ACOUSTIC EMISSION AT TRANSITIONS IN LIQUID CRYSTALS

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ACOUSTIC EMISSION AT TRANSITIONS IN LIQUID CRYSTALS

F. Scudieri, T. Papa, D. Sette, M. Bertolotti, E. Sturla

To cite this version:

F. Scudieri, T. Papa, D. Sette, M. Bertolotti, E. Sturla. ACOUSTIC EMISSION AT TRANSI- TIONS IN LIQUID CRYSTALS. Journal de Physique Colloques, 1979, 40 (C3), pp.C3-392-C3-395.

�10.1051/jphyscol:1979378�. �jpa-00218774�

JOURNAL DE PHYSIQUE Colloque C3, supplkment au no 4, Tome 40, Avril 1979, page C3-392

ACOUSTIC EMISSION AT TRANSITIONS IN LIQUID CRYSTALS

F. SCUDIERI, T. PAPA, D. SETTE, M. BERTOLOTTI and E. STURLA Istituto di Fisica-Facolta' di Ingegneria-Universita' di Roma, Roma, Italy

RBsum6. -On prtsente des mesures d'tmission acoustique dans les nkmatiques et les smectiques A indiquant que le phtnomkne est corrklt avec la dynamique des dCfauts dans les matkriaux. Une methode thermo-optique est utilisee pour avoir des dkfauts de fa~on contrblke ainsi que des tubes capillaires.

Abstract. ^- Measurements of acoustic emission (a.e.) in the nematic and smectic A phases are described which give evidence that the phenomenon is connected to the dynamics of defects in the material. A thermo-optical method and the use of capillary pipes are described to obtain defects in a controlled way.

1. Introduction. - A sudden release of elastic energy in a solid generally can produce a detectable amount of acoustic emission (a.e.). In particular a large acoustic activity is connected to the dynamics of defects in solids, as produced by an applied stress field or in certain phase transitions. When the activity is sufficiently strong it is possible to obtain information on the elastic energy released and its power spectrum.

Characteristics times involved in such processes can furthermore be determined. Up to now however the resonant nature of the piezoelectric transducers used as detectors limits the amount of available informations. In liquid materials it is not possible to observe any acoustic emission because the possible elastic energy release is generally slow and damped

by viscosity; up to now only cavitation phenomena exhibit an acoustic activity. In the liquid crystalline materials the coexistence of both solid like elastic and viscous properties offers the possibility to extend the study of defects dynamics by a.e. technique to such substances. In fact the existence of acoustic emission was already shown by the authors in phase transitions of liquid crystals without or with an external applied field [l, 21.

In the present paper we wish to investigate about a.e. produced by introducing controlled defects in the sample. In particular we have restricted our attention to the localized defects in smectic A phase, using boundary controlled and thermo-optically excited defects.

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FIG. 1. - A.E. activity for an unoriented sample of CBOOA increasing the temperature as time increases.

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Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979378

ACOUSTIC EMISSION AT TRANSITIONS I N LIQUID CRYSTALS

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FIG. 2. - A.E. activity for an unoriented sample of COOB increasing the temperature as time increases.

A+ l a s e r 514 nm

glass A E detector

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FIG. 3A. - Experimental set up for a.e. connected with thermos optic effect.

FIG. 3B. - Scattering pattern (unpolarized He-Ne laser beam) : a ) above the intensity threshold for buckling instability ; b) at the threshold for swallow tailed splitting ; c ) swallow tailed splitted pattern : note the circumference arc pattern passing through the

laser spot.

C3-394 F. SCUDIERI, T. PAPA, D . SETTE, M . BERTOLOTTI A N D E. STURLA

2. General considerations. - The acoustic emis- sion connected with phase transitions of unoriented samples of liquid crystals is easily detected with standard techniques as described in reference [l]. To show the general behavior of the emission, in figures 1 and 2 the a.e. rate is plotted as a function of time, when the sample temperature was increased.

The used materials were CBOOA and COOB (M24).

The materials were contained in a cylindrical glass holder (diameter 15 mm, thickness 0.998 mm).

The holder cell was put into acoustic contact with an a.e. detector, whose best efficiency was in the 500 KHz frequency range. Typical noise rate was much lower than one pulse per second. Optical inspection of the sample was also provided.

Maxima of activity are clearly seen in correspon- dence of phase transitions. A tentative explanation was given [l] by.assuming that the increase of activity is connected with the dynamics of distorted regions andlor with the depinning of localized defects. To decide which processes are effective we have carried out some experiments, which will be described in the following.

3. Experimental set-up and results. - A first expe- riment has been performed in which buckling insta- bilities have been thermally excited.

It is well known that a smectic A, in the homeo- tropic alignment, exhibits a buckling instability under negative pressure stress [3-51. Unfortunately such type of excitation cannot be used in our case because the a.e. detector is very sensitive to changes in pres- sure and a.e. can be also excited in the holder of the liquid crystal samples.

