On some elastic instabilities in biaxial nematics

Within the framework of the continuum elastic theory of biaxial nematic liquid crystals, we have addressed ourselves to the structure, stability and energetics of some singular and non-singular topological defects, and certain director configurations. We find that certain non-singular hybrid disclinations could be energetically favourable relative to certain half-strength disclinations. The interaction between singular hybrids depends strongly on the biaxial elastic anisotropy. We suggest possible defect structures that can exist in spherical droplets of biaxial nematics. Further we find structural instabilities, in confined geometries, arising due to the inherent biaxiality of the system

Influence of shear flow on vesicles near a wall: a numerical study

We describe the dynamics of three-dimensional fluid vesicles in steady shear flow in the vicinity of a wall. This is analyzed numerically at low Reynolds numbers using a boundary element method. The area-incompressible vesicle exhibits bending elasticity. Forces due to adhesion or gravity oppose the hydrodynamic lift force driving the vesicle away from a wall. We investigate three cases. First, a neutrally buoyant vesicle is placed in the vicinity of a wall which acts only as a geometrical constraint. We find that the lift velocity is linearly proportional to shear rate and decreases with increasing distance between the vesicle and the wall. Second, with a vesicle filled with a denser fluid, we find a stationary hovering state. We present an estimate of the viscous lift force which seems to agree with recent experiments of Lorz et al. [Europhys. Lett., vol. 51, 468 (2000)]. Third, if the wall exerts an additional adhesive force, we investigate the dynamical unbinding transition which occurs at an adhesion strength linearly proportional to the shear rate.Comment: 17 pages (incl. 10 figures), RevTeX (figures in PostScript

Hydrodynamics of smectic-C liquid crystals: field and flow induced instabilities in confined geometries

Following the Ericksen-Leslie approach, we formulate a complete nonlinear macroscopic theory of the isothermal hydrodynamics of smectic-C liquid crystals. We assume an asymmetric stress tensor and incorporate the essential features of a hydrodynamic theory of a smectic phase, i.e., permeation and variations in layer spacing. Using Onsager’s reciprocity relations, we find that entropy production is described by 16 viscosity coefficients and a permeation constant associated with the dissipative dynamics of the layered system. We study the reorientation dynamics of the c vector under the destabilizing influence of an external field. We stress that permeation is important and that transverse flows along and normal to the layers exist. We have also studied certain instabilities that can arise in shear flows. As a consequence of permeation, in Poiseuille flow with the layers parallel to the plates, we find that the length of the inlet section can be very large being many times the lateral dimension. When the layers are perpendicular to the plates, an analog of the nematic Hall effect is shown to exist even in the absence of an aligning external field

Dynamics of kinks in smectic-C liquid crystals in periodically varying external fields

We have considered the dynamics of kinks in smectic-C liquid crystal in uniformly rotating and oscillating electric or magnetic fields. In tilted oscillating fields our results are at variance with those of earlier investigations which predict chaos in this geometry for a π kink. On the other hand, we find that the velocity and the structure of a π kink are oscillatory with the frequency of the varying field. The average velocity as a function of the tilt angle of the field indicates a threshold angle beyond which there is a crossover in velocity selection with the system choosing from a family of solutions. Surprisingly, in a nonoscillating field rotating in a plane normal to the layers, a π kink has a drift velocity whose direction is dependent upon the sense of rotation. As a result of this, a 2π kink could be in a bound oscillating state or it could split itself into two oppositely drifting oscillating π kinks. In a nonoscillating tilted field rotating about the layer normal, in the synchronous regime we find an instability of a kink connecting a stable state and a metastable state. In the asynchronous regime, a kink connecting two stable states has aperiodic motion. Interestingly, in the same geometry in oscillating fields, we find that a kink joining a stable state and a metastable state is more stable. In this geometry, periodic and aperiodic fluctuating kink structures are also possible. We have suggested a simple way of understanding the general dynamical features of a kink on the basis of the dynamics of the uniform director state which is found to be very sensitive to parameters of the system

Robustness of FTIR-Based Ultrarapid COVID-19 Diagnosis Using PLS-DA

The World Health Organization (WHO) declared the Omicron variant (B.1.1.529) of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the pathogen responsible for the Coronavirus disease 2019 (COVID-19) pandemic, as a variant of concern on 26 November 2021. By this time, 42% of the world’s population had received at least one dose of the vaccine against COVID-19. As on 1 October 2022, only 68% of the world population got the first dose of the vaccine. Although the vaccination is incredibly protective against severe complications of the disease and death, the highly contagious Omicron variant, compared to the Delta variant (B.1.617.2), has led the whole world into more chaotic situations. Furthermore, the virus has a high mutation rate, and hence, the possibility of a new variant of concern in the future cannot be ruled out. To face such a challenging situation, paramount importance should be given to rapid diagnosis and isolation of the infected patient. Current diagnosis methods, including reverse transcription-polymerase chain reaction and rapid antigen tests, face significant burdens during a COVID-19 wave. However, studies reported ultrarapid, reagent-free, cost-efficient, and non-destructive diagnosis methods based on chemometrics for COVID-19 and COVID-19 severity diagnosis. These studies used a smaller sample cohort to construct the diagnosis model and failed to discuss the robustness of the model. The current study systematically evaluated the robustness of the diagnosis models trained using smaller (real and augmented spectra) and larger (augmented spectra) datasets. The Monte Carlo cross-validation and permutation test results suggest that diagnosis using models trained by larger datasets was accurate and statistically significant (Q2 > 99% and AUROC = 100%)

Persistence of Symptoms after Acute COVID-19 Infection- An Experience from a Tertiary Care Centre in South India

Introduction: COVID-19 pandemic is posing a challenge not only with sheer number of people infected but also with the large number of patients with persistent symptoms of COVID19 infection. A proper understanding of the magnitude and associated factors of persistent COVID-19 symptoms will go a long way in planning treatment and control strategies. Aim: To determine the proportion of patients who have persistent symptoms post-acute COVID-19 infection and to determine the factors associated with it, among those who have been discharged from Government Medical College, Thrissur. Materials and Methods: The current cross-sectional study was conducted among 335 patients who were admitted and discharged with COVID-19 infection in Government Medical College, Thrissur, Kerala, a tertiary care institution in southern India between December 2020 and February 2021. They were contacted through a telephonic interview 28 days from symptom onset through a semi-structured interview schedule. The questions included basic demographic details, symptomatology at admission, persistent symptoms at 28 days after onset and other clinical details including comorbidities. For defining postCOVID-19 symptom persistence National Institute for Health and Care (NICE) guidelines were used. Association between persistent symptoms and selected factors was done by Chisquare test. Results: Out of the 335 patients, the persistence of symptoms of COVID-19 infection after 28days of symptom onset was 221 (66%) C.I -60.7-70.8%). Persistence of two or more COVID-19 symptoms after 28 days of onset was seen in 120 (35.8%). The most common persistent symptoms among the patients were fatigue in 109 (32.5%) of people followed by dyspnoea in 77 (23%), cough in 45 (13.4%) and myalgia in 37 (11%) patients Highest persistence was seen in Category C patients where symptoms were persisting in 75%. Persistence was also higher in those with diabetes mellitus, those who received oxygen support and those who were in Intensive Care Unit (ICU) or ventilator and the association was statistically significant(p-value<0.05). Conclusion: The study shows that 2/3rd of patients still continues to have persistent symptoms even after 28 days of symptom onset. Health systems should be prepared to face the consequences of morbidities caused by post COVID-19 syndrome