Papers by Rema Krishnaswamy
arXiv (Cornell University), Aug 19, 2020
Mixed surfactant systems show many interesting phases such as the random mesh phase consisting of... more Mixed surfactant systems show many interesting phases such as the random mesh phase consisting of a disordered array of defects (water-filled nano-pores in the bilayers). The present study addresses the non-equilibrium phase transition of the random mesh phase under shear to an ordered mesh phase with high degree of coherence between defects in three dimensions. In-situ small-angle synchrotron X-ray diffraction under different shear stress conditions shows sharp Bragg peaks in X-ray diffraction, successfully indexed to the rhombohedral lattice with the R3m space group symmetry. The ordered mesh phase shows isomorphic twinning and buckling at higher shear stress. Our experimental studies bring out rich phase transitions in concentrated mixed surfactant systems hitherto not well explored and provide motivation for a quantitative understanding.
Langmuir, Feb 24, 2007
We report the observation of an intermediate mesh phase with rhombohedral symmetry, corresponding... more We report the observation of an intermediate mesh phase with rhombohedral symmetry, corresponding to the space group R3 hm, in a mixed surfactant system formed by the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the organic salt 3-sodium-2-hydroxy naphthoate (SHN). It occurs between a random mesh phase (L R D) and a lamellar phase (L R) at low temperatures; at higher temperatures, the L R D phase transforms continuously into the L R phase with an increasing surfactant concentration (φ s). To separate the effects of salt and φ s on the phase behavior, the ternary system consisting of cetyltrimethylammonium 3-hydroxy-naphthalene-2-carboxylate (CTAHN), sodium bromide (NaBr), and water was studied. The intermediate mesh phase is found in this system at high NaBr concentrations. The micellar aggregates, both in the intermediate and random mesh phases, are found to be made up of a twodimensional network of rod-like segments, with three rods meeting at each node. The average mesh size increases with φ s , and the transition from the random mesh phase to the intermediate phase is found to occur when it is approximately 1.5 times the lamellar periodicity. The intermediate mesh phase is absent in the equimolar dodecyltrimethylammonium bromide (DTAB)-SHN system, indicating the role of the surfactant chain length in the formation of this phase. This system exhibits a random mesh phase over a very wide range of water content, with the average mesh size decreasing upon an increasing φ s , contrary to the trend seen in the CTAB-SHN system.
Experimental signatures of a nonequilibrium phase transition near the crossover point of a Langmuir monolayer
Journal of Physics: Condensed Matter, Oct 3, 2019
We investigate the response of the two-dimensional (2D) continuous non-particulate film of surfac... more We investigate the response of the two-dimensional (2D) continuous non-particulate film of surfactant sorbitan tristearate confined at the air-water interface under oscillatory shear deformation. The time dependence of various rheological parameters show critical-like behavior at a value of strain amplitude close to the crossover point of elastic (G') and viscous (G'') shear moduli. Imposing oscillatory shear of different strain amplitudes (γ0) above and below the crossover strain amplitude (γf) over a large number of cycles, we quantify the temporal dependence of interfacial viscous modulus, phase angle (G''/G') as well as higher harmonic components of stress. The number of shear cycles (τR) required for these quantities to reach the steady state value diverges near γf. The steady state values of the third harmonic (I3/I1) show order parameter like behavior indicating the importance of higher order harmonics near the nonequilibrium transition. We further show that the energy dissipation per cycle per unit volume has a marked change near γf, consistent with continuum level nonequilibrium shear-transformation-zone (STZ) model of amorphous viscoplasticity.
arXiv (Cornell University), Aug 14, 2020
Mixed surfactant systems with strongly bound counterions show many interesting phases such as the... more Mixed surfactant systems with strongly bound counterions show many interesting phases such as the random mesh phase consisting of a disordered array of defects (water-filled nano-pores in the bilayers). The present study addresses the non-equilibrium phase transition of the random mesh phase under shear to an ordered mesh phase with a high degree of coherence between nano-pores in three-dimension. In-situ small-angle synchrotron X-ray study under different shear stress conditions shows sharp Bragg peaks in the X-ray diffraction, successfully indexed to the rhombohedral lattice with R3m space group symmetry. The ordered mesh phase shows isomorphic twinning and buckling at higher shear stress. Our experimental studies bring out rich non-equilibrium phase transitions in concentrated cationic surfactant systems with strongly bound counterions hitherto not wellexplored and provide motivation for a quantitative understanding.
