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Physical Review E 03/2013; 87:030501. · 2.26 Impact Factor
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ABSTRACT: We report experimental studies on small angle light scattering (SALS), and rheodielectric and electrorheological properties of a binary mixture of octyloxy cyanobiphenyl and hexyloxy cyanobiphenyl liquid crystals. The mixture exhibits nematic (N) to smectic-A (SmA) phase transitions, and then again to a reentrant nematic (NR) phase transition. Rapid shear thinning in the quenched samples in the low shear rate region in the N and SmA phases
observed from SALS experiments is attributed to the realignment of the director within the domains. The domains are elongated along the shear direction at higher shear rates. The temperature variation of the effective viscosity and static dielectric constant reveals the changes in the director orientation across N-SmA-NR phase transitions. At a steady shear rate the effective viscosity increases with the electric field in all the phases and saturates at much
higher fields. It also exhibits two anomalous peaks across N-SmA-NR phase transitions beyond a particular field. The shear modulus of the SmA phase in an intermediate field is significantly larger than that measured at both low and high fields. This enhanced viscoelasticity of the SmA phase is argued to originate from the increased dislocation density.
Physical Review E 07/2012; 86:011710. · 2.26 Impact Factor
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ABSTRACT: We report experimental studies on small-angle light scattering (SALS), rheodielectric and elec-
trorheological properties of a binary mixture of octyloxy cyanobiphenyl (8OCB) and hexyloxy
cyanobiphenyl (6OCB) liquid crystals. The mixture exhibits nematic (N) to smectic-A (SmA), and
then again to a re-entrant nematic (NR ) phase transitions. Rapid shear thinning in the quenched
samples in the low shear rate region in the N and SmA phases observed from rheo-SALS experiments
is attributed to the realignment of director within the domains. The domains are elongated along
the shear direction at higher shear rates. The temperature variation of effective viscosity and static
dielectric constant, reveals the changes in the director orientation across N-SmA-NR phase transi-
tions. At a steady shear rate the effective viscosity increases with the electric field in all the phases
and saturates at much higher fields. It also exhibits two anomalous peaks across N-SmA-NR phase
transitions in the intermediate field range. Shear modulus of the SmA phase in the intermediate
field range is significantly larger than that measured at both low and high fields. This enhanced
viscoelasticity of the SmA phase is argued to originate from the increased dislocation density.
Physical Review E (Submitted). 02/2012;
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ABSTRACT: We report simultaneous measurements of shear viscosity (η) and dielectric constant (ε) of octyloxy cyanobiphenyl (8OCB) in the nematic (N) and smectic-A (SmA) phases as functions of temperature and electric field. With increasing electric field η increases in the N phase whereas it decreases in the SmA phase and saturates beyond a particular field in both the phases. The flow curves in the intermediate-field range show two Newtonian regimes in the N phase. The temperature-dependent behavior of η and ε at zero or at small electric field suggests the occurrence of several structures that results from precessional motion of the director along the neutral direction as reported in similar other system. We show that the precessional motions are gradually suppressed with increasing electric field and the effective viscosity resembles with the Miesowicz viscosity η1 at high enough electric field. In the intermediate field range the temperature-dependent η exhibits anomalous behavior across the N-SmA phase transition which is attributed to the large contribution of Leslie coefficient α1.
The European Physical Journal E 08/2011; 34(8):74. · 1.94 Impact Factor
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ABSTRACT: We measured rheological properties of cholesteryl nonanoate and its mixture with
40-octyloxy-4-cyanobiphenyl. In both systems, wider temperature range of blue
phase under shear with anomalous increase in viscosity associated with non- Newtonian
flow behavior has been observed. Shear thinning followed by Newtonian flow is
found in cholesteric phase and non-Newtonian flow behavior at high shear rates is
seen in smectic-A phase. In cholesteryl nonanoate a small peak in the viscosity at
cholesteric to smectic-A transition along with a shear rate dependent viscosity at
higher shear rate is attributed to the type-II character of the compound. We find that
the type-II character of the compound is further enhanced by mixing a small amount
(20 wt%) of 40-octyloxy-4-cyanobiphenyl in cholesteryl nonanoate. No significant
change in the temperature dependent rheological behavior is observed at the twistgrain-
boundary-A to smectic-A transition. It is suggested that the small anomalous
peak in the viscosity just above the twist-grain-boundary-A phase is due the existence
of chiral line liquid phase.
Molecular Crystals and Liquid Crystals 06/2011; 547:39-45. · 0.58 Impact Factor
