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PubMed1981

Diazepam effects and kinetics in Caucasians and Orientals.

Mental and psychomotor effects and diazepam kinetics were studied in Caucasian and Orientals. 12 Caucasian and 13 Oriental young adults received on one of two occasions, separated by 2 weeks, either 0.2-mg/kg diazepam or saline intravenously. Serum diazepam and desmethyldiazepam concentrations were measured by electron-capture gas-liquid chromatography in samples drawn up to 72 hr after injection. Serum protein binding was measured by equilibrium dialysis. Subjects were tested on a battery of psychological tests before and 0.5, 2, and 4 hr after treatment. While the free fraction of diazepam was identical in both races (0.02), volume of distribution at steady state (Vdss) was different when calculated as absolute volume (Vdss = 76.55 +/- 9.63 l in Caucasians and 54.96 +/- 4.55 l in Orientals, p = 0.04) and marginally significant when corrected for body weight (Vdssl/kg = 1.10 +/- 0.11 in Caucasian and 0.88 +/- 0.05 in Orientals, p = 0.07). total body clearance (Cl), but not elimination half-life (t 1/2), was higher in Caucasians than Orientals, p less than 0.01; t 1/2 = 37.70 +2- 5.53 hr in Caucasians and 41.77 +/- 3.80 in Orientals). Desmethyldiazepam levels were higher in Orientals than Caucasians. Mental and psychomotor effects were maximal at the first session (0.5 hr), followed by complete recovery by the 4-hr session. Effects were similar in both groups. If repeated dosing causes a higher rate of cumulated diazepam serum levels in Orientals, as expected, there might be deeper brain depression in that group.

Adult↗
PubMed2004

Tension between two kinetochores suffices for their bi-orientation on the mitotic spindle.

The movement of sister chromatids to opposite spindle poles during anaphase depends on the prior capture of sister kinetochores by microtubules with opposing orientations (amphitelic attachment or bi-orientation). In addition to proteins necessary for the kinetochore-microtubule attachment, bi-orientation requires the Ipl1 (Aurora B in animal cells) protein kinase and tethering of sister chromatids by cohesin. Syntelic attachments, in which sister kinetochores attach to microtubules with the same orientation, must be either 'avoided' or 'corrected'. Avoidance might be facilitated by the juxtaposition of sister kinetochores such that they face in opposite directions; kinetochore geometry is therefore deemed important. Error correction, by contrast, is thought to stem from the stabilization of kinetochore-spindle pole connections by tension in microtubules, kinetochores, or the surrounding chromatin arising from amphitelic but not syntelic attachment. The tension model predicts that any type of connection between two kinetochores suffices for efficient bi-orientation. Here we show that the two kinetochores of engineered, unreplicated dicentric chromosomes in Saccharomyces cerevisiae bi-orient efficiently, implying that sister kinetochore geometry is dispensable for bi-orientation. We also show that Ipl1 facilitates bi-orientation by promoting the turnover of kinetochore-spindle pole connections in a tension-dependent manner.

Anaphase↗
PubMed2002

The effect of ablation electrode length and catheter tip to endocardial orientation on radiofrequency lesion size in the canine right atrium.