Interfacial Rheology of an Ultrathin Nanocrystalline Film Formed at the Liquid/Liquid Interface
Langmuir, 2007
We report the interfacial properties of monolayers of Ag nanoparticles 10-50 nm in diameter forme... more We report the interfacial properties of monolayers of Ag nanoparticles 10-50 nm in diameter formed at the toluene-water interface under steady as well as oscillatory shear. Strain amplitude sweep measurements carried out on the film reveal a shear thickening peak in the loss moduli (G") at large amplitudes followed by a power law decay of the storage (G') and loss moduli with exponents in the ratio 2:1. In the frequency sweep measurements at low frequencies, the storage modulus remains nearly independent of the angular frequency, whereas G" reveals a power law dependence with a negative slope, a behavior reminiscent of soft glassy systems. Under steady shear, a finite yield stress is observed in the limit of shear rate .gamma going to zero. However, for .gamma > 1 s-1, the shear stress increases gradually. In addition, a significant deviation from the Cox-Merz rule confirms that the monolayer of Ag nanoparticles at the toluene-water interface forms a soft two-dimensional colloidal glass.
This is a review of the book “Beautiful, Simple, Exact, Crazy” written by Apoorva Khare and Anna ... more This is a review of the book “Beautiful, Simple, Exact, Crazy” written by Apoorva Khare and Anna Lachowska. The authors write in the preface that this book arose out of an introductory course called Mathematics in the Real World which they co-designed (and taught at Stanford and Yale University, respectively). The target audience of that course consisted mainly of undergraduates of humanities and social sciences – students whose principal interests lay outside of mathematics and the sciences
Langmuir, 2021
Mixed surfactant systems with strongly bound counterions show many interesting phases such as the... more Mixed surfactant systems with strongly bound counterions show many interesting phases such as the random mesh phase consisting of a disordered array of defects (water-filled nano-pores in the bilayers). The present study addresses the non-equilibrium phase transition of the random mesh phase under shear to an ordered mesh phase with a high degree of coherence between nano-pores in three-dimension. In-situ small-angle synchrotron X-ray study under different shear stress conditions shows sharp Bragg peaks in the X-ray diffraction, successfully indexed to the rhombohedral lattice with R3m space group symmetry. The ordered mesh phase shows isomorphic twinning and buckling at higher shear stress. Our experimental studies bring out rich non-equilibrium phase transitions in concentrated cationic surfactant systems with strongly bound counterions hitherto not wellexplored and provide motivation for a quantitative understanding.
Aggregation of a Peptide Antibiotic Alamethicin at the Air−Water Interface and Its Influence on the Viscoelasticity of Phospholipid Monolayers
Langmuir, Oct 1, 2008
The aggregation properties of an antibiotic membrane-active peptide alamethicin at the air-water ... more The aggregation properties of an antibiotic membrane-active peptide alamethicin at the air-water interface have been studied using interfacial rheology and fluorescence microscopy techniques. Fluorescence microscopy of alamethicin monolayers revealed a coexistence of liquid expanded (LE) and solid phases at the surface concentrations studied. Interfacial oscillatory shear measurements on alamethicin monolayers indicate that its viscoelastic properties are determined by the area fraction of the solid domains. The role of zwitterionic phospholipids dioleoylphosphatidyl choline (DOPC) and dioleoylphosphatidyl ethanolamine (DOPE) on the peptide aggregation behavior was also investigated. Fluorescence microscopy of alamethicin/phospholipid monolayers revealed an intermediate phase (I) in addition to the solid and LE phase. In mixed monolayers of phospholipid (L)/alamethicin (P), with increase in L/P, the monolayer transforms from a viscoelastic to a viscous fluid with the increase in area fraction of the intermediate phase. Further, a homogeneous mixing of alamethicin/lipid molecules is observed at L/P > 4. Our studies also confirm that the viscoelasticity of alamethicin/phospholipid monolayers is closely related to the alamethicin/phospholipid interactions at the air-water interface.
