Comparing responses of dairy cows to short-term and long-term heat stress in climate-controlled chambers.

Heat stress (HS) in dairy cows can be classified into short-term heat stress (STHS) and long-term heat stress (LTHS) according to the number of consecutive days in HS. The comparative study of these 2 types of HS is limited in terms of their effects on the production and energy metabolism of cows. In this study, 4 lactating Holstein cows (102.5 ± 12 days in milk, 605 ± 22 kg of body weight, second parity) fitted with rumen fistulae were randomly assigned to 1 of 2 groups in a 2 × 2 crossover design and allocated to 1 of 2 climate-controlled chambers. This study contained 2 periods, each with a control phase and a HS phase. There was a recovery phase between 2 periods. The HS phase comprised either STHS (3 d) or LTHS (7 d) treatments. Data collected from the 3 d of STHS and the last 3 d of LTHS were compared. The chambers were set at thermal neutral conditions (20°C, 50% humidity) during the control and recovery phases or cyclical HS conditions (26-38°C, 50% humidity) during the HS phase. Compared with STHS, LTHS decreased milk yield by 17.2% and dry matter intake by 12.6%, indicating that LTHS caused a more severe decline in milk production and feed intake. In addition, LTHS decreased milk protein concentration by 6.8% and milk protein yield by 22.4%. In comparison with STHS, LTHS decreased rumen liquor volatile fatty acid (29.7%), blood glucose (11.6%), and nonesterified fatty acid (13.6%) concentrations, but increased milk urea nitrogen by 15.1%, blood urea nitrogen by 8.6%, and creatine concentrations by 15.4%. Our results suggest that although reduced feed intake may be mainly responsible for reduced milk production during STHS, impaired rumen metabolism and suppressed mobilization of adipose tissue could be the main reasons for further reduction in milk yield during LTHS.

Proton NMR relaxation study on the nematic-nematic phase transition in A131 liquid crystal.

A study of the proton NMR spin-lattice relaxation time, T(1), of the A131 liquid crystal compound as a function of temperature and Larmor frequency, using a combination of fast field-cycling and standard NMR techniques, is presented. The frequency dispersion in a wide range (from 10 kHz to 300 MHz) at different temperatures and the temperature variation of T(1), in several frequency conditions, were analyzed considering the contributions of the molecular movements generally detected in liquid crystals. In the case of nematic phases of calamitic liquid crystals, the nuclear spin relaxation is dominated by collective movements and local molecular reorientations. The experimental results clearly show a transition within the nematic range of this compound, previously identified as one from the uniaxial to the biaxial phase. This transition can be associated with a slowing down of the molecular rotations around the long molecular axis, where the preferred orientation defines the principal director as detected in the T(1) dispersion analysis.

Orientational order of a liquid crystal with three chiral centers by a combined 13C NMR and DFT approach.

In this work, the liquid crystal (S)-2-methylbutyl-[4'-(4' '-heptyloxyphenyl)-benzoyl-4-oxy-(S)-2-((S)-2')-benzoyl)-propionyl)]-propionate (ZLL 7/*) was investigated by means of 13C NMR spectroscopy. This compound has a very peculiar mesomorphic behavior, showing the following phases: paraelectric SmA, ferroelectric SmC*, antiferroelectric SmC*A, re-entrant ferroelectric SmC*re, and ferroelectric hexatic Sm*HEX. The structural and orientational ordering properties of ZLL 7/* have been determined by exploiting the nuclear chemical shielding properties of 13C. To this aim, solid-state NMR techniques such as CP, SPINAL-64, and SUPER have been used in combination with DFT calculations. The agreement between experimental and in vacuo DFT shielding parameters appears to be satisfactory. The orientational order parameters obtained from the 13C shielding analysis have been discussed, taking into account different data analysis approaches and comparing them to those previously obtained from an independent 2H NMR study.

Nematiclike viscosity coefficients of ferroelectric liquid crystals in their smectic-C* phase.

The five nematiclike viscosity coefficients corresponding to the Smectic-C* (SmC*) phase are calculated by a combination of the existing statistical-mechanical approach and NMR theory, based on a rotational diffusion model for two ferroelectric smectogens. The order parameter S2, the smectic tilt angle Theta, and the rotational diffusion coefficient D(perpendicular), corresponding to molecular tumbling in the SmC* phase, for the ferroelectric smectogens 4-[4'-(1-methylheptyloxy)]biphenyl'-(10-undecenyloxy)benzoate and (S)-[4-(2-methylbutyl)phenyl]-4'-n-octylbiphenylcarboxylate have been obtained by means of 2H NMR spectroscopy and these data have been used to calculate viscosity quantities. For practical purposes a specific form of the functional dependence of S4 on S2 is used. As the main result, our calculations also predict a laminar flow regime in a high shear flow for these liquid crystal compounds.

On the role of collective and local molecular fluctuations in the relaxation of proton intrapair dipolar order in nematic 5CB.