Although the determinants of radiofrequency lesion size have been characterized in vitro and in ventricular tissue in situ, the effects of catheter tip length and endocardial surface orientation on lesion generation in atrial tissue have not been studied. Therefore, the dimensions of radiofrequency lesions produced with 4-, 6-, 8-, 10-, and 12-mm distal electrode lengths were characterized in 26 closed-chested dogs. The impact of parallel versus perpendicular catheter tip/endocardial surface orientation, established by biplane fluoroscopy and/or intracardiac echocardiography, on lesion dimensions was also assessed. Radiofrequency voltage was titrated to maintain a steady catheter tip temperature of 75 degrees C for 60 seconds. With a perpendicular catheter tip/tissue orientation, the lesion area increased from 29 +/- 7 mm2 with a 4-mm tip to 42 +/- 12 mm2 with the 10-mm tip, but decreased to 29 +/- 8 mm2 with ablation via a 12-mm tip. With a parallel distal tip/endocardial surface orientation, lesion areas were significantly greater: 54 +/- 22 mm2 with a 4-mm tip, 96 +/- 28 mm2 with a 10-mm tip and 68 +/- 24 mm2 with a 12-mm tip (all P < 0.001 vs perpendicular orientation). Lesion lengths and apparent volumes were larger with parallel, compared to perpendicular tip/tissue orientations, although lesion depth was independent of catheter tip length with both catheter tip/tissue orientations. Electrode edge effects were not observed with any tip length. Direct visualization using intracardiac ultrasound guidance was subjectively helpful in insuring an appropriate catheter tip/tissue interface needed to maximize lesion size. Although atrial lesion size is critically dependent on catheter tip length, it is more influenced by the catheter orientation to the endocardial surface. This information may also be helpful in designing electrode arrays for the creation of continuous linear lesions for the elimination of complex atrial tachyarrhythmias.

Analysis of Variance↗
PubMed2000

Changes in students' orientations to nursing during nursing education.

The purpose of this study was to describe the changes that took place in students' orientations to nursing during nursing education. The theoretical background of the research lay in the model of nursing students' orientation to nursing, which defines students' orientations in terms of caring, nursing expertise and life orientation. The orientations differ from each other as regards the student's pre-educational caring and nursing experiences, the meaning of caring and nursing and the expectations for a career in nursing. The research was conducted qualitatively, but quantitative measures were also used. The interviews of 19 voluntary nursing students concerning their orientation to nursing both at the beginning and at the end of education were analyzed by content analysis. The categories were derived deductively from the model of nursing students' orientation to nursing. The categories were quantified, and paired t-test and Wilcoxon test were performed to test the significance of the changes in students' orientations between the beginning and the end of education.

Education, Nursing, Baccalaureate↗
PubMed2004

Monte Carlo simulations of antibody adsorption and orientation on charged surfaces.

Monte Carlo simulations were performed to study the adsorption and orientation of antibodies on charged surfaces based on both colloidal and all-atom models. The colloidal model antibody consists of 12 connected beads representing the 12 domains of an antibody molecule. The structure of the all-atom antibody model was taken from the protein databank. The effects of the surface charge sign and density, the solution pH and ionic strength on the adsorption and orientation of different colloidal model antibodies with different dipole moments were examined. Simulation results show that both the 12-bead and the all-atom models of the antibody, for which the dipole moment points from the Fc to (Fab)2 fragments, tend to have the desired "end-on" orientation on positively charged surfaces and undesired "head-on" orientation on negatively charged surfaces at high surface charge density and low solution ionic strength where electrostatic interactions dominate. At low surface charge density and high solution ionic strength where van der Waals interactions dominate, 12-bead model antibodies tend to have "lying-flat" orientation on surfaces. The orientation of adsorbed antibodies results from the compromise between electrostatic and van der Waals interactions. The dipole moment of an antibody is an important factor for antibody orientation on charged surfaces when electrostatic interactions dominate. This charge-driven protein orientation hypothesis was verified by our simulations results in this work. It was further confirmed by surface plasmon resonance biosensor and time-of-flight secondary ion mass spectrometry experiments reported elsewhere.

Adsorption↗
PubMed2006

Nonequilibrium molecular dynamics simulation study on the orientation transition in the amphiphilic lamellar phase under shear flow.