Physical Review E Statistical Nonlinear and Soft Matter Physics, Mar 1, 2006
We have probed structures of the complexes formed by DNA with the cationic surfactant cetyltrimet... more We have probed structures of the complexes formed by DNA with the cationic surfactant cetyltrimethylammonium bromide in the presence of the cosurfactant hexanol, using small angle x-ray diffraction techniques. They are found to exhibit a hexagonal→ lamellar→ hexagonal transition with increasing hexanol content. Quantitative analysis of the diffraction data shows that the complexes formed at low hexanol concentrations have an intercalated hexagonal ͑H I ͒ structure, whereas those formed at higher hexanol content have an inverted hexagonal ͑H II ͒ structure.
Experimental signatures of a nonequilibrium phase transition governing the yielding of a soft glass
Physical Review E, 2014
We present direct experimental signatures of a nonequilibrium phase transition associated with th... more We present direct experimental signatures of a nonequilibrium phase transition associated with the yield point of a prototypical soft solid-a binary colloidal glass. By simultaneously quantifying single-particle dynamics and bulk mechanical response, we identified the threshold for the onset of irreversibility with the yield strain. We extracted the relaxation time from the transient behavior of the loss modulus and found that it diverges in the vicinity of the yield strain. This critical slowing down is accompanied by a growing correlation length associated with the size of regions of high Debye-Waller factor, which are precursors to yield events in glasses. Our results affirm that the paradigm of nonequilibrium critical phenomena is instrumental in achieving a holistic understanding of yielding in soft solids.
RSC Advances, 2011
A lamellar surfactant mesophase is used as a soft confinement medium for gold nanoparticles that ... more A lamellar surfactant mesophase is used as a soft confinement medium for gold nanoparticles that are directly synthesized inside the lamellar mesophase by the radiolytic reduction of a gold salt incorporated into the water medium. By increasing the water layer thickness of the mesophase, spherical gold nanoparticles of increasing size are obtained. The same soft confinement medium is used to synthesize directly in situ by radiolysis gold nanorods (aspect ratio about 10). It is also shown that gold nanorods can be inserted by simple mixing into a sufficiently swollen lamellar phase. In all cases, the structure of the lamellar phase is preserved in the presence of the nanoparticles.
Shear banding in a yield stress bearing Langmuir monolayer
Soft Matter, 2011
We examine the shear-thinning behaviour of a two dimensional yield stress bearing monolayer of so... more We examine the shear-thinning behaviour of a two dimensional yield stress bearing monolayer of sorbitan tristearate at air/water interface. The flow curve consists of a linear region at low shear stresses/shear rates, followed by a stress plateau at higher values. The velocity profile obtained from particle imaging velocimetry indicates that shear banding occurs, showing coexistence of the fluidized region near the rotor and solid region with vanishing shear-rate away from the rotor. In the fluidized region, the velocity profile, which ...
Yield stress, thixotropy and shear banding in a dilute aqueous suspension of few layer graphene oxide platelets
Soft Matter, 2013
ABSTRACT We demonstrate a rigidity percolation transition and the onset of yield stress in a dilu... more ABSTRACT We demonstrate a rigidity percolation transition and the onset of yield stress in a dilute aqueous dispersion of graphene oxide platelets (aspect ratio 5000) above a critical volume fraction of 3.75 × 10−4 with a percolation exponent of 2.4 ± 0.1. The viscoelastic moduli of the gel at rest measured as a function of time indicate the absence of structural evolution of the 3D percolated network of disks. However a shear-induced aging giving rise to a compact jammed state and shear rejuvenation indicating a homogenous flow is observed when a steady shear stress (σ) is imposed in creep experiments. We construct a shear diagram (σ vs. volume fraction ) and the critical stress above which shear rejuvenation occurs is identified as the yield stress σy of the gel. The minimum steady state shear rate m obtained from creep experiments agrees well with the end of the plateau region in a controlled shear rate flow curve, indicating a shear localization below m. A steady state shear banding in the plateau region of the flow curve observed in particle velocimetry measurements in a Couette geometry confirms that the dilute suspensions of GO platelets form a thixotropic yield stress fluid.
