the temperature is reduced from 2.167 OK (upper photo) to 2.161 OK (lower photo). Slightly curved stripes near the heater on the upper photograph (secondary sound packets) are generated only in He11

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SCHLIEREN PHOTOGRAPHY IN LIQUID 4He C 3

-

¹⁰⁷

and partial reflection from the surface of the helix.

When a sound wave strikes the liquid-vapour interface, a reflected wave (of the reversed sign) in the liquid and a refracted wave in the vapour are formed as it is seen on the upper left photograph in figure 5. A wave of the second sound incident upon the interface (see the upper right photograph) gives a reflected second sound wave in the liquid, but in the vapour at the same time a sound wave is generated which propagates then with the velocity exceeding the second sound velocity. The lower photograph in figure 5 illustrates the transformation of a sound wave incident from the

just below T,, and they are the first signs of the

A-

transition (second sound is not yet seen). At lower temperatures clear-cut secondary packets are formed earlier and they are seen near the back fronts of the primary sound packets. Conditions for the formation of these secondary sound packets are not quite unders- tood.

Figure 5 shows pictures of interactions of cylindrical waves with the liquid-vapour interface. Such waves have been initiated by a single current pulse supplied

to a helical cylindrical heater

-

5 mm in diameter _FIG._5.

(constantan ribbon 0.02 mm thick, helical-wound with the pitch of

-

¹mm). That cylindrical wave

which travels first to the axis of the heater, multi- vapour on the interface into a wave of the second plies itself with the pitch equal to the heater's dia- sound in He11 (reflected sound waves in the vapour are meter due to reflection from the geometrical axis considerably weekened).

Reference [I] GOULYAEV (A. I.), Zh. Eksperim. i. Teor. Fiz., 1969,57,

1 (7), 59.