By the extensive large-scale nonequilibrium molecular dynamics simulation on an effective generic model-A2B2 tetramer for amphiphiles, we investigate the shear-induced parallel to perpendicular orientation transition in the lamellar phase as a function of segregation degree and shear rate. Under low rate shear flow the evolution of parallel lamellar configurations at different segregation strengths shows a similar kinetic pathway independent of the segregation degree. While under high rate shear flow in which the lifetime of undulation instability exceeds the characteristic time of the applied shear flow, the kinetic pathway of the shear-induced parallel-to-perpendicular orientation transition in lamellar systems is the segregation degree dependent. Comparing the temporal mesoscopic domain morphology, the microscopic chain conformation, and macroscopic observable-viscosity changes with the experimentally proposed mechanisms, we find that the undulation instability, partial breakup of monodomain, grain rotation, and recombination combined with defect migration and annihilation are the kinetic pathway for the parallel-to-perpendicular orientation transition in the lamellar phase in or near the intermediate segregation limit, and that the undulation instability, domain dissolution, and reformation along the preferred direction combined with defect migration and annihilation are the kinetic pathway for the parallel-to-perpendicular orientation transition in the lamellar phase close to the order-to-disorder phase transition point. A detailed underlying microscopic picture of the alignment process illustrates that the orientation transition is driven by the alignment of molecules with shear flow. The orientation diagram that characterizes the steady-state orientations as a function of shear rate and attractive potential depth is built, in which the attractive potential depth takes the role of an inverse temperature, somewhat like the Flory-Huggins interaction parameter. The microscopic mechanism of the critical orientation transition condition is discussed.

Journal Article↗
PubMed2002

Cognitive interference during competition among volleyball players with different goal orientation profiles.

In this study, we examined interfering thoughts during sport competition among university volleyball players as a function of dominant goal orientation and outcome. In particular, we investigated the performance worries and thoughts of escape experienced while performing in athletes with high task and low ego orientation and athletes with high ego and low task orientation. Goal orientations were assessed before the start of a volleyball tournament, whereas cognitive interference was assessed on three different occasions after games. The results revealed no consistent differences for performance worries. In contrast, in all analyses we found that athletes with high ego and low task orientations reported more thoughts of escape when losing than when winning, and more thoughts of escape than athletes with high task and low ego orientations when winning or losing. The results support in part the suggestion that a high ego orientation, when not accompanied by a high task orientation, can be linked to motivationally maladaptive cognitions.

Adult↗
PubMed1994

Psychosocial predictors of goal orientations in youth soccer.

Little is known about the nature of task and ego orientations that are key motivation constructs. The purpose of this study, therefore, was to examine the extent to which perceived social, contextual, and personal factors predicted the goal orientations of youth sport participants. The sample consisted of 166 male and female adolescent soccer players, who completed self-report measures at the end of a 7-week competitive season. A canonical correlation analysis revealed that the set of predictor variables accounted for 24% of the variance in player goal orientations. Higher scores on perceived soccer competence, perceived parent task orientation, and particularly perceived parent ego orientation were primarily associated with higher scores on player ego orientation. In addition, higher scores on perceived soccer competence, perceived parent task orientation, and perceived mastery climate, as well as lower scores on perceived performance climate, were associated with a higher level of player task orientation. These findings are interpreted and discussed in terms of future research directions.

Adolescent↗
PubMed1989

Changes in microtubule polarity orientation during the development of hippocampal neurons in culture.

Microtubules in the dendrites of cultured hippocampal neurons are of nonuniform polarity orientation. About half of the microtubules have their plus ends oriented distal to the cell body, and the other half have their minus ends distal; in contrast, microtubules in the axon are of uniform polarity orientation, all having their plus ends distal (Baas, P.W., J.S. Deitch, M. M. Black, and G. A. Banker. 1988. Proc. Natl. Acad. Sci. USA. 85:8335-8339). Here we describe the developmental changes that give rise to the distinct microtubule patterns of axons and dendrites. Cultured hippocampal neurons initially extend several short processes, any one of which can apparently become the axon (Dotti, C. G., and G. A. Banker. 1987. Nature [Lond.]. 330:477-479). A few days after the axon has begun its rapid growth, the remaining processes differentiate into dendrites (Dotti, C. G., C. A. Sullivan, and G. A. Banker. 1988. J. Neurosci. 8:1454-1468). The polarity orientation of the microtubules in all of the initial processes is uniform, with plus ends distal to the cell body, even through most of these processes will become dendrites. This uniform microtubule polarity orientation is maintained in the axon at all stages of its growth. The polarity orientation of the microtubules in the other processes remains uniform until they begin to grow and acquire the morphological characteristics of dendrites. It is during this period that microtubules with minus ends distal to the cell body first appear in these processes. The proportion of minus end-distal microtubules gradually increases until, by 7 d in culture, about equal numbers of dendritic microtubules are oriented in each direction. Thus, the establishment of regional differences in microtubule polarity orientation occurs after the initial polarization of the neuron and is temporally correlated with the differentiation of the dendrites.