Proceedings of the National Academy of Sciences, 2011
Proceedings of the National Academy of Sciences, 2013
Pramana, 2003
Structures of complexes formed in aqueous solutions by some anionic polyelectrolytes (double and ... more Structures of complexes formed in aqueous solutions by some anionic polyelectrolytes (double and single stranded (ds and ss) DNA, poly(vinyl sulfonate) (PVS), and poly(styrene sulfonate) (PSS)) with a cationic surfactant system consisting of cetyltrimethylammonium bromide (CTAB) and sodium 3-hydroxy-2-naphthoate (SHN) have been determined using small angle Xray diffraction. All complexes are found to have a two-dimensional (2-D) hexagonal structure at low SHN concentrations. Analysis of the diffraction data shows that the ds DNA-CTAB complex has an intercalated structure, with each DNA strand surrounded by three cylindrical micelles. On increasing SHN concentration, DNA-CTAB-SHN complexes exhibit a hexagonal-to-lamellar transition, whereas PVS complexes show a hexagonal centered rectangular lamellar transition. PSS complexes show yet another sequence of structures. These results indicate the significant influence of the chemical nature of the polyelectrolyte on the structure of the complexes.
PLoS ONE, 2013
Lamins are intermediate filament proteins of type V constituting a nuclear lamina or filamentous ... more Lamins are intermediate filament proteins of type V constituting a nuclear lamina or filamentous meshwork which lines the nucleoplasmic side of the inner nuclear membrane. This protein mesh provides a supporting scaffold for the nuclear envelope and tethers interphase chromosome to the nuclear periphery. Mutations of mainly A-type lamins are found to be causative for at least 11 human diseases collectively termed as laminopathies majority of which are characterised by aberrant nuclei with altered structural rigidity, deformability and poor mechanotransduction behaviour. But the investigation of viscoelastic behavior of lamin A continues to elude the field. In order to address this problem, we hereby present the very first report on viscoelastic properties of wild type human lamin A and some of its mutants linked with Dilated cardiomyopathy (DCM) using quantitative rheological measurements. We observed a dramatic strain-softening effect on lamin A network as an outcome of the strain amplitude sweep measurements which could arise from the large compliance of the quasi-cross-links in the network or that of the lamin A rods. In addition, the drastic stiffening of the differential elastic moduli on superposition of rotational and oscillatory shear stress reflect the increase in the stiffness of the laterally associated lamin A rods. These findings present a preliminary insight into distinct biomechanical properties of wild type lamin A protein and its mutants which in turn revealed interesting differences.
Physical Review E, 2004
Complexes of double-stranded DNA with the cationic surfactant cetyltrimethylammonium bromide have... more Complexes of double-stranded DNA with the cationic surfactant cetyltrimethylammonium bromide have been studied using small angle x-ray diffraction at varying concentrations of DNA and the cosurfactant hexanol. At low DNA concentrations, an intercalated hexagonal (H I c)→lamellar (L ␣ c)→inverted hexagonal (H II c) transformation is found on increasing hexanol content. The H II c structure is converted into L ␣ c on adding more DNA. Further increase in hexanol content leads to a phase separation in the surfactant solution, and a reentrant L ␣ c →H II c →L ␣ c transition is observed as DNA concentration is increased. Such structural transformations of DNA-surfactant complexes, driven by DNA concentration, have not been reported until now.
Aggregation of a Peptide Antibiotic Alamethicin at the Air−Water Interface and Its Influence on the Viscoelasticity of Phospholipid Monolayers
Langmuir, 2008
The aggregation properties of an antibiotic membrane-active peptide alamethicin at the air-water ... more The aggregation properties of an antibiotic membrane-active peptide alamethicin at the air-water interface have been studied using interfacial rheology and fluorescence microscopy techniques. Fluorescence microscopy of alamethicin monolayers revealed a coexistence of liquid expanded (LE) and solid phases at the surface concentrations studied. Interfacial oscillatory shear measurements on alamethicin monolayers indicate that its viscoelastic properties are determined by the area fraction of the solid domains. The role of zwitterionic phospholipids dioleoylphosphatidyl choline (DOPC) and dioleoylphosphatidyl ethanolamine (DOPE) on the peptide aggregation behavior was also investigated. Fluorescence microscopy of alamethicin/phospholipid monolayers revealed an intermediate phase (I) in addition to the solid and LE phase. In mixed monolayers of phospholipid (L)/alamethicin (P), with increase in L/P, the monolayer transforms from a viscoelastic to a viscous fluid with the increase in area fraction of the intermediate phase. Further, a homogeneous mixing of alamethicin/lipid molecules is observed at L/P > 4. Our studies also confirm that the viscoelasticity of alamethicin/phospholipid monolayers is closely related to the alamethicin/phospholipid interactions at the air-water interface.
Uploads
Papers by Rema Krishnaswamy