Biomedical subjects
Publications and source records attributed to P Oswald.
In this article we report our systematic studies of the dependence on the sample thickness of the onset parameters of the instability of the nematic-isotropic interface during directional growth and melting, in homeotropic or planar anchoring.
In this paper we discuss the formation and shape of the meniscus between a free-standing film of a smectic-A phase and a wall (in practice the frame that supports the film). The wall may be flat or circular, and the system with or without a reservoir of particles. The formation of the meniscus is always an irreversible thermodynamic process, since it involves the creation of dislocations in the bulk (therefore it involves friction). The four basic shapes of meniscus discussed are the following: exponential, algebraic (x(3/2)), circular, and catenoid. Three principal regions of the whole meniscus must be distinguished: close to the wall with a high density of dislocations, away from the wall with medium density of dislocations, and far from the wall (i.e., close to the film) with a low density of dislocations (vicinal regime). The region with medium density of dislocations is observable using a microscope, and is determined by the competition between surface tension, energy of dislocations, and pressure difference set by the mass of the meniscus or by the reservoir. Its profile is circular as observed in recent experiments [J.-C. Geminard, R. Holyst, and P. Oswald, Phys. Rev. Lett. 78, 1924 (1997)]. By contrast, the vicinal regime with low density of dislocations is never observable with an optical microscope. In the regime with a high density of dislocations, the reasons why the dislocations tend to gather by forming giant dislocations and rows of focal conics are discussed. Finally, we discuss the stability of a smectic film with respect to the formation of a dislocation loop. We show experimentally that the critical radius of the loop is proportional to the curvature radius of the meniscus in its circular part, in agreement with the theory. In addition, we show that the mobility of edge dislocations measured in thick films is in agreement with that found in bulk samples from a creep experiment. This result confirms again our model of the meniscus.
The line tension of a dislocation is measured in a vertical smectic-A film as a function of temperature and film thickness. There are two contributions to the line tension: a bulk contribution that corresponds to the energy of the dislocation in an infinite medium and a surface correction that accounts for interactions with the two free surfaces. Both terms are measured in pure 8CB (octylcyanobiphenyl) as a function of temperature when the bulk nematic-smectic-A transition temperature T(c) is approached.
Localized structures named "fingers" form in the vicinity of the unwinding transition of a cholesteric liquid crystal subjected to an electric field and to homeotropic boundary conditions. Several types of fingers exist, with different static and dynamic properties. For instance, cholesteric fingers of the second species (CF-2) can drift perpendicular to their axes and form spirals in ac electric fields, whereas fingers of the first species (CF-1) crawl along their axes. In this article we show that CF-2's are much easier to nucleate in thick samples (with respect to the pitch) than in thin ones and may form loops like the CF-1's, with or without defects. We show that looped CF-1's always collapse in thick samples at increasing voltage, whereas they can form cholesteric bubbles in thin samples. By contrast, we never observe the formation of a bubble from a loop of a CF-2 except when it possesses a point defect. We also recall that CF-1 segments always collapse at increasing voltage, whereas CF-2 segments systematically give cholesteric bubbles in similar conditions. To qualitatively explain these transformations, we use a simplified representation on the unit sphere S2 of the director field within the fingers. While the CF-1's are described within the standard model of Press and Arrot, we use for the CF-2's a recent model of Gil and Gilli, which we prove to explain most observations. We also describe the growth and collapse dynamics of a loop of a CF-2 in close connection with the spiral dynamics. Finally, we show experimentally and numerically that the CF-2's get abruptly thinner when the electric field exceeds the spinodal limit of the CF-1's. This transformation is reversible, but strongly hysteretic.
We show that the drift velocity of the cholesteric fingers of the second species in ac electric field strongly depends on the conductivity of the liquid crystal chosen and exponentially vanishes above a cutoff frequency we found to be equal (within a numerical factor close to 1) to the charge relaxation frequency of the sample. We further show that the drift velocity is proportional to the applied electric field, provided that the confinement ratio is kept constant.
Superparamagnetic nanoparticles have both high r1 and r2 relaxivities responsible for positive or negative enhancement properties. The aim of this study was to investigate to what extent perfusion (circulating particles) and uptake (clustered particles) mechanisms contribute to liver positive or negative enhancement using two different particles, superparamagnetic iron oxides (ferumoxides, AMI 25) and ultrasmall superparamagnetic iron oxides (ferumoxtran, AMI-227). Uptake kinetics were studied after intravenous injection of 20 micromol Fe/kg ferumoxtran on a washout liver model. Livers of 82 rats were surgically isolated and washed with saline infusion. Imaging was performed ex vivo at 0.5T with T1- and T2-weighted sequences. Enhancement kinetics of the liver were studied in vivo using MRI up to 180 min post injection of 20 micromol Fe/kg ferumoxtran (time response study) or 10, 20, 40 micromol Fe/kg ferumoxtran and 20 micromol Fe/kg ferumoxides (dose response study.) Particle uptake occurred early and resulted in a negative enhancement of the washed livers 15 min after injection of both T1 and T2 sequences. In vivo, a positive enhancement was only seen during the first five min with the lowest dose of ultrasmall superparamagnetic iron oxides and the T1 sequence. Uptake and clustering of the particles induced a negative liver enhancement. During the first minutes after injection, when uptake has not significantly occurred, perfusion imaging of the liver at a dose of 10 micromol Fe/kg results in a positive enhancement with T1-weighted sequences.
Haemorrhage is the major complication of renal angiomyolipoma and is classically treated surgically, but embolization constitutes an alternative treatment. Improvement of catheters and embolization materials now allows highly selective embolization. The authors present a case in which the use of a variable stiffness catheter and platinum microcoils allowed highly selective embolization of a haemorrhagic renal angiomyolipoma while preserving the functional renal parenchyma.
Explore the source record for details and available documents.
A case of hepatosplenic silicosis complicating a pulmonary silicosis is reported. Numerous calcified splenic nodules seen with conventional radiology, ultrasound or computed tomography, hepatic microcalcifications and "egg shell" abdominal lymph nodes best seen with computed tomography represent the radiologic signs. However, hepatosplenic silicosis can only be confirmed by pathological examination of liver biopsies showing birefringent particles within hyalinized nodules.
Explore the source record for details and available documents.
The authors evaluate the merits of computer reconstructions in the coronal and sagittal planes to assess the anatomy of the facial structures. If the patient can be kept motionless, the coronal plane provides information similar to those obtained by a study with direct sections. The ethmoidofrontal recesses, the lamina cribrosa, the thin relief of the middle meatus, the floor of the orbit are perfectly examined. The sagittal plane provides a new approach of the anterior and posterior cortical bone, of the frontal sinus, of the nasofrontal canal, of the ethmoidal cells, of the sphenoid sinus and of the vault of the palate.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
The influence of chlorpromazine hydrochloride on failing, cranially-based dorsal flaps of female rats (Uje:WIST) was studied. The drug was injected subcutaneously into the abdomen pre- and postoperatively. The dosage was 1.5 mg/100 g preoperatively and 0.75 mg/100 g body weight every 12 hours following flap replantation for 10 consecutive days. No improvement in the survival rate was achieved by administration of chlorpromazine hydrochloride in this rat strain.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.