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Sparavigna, A. (2016). From an Isotropic Liquid to a Nematic or a Smectic Mesophase. PHILICA.COM Article number 554.

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From an Isotropic Liquid to a Nematic or a Smectic Mesophase

Amelia Carolina Sparavignaunconfirmed user (Department of Applied Science and Technology, Politecnico di Torino)

Published in physic.philica.com

Abstract
Abstract: The paper is proposing a discussion on transitions from the isotropic liquid phase into a nematic or a smectic mesophase, as they appear when observed by means of a polarized light microscope. This discussion can be useful for a student laboratory, which is concerning the physics of mesophases.
Keywords: Liquid Crystals, Polarized Light Microscopy

Article body

 

 

From an Isotropic Liquid to a Nematic or a Smectic Mesophase

 

Amelia Carolina Sparavigna

Department of Applied Science and Technology, Politecnico di Torino, Torino, Italy

 

 Abstract: The paper is proposing a discussion on transitions from the isotropic liquid phase into a nematic or a smectic mesophase, as they appear when observed by means of a polarized light microscope. This discussion can be useful for a student laboratory, which is concerning the physics of mesophases.

Keywords: Liquid Crystals, Polarized Light Microscopy

 

Introduction: Liquid crystals are materials composed of molecules having a form which is not spherical. Then, these materials are made of rod-like, disk-like or more complex shapes [1]. Quite interesting are those made of molecules possessing a core-bent [2], so that the molecules look like being banana-shaped [3,4]. The liquid crystals are materials characterized by the presence of mesophases between the isotropic liquid phase and the crystalline phase. Having the mesophases features of both liquid and crystal phases, the materials possessing them were called “liquid crystals”. Typical mesophases are nematic and smectic phases. The nematic phase has the centers of molecules that can assume arbitrary positions, while the axes of them tend to orient in the same direction. In smectic phases, molecules are arranged with their centers on specific layers, with axes of molecules being perpendicular or tilted with respect to the layer. If the direction is perpendicular, it is said that the phase is smectic A; if the axis is tilted, the phase is smectic C. Smectic B has the molecules perpendicular to the layers, with their centers arranged on a regular lattice. In general then, the smectic phase is more ordered than the nematic phase, and more disordered than a crystal [5-7]. Let us note that, besides the three abovementioned smectic phases, several other exist [8].

Since the nematic phase is quite disordered, it is usually observed on cooling from the liquid isotropic phase, where the molecules are disordered both in position and orientation. However, some materials do not show a nematic phase, but only a smectic phase. If we consider a thermotropic liquid crystal, that is a material having the molecular order determined or changed by temperature, on heating or cooling the sample, we can see a direct transition from smectic to isotropic liquid (on heating) and from isotropic liquid to smectic (on cooling). In the following, we will discuss what we can see by means of a polarized liquid microscope and a suitable thermostat. 

 

From isotropic liquid to nematic phase

To know the mesophases displayed by a given material, the observation of a liquid crystal by a polarized light microscope is fundamental. In each mesophase, the liquid crystal shows typical patterns that are defined as “textures”.  For the textures displayed by the different mesophases, fundamental books exist giving color plates of textures obtained from polarized light microscopy, for instance [9,10]. A polarized light microscope is a microscope where the sample is illuminated with polarized light. The light transmitted by the sample can, optionally, be blocked with a polarizer orientated at 90 degrees to the illumination. In the case of liquid crystals, the sample usually consists of a thin film of material sandwiched between two microscope slides. The material thickness is of a few microns. If we need to change the temperature of the sample, a suitable temperature control thermostat is necessary, having two windows for the transmission of light across the sample.

Let us see what can be observed with a polarizing microscope, when a sample passes from the isotropic liquid phase, where the molecules are disordered both in position and orientation, to a liquid-crystalline phase. The sample is heated until it reaches the liquid phase. This phase, when viewed by a microscope with crossed polarizing filters, is of a uniform black. The isotropic optical material allows the complete extinction of light. If the liquid is cooled and it passes in the nematic phase, colored bubbles appear (Figure 1). The colours are due to the birefringence of the nematic phase, which is uniaxial. When we see the bubbles, the material is nematic. The bubbles grow until all the material is nematic. An example is given in the picture of Figure 1. The liquid crystal  is the 12OBBAC acid of the alkyloxybenzoic family [11-13]. In the Figure 2, we can see a nematic which appears from the liquid, with the presence of typical defects [14-18]. The nematic is an acid of the hexylcyclohexane-carboxylic family [19].

