Surface anchoring energy measurement

D3 Other Data on the Surface Anchoring Energy Measured for Nematics... 

Contact angle and surface free energy measurements offer the first insight into the potential use of the PDLC as biomaterial and also provide information about the physical interactions inside the material that determine the droplet anchoring. Usually, for a material to be considered as potentially biocompatible, the water contact angle value must be in the range of 60-90 degrees, which is considered within the domain of moderate wettability (Ikada 1994 Vasile and Pascu 2007). These values ensure an appropriate balance of hydrophobic/hydrophilic forces that will favor cellular adhesion and prevent the rejection of the implanted material. 

The main defect in this kind of theory is the assumption that the orientational order is nematic like in the interfacial region, which is generally not true (see Section 10.3.1). Another problem is linked to the fact that the surface anchoring energy is unknown. This surface anchoring energy is difficult to measure directly and the result of any indirect measurement depends on how the response of the system to a disorientation is modeled. One way out is to use the orientational distribution of the molecules in the surface layer as a boundary condition of the nematic order [47] (see also Section 10.3.2]. 

D MEASUREMENT TECHNIQUES OF THE SURFACE ANCHORING ENERGY D1 External Field OffMethod... 

There are various experimental techniques for measuring the anchoring energy which may be divided into two different groups according to whether an external perturbating field is applied (fleld-on techniques) or not (fleld-off techniques). In any case, the kind of distortion of the director fleld in a surface layer is of great importance. In different experiments for the determination of W the parameters varied were ... 

The TIR and ATR effects are not the only possible phenomena which are useful in studying the surface properties such as the director orientation at the boundaries, the surface order parameter, the anchoring energy, etc. Such useful techniques as Brewster angle measurements [215] and ellipso-metric studies [216] should also be mentioned. But the discussion of these methods is beyond the firamework of this book. 

The bistability phenomena in the FLC structures will be considered below.) The results of these two methods coincide with each other to an accuracy of 30%. The anchoring energy of an FLC mixture (ZhK-224) for different conducting surfaces is shown in Table 7.2. The values of Wd, measured according to (7.40), (7.41), were shown to depend linearly on the temperature near the A C phase transition point... 

In a chiral compound, the liquid crystalline structure itself possesses a polarity which interacts with the surface. At the surface of a smectic C liquid crystal, the ferroelectric polarization P points preferably either towards or away from the surface, depending on the material [104, 105]. Conversely, if the surface director of a chiral nematic liquid crystal is tilted, a polarization (dependent on the tilt angle) is created perpendicular to the tilt plane [106]. In principle, the presence of this polarization makes a contribution to the anchoring energy this chiral contribution is, however, too small to be measured [107]. 

To avoid the formation of surface walls and to provide a fast switching ofF time, the anchoring energy should be sufficiently high, e.g. > 10 " J/m, comparable with that of the rubbed polymer surface. This is not really observed for polyvinyl-cinnamate (PVCN) derivatives [27-29]. The recent measurements also confirm that 10 to 10 J/m, i.e. about one to two orders of mapitude smaller than the required value [1]. The in-plane sliding mode (IPSL) was proposed in a... 

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