Animals↗
PubMed1977

Structural analysis of human neutrophil migration. Centriole, microtubule, and microfilament orientation and function during chemotaxis.

Orientation of nucleus, centriole, microtubules, and microfilaments within human neutrophils in a gradient of chemoattractant (5 percent Escherichia coli endotoxin-activated serum) was evaluated by electron microscopy. Purified neutropils (hypaque-Ficoll) were placed in the upper compartment of chemotactic chambers. Use of small pore (0.45 mum) micropore filters permitted pseudopod penetration, but impeded migration. Under conditions of chemotaxis with activated serum beneath the filter, the neutrophil population oriented at the filter surface with nuclei located away from the stimulus, centrioles and associated radial array of microtubules beneath the nuclei, and microfilament-rich pseudopods penetrating the filter pores. Reversal of the direction of the gradient of the stimulus (activated serum above cells) resulted in a reorientation of internal structure which preceded pseudopod formation toward the activated serum and migration off the filter. Coordinated orientation of the entire neutrophil population did not occur in buffer (random migration) or in a uniform concentration of activated serum (activated random migration). Conditions of activated random migration resulted in increased numbers of cells with locomotory morphology, i.e. cellular asymmetry with linear alignment of nucleus, centriole, microtubule array, and pseudopods. Thus, activated serum increased the number of neutrophils exhibiting locomotory morphology, and a gradient of activated serum induced the alignment of neutrophils such that this locomotory morphology was uniform in the observed neutrophil populayion. In related studies, cytochalasin B and colchicines were used to explore the role of microfilaments and microtubules in the neutrophil orientation and migration response to activated serum. Cytochalasin B (3.0 mug/ml) prevented migration and decreased the microfilaments seen, but allowed normal orientation of neutrophil structures. In an activated serum gradient, colchicines, but not lumicolchicine, decreased the orientation of nuclei and centrioles, and caused a decrease in centriole-associated microtubules in concentrations as low as 10(-8) to 10(-7) M. These colchicines effects were associated with the rounding of cells and impairment of pseudopod formation. The impaired pseudopod formation was characterized by an inability to form pseudopods in the absence of a solid substrate, a formation of narrow pseudopods within a substrate, and a defect in pseudopod orientation in an activated serum gradient. Functional studies of migration showed that colchicines, but not lumicolchicine, minimally decreased activated random migration and markedly inhibited directed migration, but had not effect on random migration. These studies show that, although functioning microfilaments are probably necessary for neutrophil migration, intact microtubules are essential for normal pseudopod formation and orientation, and maximal unidirectional migration during chemotaxis.

Blood↗
PubMed1999

Beam-orientation customization using an artificial neural network.