 

 

Figure 1: Transition from the isotropic liquid into the nematic phase, viewed by means of a polarized light microscope with crossed polarizers. The liquid is black in the image; the nematic phase appears as colored bubbles. The liquid crystal is 12OBAC alkyloxybenzoic  acid. The nematic phase is optically anisotropic. The material changes the light which had been polarized by the first filter of the microscope. In this manner, the second filter is no longer able of extinguishing the light. The material appears colored due to interference phenomena.

 

Figure 2: Here the nematic phase on cooling from liquid. Note the topological defects, which are disclinations of strength 1/2 and 1.

 

From isotropic liquid to cholesteric or blue phases

Some materials exhibit a chiral nematic phase possessing chirality. This phase is often defined as the cholesteric phase because it was first observed for cholesterol derivatives. We can see the cholesteric phase when chiral molecules are present in the sample. The phase exhibits a twisting of the average direction of molecular exes. Some images of chiral nematics growing in an isotropic liquid are shown at the web site www.personal.kent.edu/ ~bisenyuk/liquidcrystals/textures2.html. The isotropic liquid to the cholesteric phase transition is of the first order, like that of the isotropic liquid in nematic [20]. In this manner, the cholesteric phase appears through nucleation. However, Let us note that in some cholesteric materials a blue phase can exist in the temperature range between the isotropic liquid and the chiral  nematic phase [21-24]. Blue phases have a regular cubic structure of defects which are reflecting the blue light. They were first observed as early as 1888 by Friedrich Reinitzer, who reported: ‘On cooling of the molten compound a bright blue-violet colour phenomenon appears which disappears rapidly followed by a non-uniform turbidity’ [25]. Usually the blue phase exists within a narrow temperature range, of some degrees.

 

Smectic A mesophase

What can we see when we have a transition from the isotropic liquid into a smectic mesophase? Since there are different smectics, the transition depends on the specific mesophase. Let us start from a material having a smectic A phase. We use one of the oxadiazoles studied in [26-28]. By means of the polarized light microscope, this material shows a smectic A texture characterized by fan domains. These domains are also known as "focal-conic" domains.

 

 

 

Figure 3: The  “focal-conic domains of the smectic phase.

 

Increasing the temperature, we bring the sample in the isotropic liquid phase. The sample, viewed between crossed polarizer looks black. Cooling slowly the sample, we bring it in smectic phase again. No nematic phase is observed. In the field of view of the microscope, some “bâtonnets” begin to grow in the black of the isotropic liquid phase (Figure 4). These domains grow until they form the texture made of focal-conic domains (Figure 3).

 

Figure 4: Bâtonnets of the smectic phase are growing in the isotropic liquid melt.

 

Smectic  C mesophase

Besides the focal-conic domains, other textures are shown by the smectic phase. Let us consider another material, having the direct transition isotropic liquid - smectic phase. It is the 16OBAC acid of alkyloxybenzoic family [29]. The material is crystalline up to 90 degrees and then passes into a smectic C phase. At the temperature of 131 degrees, it becomes an isotropic liquid. This material then does not possess the nematic phase. This is due to the fact that its molecules are so long as to maintain the smectic order up to high temperatures, overcoming the disordering tendency caused by thermal motion. In cooling, the smectic phase appears from the isotropic phase: in the Figure 5 we can observe a dendritic structure that appear in black field of the sample viewed between crossed polarizers.

 

Figure 5: Here the smectic is growing in the isotropic liquid phase, showing a texture that looks like coral branches.

 

Figure 6: The sequence on the left show the evolution of the dendritic texture, when temperature is lowered of 0.5 degrees per minute (16OBAC). On the right, the smectic phase of 16OBAC: the texture looks like that of a nematic, but, only  topological defects with strength 1 are present.

 

it is interesting to note that the smectic phase which forms on cooling the sample has a texture different from that which is observed in heating [29]. The texture that we  observe on cooling looks like that of a nematic (Figure 6). Because the phase is smectic, there are only topological defects with strength 1. We can therefore distinguish it from the nematic phase, which has also defects  with strength 1/2.

If we use a mixture of two acids, 12OBAC and 16OBAC [29], we have a greater molecular disorder because there are molecules with different lengths in the mixture. For this reason, the mixture has a nematic phase between smectic and isotropic liquid. When a sample made of this mixture is slowly cooled down from the liquid, a nematic phase is observed and then the smectic appears. It is interesting to note that the smectic is growing with a dendritic texture (Figure 7), which looks like those given in the Figures 5 and 6.

