Liquid crystal phase transitions in dispersions of rod-like colloidal particles

The isotropic-nematic (I-N) phase transition in dispersions of sterically stabilized rod-like boehmite (A1OOH) colloids is studied. We have examined the influence of the steric stabilizer, the dispersion medium and the presence of non-adsorbing polymer on the phase transition process. Dispersions in cyclohexane show an I-N phase separation that proceeds by a slow sedimentation of a pinned structure, shrinking from the meniscus, finally forming the nematic phase after weeks or months, depending on the steric stabilizer used. In toluene the onset of the I-N has shifted to higher volume fractions where individual nematic droplets grow and sediment, forming the nematic phase after one week. By adding non-adsorbing polymer to dispersions in cyclohexane the onset of the I-N phase separation shifts to lower colloid volume fractions. At polymer concentrations just above the phase boundary the same scenario as in the toluene dispersions without added non-adsorbing polymer is observed. At slightly higher polymer concentrations an abrupt change in scenario occurs. Now an interconnected network is formed which starts to sediment, resembling the process in pure cyclohexane dispersions. Clearly small variations in colloidal interactions engendered by the changes in dispersion characteristics considered in this work, have a strong influence on the I-N phase transition process

Hard-core Yukawa model for two-dimensional charge stabilized colloids

The hyper-netted chain (HNC) and Percus-Yevick (PY) approximations are used to study the phase diagram of a simple hard-core Yukawa model of charge-stabilized colloidal particles in a two-dimensional system. We calculate the static structure factor and the pair distribution function over a wide range of parameters. Using the statics correlation functions we present an estimate for the liquid-solid phase diagram for the wide range of the parameters.Comment: 7 pages, 9figure

Patient-Reported Outcomes and Function after Surgical Repair of the Ulnar Collateral Ligament of the Thumb

Purpose: The purpose of this study was to report prospectively collected patient-reported outcomes of patients who underwent open thumb ulnar collateral ligament (UCL) repair and to find risk factors associated with poor patient-reported outcomes. Methods: Patients undergoing open surgical repair for a complete thumb UCL rupture were included between December 2011 and February 2021. Michigan Hand Outcomes Questionnaire (MHQ) total scores at baseline were compared to MHQ total scores at three and 12 months after surgery. Associations between the 12-month MHQ total score and several variables (i.e., sex, injury to surgery time, K-wire immobilization) were analyzed. Results: Seventy-six patients were included. From baseline to three and 12 months after surgery, patients improved significantly with a mean MHQ total score of 65 (standard deviation [SD] 15) to 78 (SD 14) and 87 (SD 12), respectively. We did not find any differences in outcomes between patients who underwent surgery in the acute (&lt;3 weeks) setting compared to a delayed setting (&lt;6 months). Conclusions: We found that patient-reported outcomes improve significantly at three and 12 months after open surgical repair of the thumb UCL compared to baseline. We did not find an association between injury to surgery time and lower MHQ total scores. This suggests that acute repair for full-thickness UCL tears might not always be necessary. Type of study/level of evidence: Therapeutic II.</p

Phase behaviour and dynamics of suspensions of hard colloidal platelets

In this thesis we aim to provide a many-sided answer to the question: what are the consequences of plate-like colloidal shape on a suspensions' physical properties? A central role in this investigation is played by the experimental model system of platelets which, building on the Van 't Hoff Laboratory's rich tradition with respect to synthesizing colloidal model systems, was developed for this purpose. Consisting of fairly monodisperse sterically stabilised gibbsite platelets, this system allows us to focus on the effect of plate-like particle shape while keeping the shape, size and interactions well-defined. In part I, we consider the dynamical properties of dilute suspensions. This yields the first results for the viscosity, sedimentation, and diffusion behaviour of such a platelet suspension as a function of the volume fraction. It will be interesting to see whether these experiments will inspire theoretical efforts towards calculating the f-coefficients in the viscosity, sedimentation, and diffusion, for which no predictions are yet available. In part II, we show that these suspensions of approximately hard platelets exhibit an isotropic to nematic phase transition. While being predicted by the Onsager theory and computer simulations, this observation clearly contrasts with the ubiquitous (but poorly understood) gel formation in suspensions of clay platelets. Furthermore, increasing the platelets' polydispersity leads to a broadening of the I-N coexistence region and the appearance of pronounced fractionation. We demonstrate that a remarkable feature of the observed I-N transition in these more polydisperse systems -where the nematic phase becomes the upper phase- can be explained on the basis of strong fractionation with respect to platelet thickness. Moreover, at high particle densities, the suspensions exhibit a nematic to columnar phase transition. Although predicted by simulations for monodisperse platelets, the stability of a columnar phase in suspensions with up to 25% polydispersity in diameter is remarkable in the light of the so-called terminal polydispersity for hard sphere crystallisation and hard rod smectisation. The stability of the columnar phase in these polydisperse systems is probably connected with the fact that the platelets are polydisperse in thickness too, which suppresses the relative stability of the competing liquid crystalline phase: the smectic. In the case of 25% polydispersity in diameter the columnar structure breaks down and a smectic-like structure appears upon increasing the density, whereas the columnar structure prevails for a lower value of the diameter polydispersity. In part III, we study the phase behaviour of mixtures of plate-like colloids and either non-adsorbing polymer or rod-like colloids. The phase behaviour of these mixtures is even richer than that of the pure platelet suspensions, as we find the appearance of an additional isotropic phase (in plate-polymer mixtures) and two additional (rod-rich) liquid crystalline phases (in rod-plate mixtures). The observed topology of these phase diagrams, which include the coexistence of up to five different phases, can be explained by the interplay between the employed types of species (plate, rod, or polymer) and their polydispersity. Looking back on the properties of the model system investigated in this thesis, it is clear that we have uncovered a number of features which are both fascinating and of fundamental interest. At the same time however, we must recognise that the role of the parameters varied in this study -such as polydispersity, attraction, and the presence of added colloidal species- does not yet allow us to understand the behaviour exhibited by the much more comple