A methodology for the constrained customization of coplanar beam orientations in radiotherapy treatment planning using an artificial neural network (ANN) has been developed. The geometry of the patients, with cancer of the prostate, was modelled by reducing the external contour, planning target volume (PTV) and organs at risk (OARs) to a set of cuboids. The coordinates and size of the cuboids were given to the ANN as inputs. A previously developed beam-orientation constrained-customization (BOCC) scheme employing a conventional computer algorithm was used to determine the customized beam orientations in a training set containing 45 patient datasets. Twelve patient datasets not involved in the training of the artificial neural network were used to test whether the ANN was able to map the inputs to customized beam orientations. Improvements from the customized beam orientations were compared with standard treatment plans with fixed gantry angles and plans produced from the BOCC scheme. The ANN produced customized beam orientations within 5 degrees of the BOCC scheme in 62.5% of cases. The average difference in the beam orientations produced by the ANN and the BOCC scheme was 7.7 degrees (+/-1.7, 1 SD). Compared with the standard treatment plans, the BOCC scheme produced plans with an increase in the average tumour control probability (TCP) of 5.7% (+/-1.4, 1 SD) whilst the ANN generated plans increased the average TCP by 3.9% (+/-1.3, 1 SD). Both figures refer to the TCP at a fixed rectal normal tissue complication probability (NTCP) of 1%. In conclusion, even using a very simple model for the geometry of the patient, an ANN was able to produce beam orientations that were similar to those produced by a conventional computer algorithm.

Humans↗
PubMed1999

Correlates of family-oriented physician communications.

BACKGROUND: Family orientation in patient care has long been one of the primary tenets of the practice of family medicine. Yet we know surprisingly little about how frequently family-oriented transactions occur in actual doctor-patient encounters, or about what other aspects of physician communication patterns might be associated with increased family orientation. The purpose of this study was to investigate both frequency and correlates of family orientation in a residency-based practice. METHODS: Sixty videotapes representing 38 second and third-year residents interviewing a range of multiethnic patients over a 2-year period at a community clinic were analysed for evidence of family-oriented communications, as well as other interaction behaviours such as information exchange and partnership building. Inter-rater agreement was 78%. RESULTS: Asking for medical information, clarifying patient information, and giving medical information and explanations were the most common types of resident actions. Family orientation was much less common, but was more frequently observed than the eliciting of a patient-centered agenda or suggestion of a psychosocial intervention or referral. Family orientation was associated with longer interviews, non-interpreted interviews, more physician questions and clarifying behaviours, and greater tendency to elicit the patient's agenda. CONCLUSIONS: Findings of this investigation suggest that family orientation in the medical interview is enhanced by having more time and a shared language, as well as a generally probing, clarifying, patient-centered style on the part of the physician.

Adult↗
PubMed1994

Estrogen metabolism and excretion in Oriental and Caucasian women.

BACKGROUND: Caucasian and Oriental women have different incidence rates of breast cancer. Among the underlying risk factors for the development of breast cancer in the women of these two groups may be their different diets and patterns of estrogen metabolism and excretion. The absolute levels and relative ratios of 16 alpha-hydroxylated estrogens and 2-hydroxylated estrogens (catechol estrogens) in the body may have a role in the etiology of breast cancer, but studies so far have provided only conflicting results. PURPOSE: Our goal was to study estrogen metabolism, in particular, the extent of 2-hydroxylation and 16 alpha-hydroxylation of estrogens in two groups of women, one Caucasian and one Oriental, with inherently different breast cancer risks. METHODS: Dietary records were analyzed over 3-day periods in the mid-follicular phase, twice, at 6-month intervals for 13 premenopausal Oriental women, recent immigrant arrivals in Hawaii with presumed low risk of breast cancer, and for 12 premenopausal Finnish women with presumed higher risk. The urinary estrogen profile was measured by gas chromatography-mass spectrometry and plasma and fecal estrogens were assayed by chromatographic radioimmunoassays. RESULTS: Mean fat intake per 1000 kcal was 73% higher (P < .001) in the Finnish women, but the mean fiber intake and fecal weights were similar to those of the Oriental women. Compared with Oriental women, Finnish women had 46% higher plasma estradiol (P < .01) and 124% higher plasma estrone sulfate (P < .01); however, after adjustment for differences in age and body mass index, only the difference in estrone sulfate remained statistically significant (P < .05). Mean plasma levels of estrone and estradiol correlated with height after adjustment for body mass index (P < .05). Mean plasma levels of estrone and sex hormone-binding globulin were similar. The Finns had higher mean urinary estrone (193%), estradiol (166%), various catechol estrogens (130%-439%), and total estrogen excretion (123%) (all P < .001), but similar 16 alpha-hydroxylated estrogen excretion. As calculated, 16 alpha-hydroxylation of estrone was significantly increased (P < .01) in the Oriental women, but 2-hydroxylation, 4-hydroxylation, and 16 beta-hydroxylation of estrone were similar in both groups. The ratio of catechol estrogen to 16 alpha-hydroxylated estrogen was four to five times higher (P < .001) in the Finnish women. The Oriental women had two to three times higher fecal excretion of estrogens than the Finnish women (P < .01). CONCLUSIONS: Our results indicate that high catechol estrogen formation may be a greater risk factor for breast cancer than high 16 alpha-hydroxylation of estrogens. However, the main risk factor for the Finnish women, as opposed to the Oriental women, may be their higher estrogen levels that result from a higher fat diet, higher estrogen production related to their greater height, and lower fecal estrogen excretion.