 

Figure 7: On cooling a 12OCAC-16OBAC mixture, the smectic phase in growing in the nematic phase with a dendritic texture, as in the case of smectic in the isotropic liquid [29].

 

Discussion

The process of growth of a mesophase on cooling from the isotropic phase is an interesting problem, and perhaps, little studied [30]. This process is made of two steps, the nucleation and the growth. These processes had been well studied for crystallizations from isotropic liquids. In the nucleation step, some centers of nucleation, the clusters, appear in the liquid where the local concentration is higher. Growing up, the clusters create nuclei that fairly reflect the order of the crystal. They are small crystals of microscopic size.

Mesophases have their nuclei too. For what concerns nematics, usually very small circular droplets are observed, which are formed in the isotropic liquid and then combine to form the nematic phase. Although the anisotropic nematic has an orientational order, the position of the molecules is disordered. Imagine the molecules of the nematic as little rods. When they are in the liquid phase isotropic, the positions of the centers of rods are disordered, as well as their orientation. When the temperature decreases, the material reaches the phase transition where the molecules turn their long axes, without having to move their centers. This can happens in the same manner  in all the directions of space. A nucleus of nematic phase grows in the isotropic liquid with a spherical symmetry then, and therefore, in polarized light, we see circular colored droplets growing in the black of the liquid. In the smectic phases, the local order is very different. The smectic phase has microscopic planes, on which the molecules have to arrange their centers.  The molecules must orient their axes and also move their centers to form the smectic planes. Therefore, the initial clusters have a preferred direction; in the smectic A, this direction is perpendicular to the smectic planes. Hence they produces the “bâtonnets”. It is very interesting the growth of smectic C, which appears as a dendritic structure. Since the molecules are tilted in its smectic planes, and due to a disorder in the arrangement of nuclei, the texture which is organized turns out emulating that of the cholesteric ‘fingerprints’. Of course, further studies are needed to better determine the characteristics of the nucleation of this smectic phase.

 

References

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[11] Montrucchio B., Sparavigna, A., & Strigazzi A (1998). A new image processing method for enhancing the detection sensitivity of smooth transitions in liquid crystals. Liquid Crystals, 24(06), 841-852.   

[12] Sparavigna, A., Mello, A., & Montrucchio, B. (2007). Texture transitions in binary mixtures of 6OBAC with compounds of its homologous series. Phase Transitions, 80(3), 191-201.

[13] Sparavigna, A. Mello, A., & Montrucchio, B. (2006).  Texture transitions in the liquid crystalline alkyloxybenzoic acid 6OBAC. Phase Transitions, 79(4-5), 293-303.  

[14] Sparavigna, A., Sanna, A., Montrucchio, B., & Strigazzi, A. (1999). Streamline image analysis: a new tool for investigating defects in nematic liquid crystals. Liquid crystals, 26(10), 1467-1478.

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[19] Montrucchio, B., Sparavigna, A., Torgova, S.I., &  Strigazzi, A. (1998). A novel order transition inside the nematic phase of trans-4-hexylcyclohexane-1-carboxylic acid discovered by image processing. Liquid crystals, 25(5), 613-620.

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[26] Sparavigna, A.C. (2013). Some features of  liquid crystalline oxadiazoles. Int. J. Sci., 2(07), 89-95.

[27] Sparavigna, A., Mello, A., & Montrucchio, B. (2008). Growth of toric domains in the smectic phase of oxadiazoles, Phase Transitions, 81(05), 471-477.

[28] Sparavigna, A., Mello, A., & Montrucchio, B. (2007). Fan-shaped and toric textures of mesomorphic oxadiazoles, Phase Transitions, 80(09), 987-998.

[29] Sparavigna, A., Mello, A., & Massa, G. (2009). Undulation textures at the phase transitions of some alkyloxybenzoic acids. Phase Transitions, 82(5), 398-408.

[30] Dierking. I. & Russell C. (2003). Universal scaling laws for the anisotropic growth of SmA liquid crystal bâtonnets, Physica B, 325(01), 281-286.

 

 

 

 

 

 


 

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Sparavigna, A. (2016). From an Isotropic Liquid to a Nematic or a Smectic Mesophase. PHILICA.COM Article number 554.


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