However, undulations and, for strong stresses, localized defects can be excited by applying a tempe- rature gradient to the sample. The hot wire technique cannot be utilized because of the strong a.e. exhibited by metals under temperature change. A local tempe- rature increase can be also obtained with a laser light [4]. We have used this effect to introduce buckling instabilities with a disposition sketched in figure 3A.

To avoid all temperature control problems and in order to work at room temperature we have utilized a COB sample 100 pm thick, doped with Rh 6G dye (concentration about 6 X lOV3 M) to increase the optical absorption. A fringe pattern was obtained with two A+ laser beams (514 nm) which induce into the sample a periodic temperature change.

A low power-He-Ne laser beam (633 nm) is utilized as optical probe and the strong characteristic scat- tering pattern is observed onto the plane n. With this experimental arrangement different thermo- optical effects are observed as a function of the A+

laser light intensity on the sample, which are present only after the illumination is switched off. Below a certain intensity threshold Il no banana like pattern appears and no a.e. activity is detected. Above I, the characteristic banana like pattern connected

with buckling instability is observed (Fig. 3B-a : in this case unpolarized He-Ne laser light was used) : also in this case no relevant increase of a.e. was

observed after switching off the A+ laser light.

By increasing the light intensity it is possible to observe in the plane z a splitting of each banana like zone in a swallow tailed pattern (Fig. 3B-b). By further increasing the light intensity a new circular arc pattern passing through the laser beam spot (Fig. 3B-c) appears. In such conditions the sample relaxes into the original ordered state in some seconds and presents a detectable increase of a.e. activity for about one hundred seconds as shown in figure 4a.

A further increase of the A+ laser illumination induces, at a third threshold value, I, I, deformations that relaxes in a very long time (few tens of minutes) during which the banana like pattern reduces in intensity but four spots corresponding to the q, values remain visible. In figure 4b the related a.e.

rate is shown.

The results show that the simple distorted region associated with the ondulation is not able to produce a detectable a.e. Only when the distortion becomes very strong so that defects as f.e. dislocations are introduced, some acoustic emission appears. The much lower activity shown in figure 4 as compared ^' with figures 1 and 2 is simply due to the smallness of the active region which is only about 1.7 X 1OW2 of the overall sample surface. This experimental limi- tation arises from the low solubility of the used dye in cyano-biphenyl materials which makes necessary to concentrate highly the laser light to have appre- ciable effects.

Because the smectic layer undulations relaxe in a time shorter than the one over which the a.e. is present,

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FIG. 4. -a) A.E. activity for the case 3 B - c the rate for t 0 is noise ; b) A.E. activity for plastic deformations : in this case the threshold amplitude for the detectable acoustic pulses is

increased respect to the case a).

ACOUSTIC EMISSION AT TRANSITIONS IN LIQUID CRYSTALS C3-395

we can suppose that the a.e. is connected only with the defects dynamics.

To obtain further evidence of the ability of dislo- cations to produce a.e. we have considered a different experiment which makes use of the strong alignment properties of walls in capillary tubes. It is well known that the homeotropic aligned samples of smectics A and nematics inside a capillary tube differ by the presence of a localized dislocation line along the axis of the tube that disappears near the phase transition (escape in the 3th dimension of the molecular director for the nematic phase). Accordingly, we have assembled the organ pipes disposition shown in figure 5 were several capillary tubes filled with COB (diameter 455 pm) were put into acoustic contact with an a.e. detector. Alignment of mean local value of the molecular director in different phases was controlled with a differential interferometric optical technique developed by one of the authors [6]. By increasing the temperature, at the phase transition

S A - N a sudden release of a.e. was observed as shown in figure 6. By decreasing the temperature the change in orientation at N-SA transition happened in a statistical independent way among the different capil- laries therefore the a.e. was spread over a much longer time.

From the previous results it is possible to infer that in the liquid crystalline mesophases the a.e.

arises principally from the defects dynamics. The thermo-optic method described in the paper offers the possibility to obtain defects in a controlled way.

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FIG. 5. - Organ pipes experimental set-up.

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FIG. 6. - A.E. activity for COB.

Further research is now necessary to gain infor- mations on the spectral behavior of the a.e. and its connections to the different kiad of defects.

References

[l] SCUDIERI, F, PAPA, T., SETTE, D. and BERTOLOTTI, M., J. AppI.

Phys. (1979) in press.

[2] SCUDIERI, F., PAPA, T., SETTE, D. and BERTOLOTTI, M,, Ann.

Phys. 3 (1978) 263 in press.

[3] CLARK, N. A. and MEYW, R. B., AppI. Phys. Lett. 22 (1973) 493.

[4] DELAYE, M., RIBOTTA, R. and DURAND, G., Phys. Lett. 44A (1973) 139.

[5] RIBOTTA, R., Thesis, Orsay, 1975.

[6] SCUDIERI, F., Appl. Opt. (1979) in press.