Adult↗
PubMed2003

The trochlea is bilinear and oriented medially.

Malfunctioning of total knee replacements often is related to patellofemoral problems. Because the trochlea guides the patella during flexion and extension, its geometry has a major influence in patellofemoral problems. There is controversy in the literature: relative to the mechanical axis, some authors have found a laterally oriented trochlea and others have found a medially oriented trochlea. The groove of implanted prosthetic femoral components always has lateral or neutral orientations. The objectives of the current study were to clarify the controversy found in the literature, to determine whether the trochlear orientation is truly linear, and to determine whether the orientation depends on the size of the femur. The trochleae of 100 human femurs were measured using a three-dimensional measurement system. Detailed analysis of the results indicated that the trochlea is best described as bilinear, with the distal half oriented 0.2 degrees +/- 2.8 degrees laterally and the proximal half oriented 4.2 degrees +/- 3.2 degrees medially. Trochlear orientation was not dependent on bone size.

Cadaver↗
PubMed2005

Posterior instrumentation reduces differences in spine stability as a result of different cage orientations: an in vitro study.

STUDY DESIGN: A multisegmental cadaveric spine model was used to quantify the load-displacement behavior of intact spine specimens, specimens injured and stabilized using Bagby and Kuslich (BAK) cages as lumbar interbody fusion devices with or without posterior instrumentation across two levels. OBJECTIVES: To compare the stabilities imparted by the cages placed using an oblique and conventional posterior approaches and to determine the effects of supplementary posterior instrumentation. SUMMARY OF BACKGROUND DATA: The BAK cage as posterior lumbar interbody fusion (PLIF) has been used to restore disc height, reduce morbidity, provide immediate stability to the patients, and enhance fusion rates. The obliquely inserted BAK cage has the advantages of reducing exposure and precise implantation. The biomechanical efficacy of this procedure is sparse, especially in comparison to the PLIF with and without posterior instrumentation. METHODS: Nine fresh human ligamentous spines (L2-S1) were affixed within a testing frame for determining their load-displacement behaviors. Load testing in clinically relevant modes was performed sequentially for the intact and the following procedures across the L4-S1 segment: posterior destabilization, stabilization using two parallel BAK cages (CBAK group) or one oblique BAK cage (OBAK group), further stabilization with posterior instrumentation, and finally cyclic loading in flexion-extension. Spatial positions of the LEDs attached to vertebral bodies were recorded using a three-dimensional motion measurement system. RESULTS: When used alone to restore stability, the orientation of the cage affected the outcome. In flexion OBAK orientation and in extension CBAK orientation provided better stability (decrease in motion with respect to intact case), compared with the other orientation. In lateral bending, CBAK orientation was found to be better than OBAK. In axial mode, CBAK orientation was effective in both directions while OBAK was effective only in right axial rotation. With the supplementary posterior fixation, the differences in stability resulting from the orientations were not noticeable at all, both before and after cyclic tests. CONCLUSIONS: Owing to the differences in the surgical approach and the amount of dissection, the stability for the cages when used alone as a function of cage orientation was different. These subtle differences were reduced by the use of posterior fixation device, underscoring the importance of using instrumentation when cages are used as PLIFs. However, the oblique insertion may be more favorable since it requires less exposure, enables precise implantation, and is less expensive, especially when used with supplementary instrumentation.

