We reveal that under moderate shear stress (_ 0:1 Pa) red blood cells present an oscillation of their inclination (swinging) superimposed to the long-observed steady tank treading (TT) motion. A model based on a fluid ellipsoid surrounded by a viscoelastic membrane initially unstrained (shape memory) predicts all observed features of the motion: an increase of both swinging amplitude and period (1=2 the TT period) upon decreasing _ , a _-triggered transition toward a narrow _ range intermittent regime of successive swinging and tumbling, and a pure tumbling at low _ values. A human red blood cell (RBC) is a biconcave disk-shaped membrane encapsulating a Newtonian solution of hemoglobin. The membrane is composed by a fluid incom-pressible lipid bilayer underlined by a thin elastic cytoske-leton [1]. This complex structure determines the RBC behavior in shear flow, which greatly influences flow and mass transport in the microcirculation in both health and disease [2]. However, this behavior is not well understood yet and important questions remain open. First, the state of deformation of the elastic membrane at rest is still debated. Does RBC present shape memory (membrane elements of the rim and the dimples nonmechanically equivalent) as recently suggested [3]? Second, flowing RBCs were observed [4-7] only when the cells were suspended in plasma and present an unsteady tumbling (T) solidlike motion [4] or when they are subjected to a high shear stress and exhibit a droplike tank treading (TT) motion characterized by a steady orientation and membrane rotation about the internal fluid. The RBC regime of motion at smaller shear-stress and close to the T-TT transition has not been studied, although it is of crucial importance. Indeed, the simplest models, which treat RBCs like fluid ellipsoids [7-9] retrieve T and TT motions but do not capture the shear-rate dependency of the transition, thus raising the question of the role of the elasticity of the membrane on the cell behavior. Here, by using a recent method of cell imaging parallel to the shear plane [10], we explore the RBC movement close to the T-TT transition. In the TT regime, we reveal that RBCs present an oscillation of their inclination with a period equal to half the TT period that we name swinging (S). We characterize the shear-stress dependence of this oscillation down to the T-TT transition. We show that the transition to pure T is preceded by a narrow critical shear-stress regime where the RBC exhibits an intermittent S-T behavior. Finally, we propose a model, assuming an elastic nonspherical RBC membrane, which captures the main features of the observed behavior. Materials and methods.-Direct measurements of the orientation of the cells with respect to the flow direction (angle) and cell shape (lengths of the long and small axis of the cell cross section, a 1 and a 2 , respectively) are provided from side-view microscopic imaging [10] [Fig. 1(a)]. We varied the wall shear rate _ (in the range 0-5 s ÿ1) and the outer viscosity 0 by suspending RBCs in various solutions of dextran (concentration 6%, 7.5%, or 9% w=w and viscosity 22, 31, and 47 mPa s, respectively). Correspondingly, the wall shear stress, 0 _ , varies in a range from 0 to 0.25 Pa. We observed (i) the motion of flowing RBCs at a fixed 0 _ , (ii) the motion of individual cells at various 0 _ (for 8 RBCs), and (iii) the T-TT transition of 35 cells by increasing and/or decreasing 0 _. FIG. 1. Units i 0 m mPa s, Pa and _ s ÿ1. (a) Schematic drawing of a TT ellipsoid in a shear flow. (b) RBC swinging (_ 1:33, 0 47). Time sequence of 2 s. (c) Rotation of a bead (diameter 1 m) stuck on the membrane of a RBC with (_ 6, 0 47). Time sequence of 1 s. (d) The transition from S to T induced by decreasing _ is associated with a transient localized deformation (0 47, _ 2:66). Time sequence of 1 s.

The formation of soluble hydrogen-bonding interpolymer complexes between poly(acrylic acid) (PAA) and poly(acrylic acid-co-2-acrylamido-2-methyl-1-propane sulfonic acid)-graft-poly(N,N-dimethylacrylamide) (P(AA-co-AMPSA)-g-PDMAM) at pH = 2.0 was studied. A viscometric study showed that in semidilute solution a physical gel is formed due to the interconnection of the anionic P(AA-co-AMPSA) backbone of the graft copolymer, in a transient network, by means of the complexes formed between the PDMAM side chains of the graft copolymer and PAA. Dynamic and static light scattering measurements, in conjunction with small-angle neutron scattering measurements, suggest the formation of core−shell colloidal nanoparticles in dilute solution, comprised by an insoluble PAA/PDMAM core surrounded by an anionic P(AA-co-AMPSA) corona. Even if larger clusters are formed in semidilute solution, the size of the insoluble core remains practically stable. Atomic force microscopy performed under ambient conditions reveal that the particles collapse and flatten upon deposition on a substrate, with dimensions close to the ones of the dry hydrophobic core.