We investigate the role that local motions and slow cooperative fluctuations have on the relaxation of the intrapair dipolar order in the nematic 5CB. With this purpose we present a theoretical and experimental systematic study which allow us to quantify the contribution from each type of molecular fluctuation to the intrapair dipolar order relaxation time, T(1D). The experimental work includes measurements of Zeeman and intrapair dipolar order relaxation times (T(1Z) and T(1D)) as a function of temperature at conventional NMR frequencies, in three complementary samples: normal and chain deuterated 4-n-pentyl-4(')-cyanobiphenyl (5CB and 5CB(d11)) and a mixture of normal 5CB and fully deuterated 4-n-pentyl-4'-cyanobiphenyl (5CB(d19)), 50% in weight. Additionally we perform T(1Z) field-cycling Larmor frequency-dependent measurements to obtain the spectral density of the cooperative fluctuations. The obtained results are as follows. (a) The cooperative molecular fluctuations have a strong relative weight in the relaxation of the intrapair dipolar order state, even at Larmor frequencies in the range of conventional NMR. (b) Alkyl chain rotations are an important relaxation mechanism of the intrapair dipolar order at megahertz frequencies. (c) Intermolecular fluctuations mediated by translational self-diffusion of the molecules is not an efficient mechanism of relaxation of the intrapair dipolar order.

Surface polarization and effective anchoring energy in liquid crystals.

The influence of the surface polarization (SP) on the effective anchoring energy for both homeotropic and planar alignments of polar liquid crystals at a solid substrate is discussed from the energy point of view. It is shown that in a certain range of surface charge density the destabilizing SP mechanism may lead to destruction of the homeotropic or planar alignment of liquid crystalline molecules, such as 4-n-pentyl-4(')-cyanobiphenyl.

Dielectric and elastic properties of liquid crystals.

The structural properties, the static and relaxation dielectric coefficients [epsilon(j) and epsilon(j)(omega) (j= ||, perpendicular)], the rotational diffusion constants D( perpendicular) and D( ||), the orientational correlation times tau(1)(i0) (i=0,1), and the bulk elastic constants K(i) (i=1,2,3) are investigated for polar liquid crystals, such as 4-n-pentyl-4(')-cyanobiphenyl (5CB). epsilon(j) are calculated by a combination of the existing molecular theory and statistical-mechanical approach (SMA) that takes into account translational and orientational correlations as well as their coupling, whereas epsilon(j)(omega) are calculated by combining SMA and nuclear magnetic resonance relaxation theory, both based on a rotational diffusion model in which the reorientation of an individual molecule is assumed as stochastic Brownian motion in a potential of mean torque. Reasonable agreement between the calculated and experimental values of epsilon(j) and epsilon(j)(omega) for 5CB is obtained. The bulk Frank elastic constants K(i) (i=1,2,3), for splay, twist, and bend distortion modes, as well as their ratios K(3)/K(1) and K(2)/K(1) are also obtained.

Rotational viscosity, dynamic phenomena, and dielectric properties in a long-chain liquid crystal: NMR study and theoretical treatment.

The rotational diffusion constants D(perpendicular) and D(parallel), rotational viscosity coefficients gamma(i) (i=1,2), the orientational correlation times tau(L)mn, and the dielectric permittivities for nematic liquid crystals (NLCs) are investigated. gamma(i) are calculated by a combination of existing statistical-mechanical approach (SMA) and NMR relaxation theory, both based on a rotational diffusion model. In the rotational diffusion model, it is assumed that the reorientation of an individual molecule is a stochastic Brownian motion in a certain potential of mean torque. According to the SMA, gamma(i) are found to be a function of temperature, density, rotational diffusion constant for tumbling motions, and the orientational order parameters. The order parameters and rotational diffusion constant are obtained from an analysis of NMR measurements. Reasonable agreement between the calculated and experimental values of gamma(i) for 4-n-octyloxy-4'-cyanobiphenyl (8OCB) is obtained. The orientational correlation times, and the longitudinal and transverse components of the real chi'(j)(omega) and imaginary chi"(j)(omega) (j= parallel, perpendicular) parts of the complex susceptibility tensor for 8OCB molecules in the nematic phase are also obtained.

Deuteron NMR study of a long-chain smectic liquid crystal: molecular order and dynamics.

Deuteron Zeeman (T(1Z)) and quadrupolar (T(1Q)) spin-lattice relaxation times and quadrupolar splittings were measured in the nematic and smectic A phases of a chain-deuterated 4-n-octyloxy-4'-cyanobiphenyl (8OCB-d17) at 15.1 and 46 MHz. To model the NMR observables, the so-called pentane effect is used to limit the number of possible conformations in the chain, and is found to be a good approximation. The additive potential method is employed to construct the potential of mean torque using the quadrupolar splittings. A decoupled model is used to describe correlated internal motions in the chain, which are independent of the molecular reorientation. The latter motion is treated using the small-step rotational diffusion model of Nordio (Tarroni and Zannoni), while the former motion is described using a master rate equation. In the nematic phase, order director fluctuations are found to be necessary. The relaxation data in both mesophases were treated using a global target method, and the derived motional parameters are acceptable in comparison with those found for 6OCB.