Aged↗
PubMed1991

Asymmetries in oriented-line detection indicate two orthogonal filters in early vision.

Visual detection of a line target differing in orientation from a background of lines may be achieved speedily and effortlessly. Such performance is assumed to occur early in vision and to involve filter mechanisms acting in parallel over the visual field. This study establishes orientational limits on this performance and analytically derives some generic properties of the underlying filters. It was found that, in brief displays, target orientation detection thresholds increased approximately linearly with background orientation, from minima at 0 degree (vertical) and 90 degrees, whereas background orientation detection thresholds decreased approximately linearly with target orientation, from maxima at 0 degree and 90 degrees. Target and background threshold functions were exactly antisymmetric. These data are shown to indicate a model of early line processing dominated by two classes of orientation-sensitive filter with axes close to the vertical and horizontal and orientation-tuning half-widths each of approximately 30 degrees at half-height.

Adult↗
PubMed1991

Horizontal-vertical filters in early vision predict anomalous line-orientation identification frequencies.

A characteristic of early visual processing is a reduction in the effective number of filter mechanisms acting in parallel over the visual field. In the detection of a line target differing in orientation from a background of lines, performance with brief displays appears to be determined by just two classes of orientation-sensitive filter, with preferred orientations close to the vertical and horizontal. An orientation signal represented as a linear combination of responses from such filters is shown to provide a quantitative prediction of the probability density function for identifying the perceived orientation of a target line. This prediction was confirmed in an orientation-matching experiment, which showed that the precision of orientation estimates was worst near the vertical and horizontal and best at about 30 degrees each side of the vertical, a result that contrasts with the classical oblique effect in vision, when scrutiny of the image is allowed. A comparison of predicted and observed frequency distributions showed that the hypothesized orientation signal was formed as an opponent combination and horizontal and vertical filter responses.

Filtration↗
PubMed2002

Orientational structure of dipolar hard-spherical colloids.

We have studied the orientational structure of a dipolar hard-spherical colloid on a homogeneous isotropic phase. The results are expressed as a function of the dipolar strength mu and volume fraction phi of dipolar colloids, and the refractive index of the scattering medium, n(s). The study is based on the self-correlation of the orientation density of the dipolar colloids, which is the static orientational structure factor [F(q)], where q is the wave vector. The importance of this quantity is that for very low phi values, it can be probed in a depolarized light scattering experiment. We have found that the structure of the suspension is better observed for high n(s). F(q) presents a different behavior for dilute and dense concentrations, it is also observed that the position of its minimum depends on phi. The response of a dipolar colloid due to its collective orientational behavior is also studied, using as an "ordering parameter" the static orientational structure factor at q=0[F(q=0)]. The study is performed for isochores as a function of mu. We have divided the analysis into five regimes, from very low to very high phi; values, i.e., phi=0.005 24, 0.1, 0.2, 0.35, and 0.45. Our analysis suggests that the dipolar colloid evolves to an orientationally ordered phase when the dipolar strength is increased, for all concentrations except for the lowest value case, phi=0.005 24. When phi=0.1 the dipolar colloid reaches the transition suddenly, whereas for the very low regime, the slope of F(q=0) first increases as if the dipolar colloid would evolve to an orientationally ordered phase; but near the transition the slope is inverted, resulting in a no global orientational order. Thus, our results suggest that in the very low regime a dipolar colloid may have a reentrant transition.

Journal Article↗