We present the X-ray diffraction, polarizing optical microscopy and electrooptic response studies of the chiral liquid crystal siloxane DSi3-MR11 [1] with a de Vries-type behavior. In a wide temperature range (b.Delta T sim 50°C) the material shows no layer shrinkage but a weak nearly linear layer spacing variation with temperature. Neither discontinuity nor slope change occurs at the SmA*-SmC* transition. In addition, using the X-ray data, the unusual electrooptic response in both 1st and 2nd harmonics together with known thermal behavior of birefringence, we evidence the cross-over from the classical orthogonal to the de Vries-like SmA* regime in DSi3-MR11 siloxane.

Aqueous solutions of poly(vinyl acetate)-b-sodium polyacrylate (PVAc-b-NaPAA) block copolymers were characterized by capillary electrophoresis (CE). CE experiments reveal the presence of NaPAA dead chains, acetate ions (which are a byproduct of the hydrolysis of PVAc), and nonassociated copolymers (unimers). This fraction of unimers is dependent on the chemical composition of the copolymers and on the purification procedure. It remains constant when increasing copolymer concentration. The fraction of unimers was too high to correspond to a critical micellar concentration. Instead, this high content of unimers is likely due to the polydispersity in composition of the copolymers, sodium polyacrylate-rich copolymers being excluded from the micellization process. This interpretation is consistent with the study of the effect of temperature on the free copolymer fraction. Above a temperature threshold, the unimer population increases due to the destabilization of part of the micelles. Addition of a neutral surfactant leads to the formation of mixed micelles, the electrophoretic mobility of which can be modeled using recent theoretical developments on electrokinetic migration of composite objects.

We designed and synthesized new SmA* liquid crystals with different molecular architectures based on the chiral mesogen MR11, which was unsubstituted, or attached at one end and at both ends of a trisiloxane segment in the monosubstituted MSi3-MR11 and disubstituted DSi3-MR11 siloxanes, respectively. We studied the critical divergence of the magnetic susceptibility by measurements of the induced tilt angle Θind at the SmA*–SmC* transition. While all three SmA* liquid crystals presented a de Vries character, this was quite distinctive for DSi3-MR11 for which the critical exponent γ = 1.87 was maximal. X-ray diffraction experiments confirmed that the SmA*–SmC* transition occurred with essentially no layer shrinkage.

Disclinations in nematic liquid crystals usually adopt a straight shape in order to minimize their elastic energy. Once created in the course of a nonequilibrium process such as a temperature quench from the isotropic to the nematic phase, the topologically stable disclinations of half-integer strength either annihilate each other in pairs of opposite strength or form topologically unstable disclinations of integer strength. In this article, we demonstrate that the annihilation process can be inhibited and the defects can be deformed by an applied electric field. We study the disclination lines in the deep uniaxial nematic phase, located at the boundary between two different types of walls, the so-called π wall (a planar soliton stabilized by the surface anchoring) and the Brochard-Léger (BL) wall stabilized by the applied electric field. By changing the electric voltage, one can control the energy of director deformations associated with the two walls and thus control the deformation and dynamics of the disclination line. At small voltages, the disclinations are straight lines connecting the opposite plates of the cell, located at the two ends of the π walls. The π walls tend to shrink. When the voltage increases above EF, the Fréedericksz threshold, the BL walls appear and connect pairs of disclinations along a path complementary to the π wall. At E>2EF, the BL walls store sufficient energy to prevent shrinking of the π walls. Reconstruction of the three-dimensional director configuration using a fluorescent confocal polarizing microscopy demonstrates that the disclinations are strongly bent in the region between the π and the BL walls. The distortions and the related dynamics are associated with the transformation of the BL wall into two surface disclination lines; we characterize it experimentally as a function of the applied electric field, the cell thickness, and the sample temperature. A simple model captures the essential details of the experimental data.

We synthesized a chiral mesogen AC11 and two different siloxanes MSi2-AC11 and DSi3-AC11 derived from it by a hydrosilylation reaction. The effects of varying the molecular architecture were investigated with special attention being devoted to the occurrence of the de Vries SmA* phase. Strong hints that all the three molecules follow the scenario suggested by de Vries were found in the behavior of the susceptibility by studies of their electrooptical response in the SmA* phase. In particular, DSi3-AC11 containing two mesogens interconnected by a siloxane spacer presented a critical exponent γ as high as 1.97, indicating a practically pure de Vries behavior. Temperature-dependent X-ray analysis revealed that the three materials underwent a small layer shrinkage at the SmA*–SmC* transition.