Formation of nematic liquid crystals in suspensions of hard colloidal platelets

A novel model system of hard colloidal platelets was observed to phase-separate into an isotropic and a liquid crystalline phase. Polarization microscopy revealed that the liquid crystalline phase was of nematic origin. With such orientational ordering in suspensions of platelike particles already being predicted in the 1940s by Onsagerâ's theory and corroborated more recently by computer simulations, a direct comparison with experimental observations has now become possible. Furthermore, the apparently unhindered formation of a macroscopic nematic phase in a suspension of hard platelets sheds new light on the issue of gelation instead of nematic phase formation in suspensions of clay platelets

Liquid-crystal phases formed in mixed suspensions of rod- and platelike colloids

The phase behavior of rod-plate mixtures was investigated using model systems of unambiguously rod- and plate-shaped colloids. Neither at equilibrium nor in the early stages of the phase separation process do we observe the theoretically disputed biaxial nematic phase. Instead, the mixtures show demixing into an isotropic and a separate rod- and a plate-dominated uniaxial nematic phase. The high-concentration regime of the rod-plate phase diagram is exceptionally rich because of the appearance of two extra liquid crystal phases: a rod-rich (presumably) nematic phase and a plate-rich columnar phase. This leads to the appearance of several four-phase equilibria and even a five-phase equilibrium. We explain the observed topology of the phase diagram by the interplay between the rod- and platelike shape of the particles and their polydispersity

Liquid-crystalline phase behavior of a colloidal rod-plate mixture

The phase behavior of rod-plate mixtures was investigated using model systems containing unambiguously rod- and plate-shaped colloids. We find that the theoretically disputed biaxial nematic phase is unstable with respect to demixing into an isotropic and two uniaxial nematic phases. The phase behavior at very high densities is exceptionally rich and includes the coexistence of up to four different liquid crystalline phases, which stem from the coupling between the employed particle shapes and polydispersity

Isotropic-nematic phase separation in suspensions of polydisperse colloidal platelets

We are studying the phase behavior of a suspension of platelike colloids which has a very broad size distribution, particularly in thickness. This suspension exhibits an isotropic-nematic phase separation over a notably wide range of particle concentrations, displaying a remarkable phenomenon. In part of the coexistence region, phase separation yields a nematic upper phase in coexistence with an isotropic bottom phase. If the nematic phase is isolated and diluted, the reverse situation is observed such that the isotropic phase now becomes the upper phase. We show that these phenomena can be explained by a pronounced fractionation with respect to platelet thickness

Liquid crystal phase transitions in suspensions of polydisperse plate-like particles

Colloidal suspensions that form periodic self-assembling structures on sub-micrometre scales are of potential technological interest; for example, three-dimensional arrangements of spheres in colloidal crystals might serve as photonic materials, intended to manipulate light. Colloidal particles with non-spherical shapes (such as rods and plates) are of particular interest because of their ability to form liquid crystals. Nematic liquid crystals possess orientational order; smectic and columnar liquid crystals additionally exhibit positional order (in one or two dimensions respectively). However, such positional ordering may be inhibited in polydisperse colloidal suspensions. Here we describe a suspension of plate-like colloids that shows isotropic, nematic and columnar phases on increasing the particle concentration.We find that the columnar two-dimensional crystal persists for a polydispersity of up to 25%, with a cross-over to smectic-like ordering at very high particle concentrations. Our results imply that liquid crystalline order in synthetic mesoscopic materials may be easier to achieve than previously thought