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Kyongok Kang

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Dr. KK in FZJ, Germany, Soft-Condensed Matter Physicist,Interested in both equilibrium and non-equilibrium complex phenomena of phase-change materials and chiral-materials.Also interested in building up experimental platforms to explore novel foundings

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Papers by Kyongok Kang

Chiral glass of charged DNA rods, cavity loops

Journal of physics communications, Jun 1, 2021

Recently, the collective phase behaviors of charged DNA-viruses (rods) are explored in various lo... more Recently, the collective phase behaviors of charged DNA-viruses (rods) are explored in various low ionic strengths, where much slowing down relaxations are observed in equilibrium. These phases are distinguished by crossing the critical ionic strength, where the salt-dependent effective microscopic dynamics are important with non-monotonic multi-phase boundaries. This is also interested in terms of phase transition and replica symmetry breaking (RSB), realized by the divergence of relaxation time and the exponent power of correlation lengths in dynamic disorderorder transitions. RSB occurs as the result of many-body interactions of randomized orientations. Here, in this paper, one of the chiral mesophases, the X-pattern is highlighted for its long-time metastable state consisting of cavity loops bounded within the planar nematic, as a new type of a glass, noted as a chiral glass. The chiral glass is discussed in terms of the divergences for relaxations in the characteristic time as well the inverse of an optical pitch length, which are analyzed in the long-time-lapsed orientation textures via image-time correlations. The outcome highlights interesting aspects of charge effect in the ‘twist’ elasticity energy for optimizing the long-ranged repulsive electrostatic interactions among charged DNA rods.

Shockwave application enhances the effect of dentin desensitizer

Dental Materials, 2021

Orthopedic Reviews 2010; volume 2:e19

Nonuniform flow in soft glasses of colloidal rods

Physical review fluids, Apr 11, 2017

Despite our reasonably advanced understanding of the dynamics and flow of glasses made of spheric... more Despite our reasonably advanced understanding of the dynamics and flow of glasses made of spherical colloids, the role of shape, i.e., the respective behavior of glasses formed by rodlike, particles is virtually unexplored. Recently, long, thin and highly charged rods (fd-virus particles) were found to vitrify in aqueous suspensions at low ionic strength [Phys. Rev. Lett. 110, 015901 (2013)]. The glass transition of these long-ranged repulsive rods occurs at a concentration far above the isotropic-nematic coexistence region and is characterized by the unique arrest of both the dynamics of domains that constitute the chiral-nematic orientational texture, as well as individual rods inside the domains. Hence, two relevant length scales exist: the domain size of a few hundreds of microns, and the rod-cage size of a few microns, inside the domains. We show that the unique dual dynamic arrest and the existing of two widely separated length scales imparts an unprecedented, highly heterogeneous flow behavior with three distinct signatures. Beyond a weak stress plateau at very small shear rates that characterizes the glass, the kinetic arrest of the domain dynamics gives rise to internal fracture, as a result of domain-domain interactions, as well as wall partial slip. It is shown that, on increasing the shear rate, the fractured plug flow changes to a shear-banded flow profile due to the stress response of the kinetically arrested aligned rods within the domains. Shear-gradient banding occurs due to the strong thinning of the uniform chiral-nematic phase within the domains, i.e., complying with the classic shear-banding scenario, giving rise to a stress plateau in the flow curve. Finally, a linear (uniform) velocity profile is found at the highest shear rates. Vorticity banding is also observed at intermediate and high shear rates. These results point to the crucial role of particle shape in tailoring the flow properties of dense colloidal suspensions. Moreover, they strongly support the argument that the origin of shear banding in soft-particle glasses with long-ranged repulsive interactions is fundamentally different from that of hard-particle glasses with short-ranged repulsive interactions.

Shear-banding instabilities

Korea-australia Rheology Journal, 2010

Correction: An electric-field induced dynamical state in dispersions of charged colloidal rods

Soft Matter, 2015

Anisotropic diffusion and phase behaviour of cellulose nanocrystal suspensions

256th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nanoscience, Nanotechnology and Beyond, Date: 2018/08/19 - 2018/08/23, Location: Boston, MA, Aug 1, 2018

Dynamic Frequency Response of Charged Chiral Rods

Various phases and dynamical states are induced by an external AC electric fleld in concentrated ... more Various phases and dynamical states are induced by an external AC electric fleld in concentrated suspensions of charged, flbrous viruses (fd). The observed phases and dynam- ical states are the result of chiral rod-rod interactions between polarized electric double layers, polarized layers of condensed ions and electro-osmotic ∞ow.

Electric-field induced modulation of amorphous protein aggregates: polarization, deformation, and reorientation

Scientific Reports

Proteins in their native state are only marginally stable and tend to aggregate. However, protein... more Proteins in their native state are only marginally stable and tend to aggregate. However, protein misfolding and condensation are often associated with undesired processes, such as pathogenesis, or unwanted properties, such as reduced biological activity, immunogenicity, or uncontrolled materials properties. Therefore, controlling protein aggregation is very important, but still a major challenge in various fields, including medicine, pharmacology, food processing, and materials science. Here, flexible, amorphous, micron-sized protein aggregates composed of lysozyme molecules reduced by dithiothreitol are used as a model system. The preformed amorphous protein aggregates are exposed to a weak alternating current electric field. Their field response is followed in situ by time-resolved polarized optical microscopy, revealing field-induced deformation, reorientation and enhanced polarization as well as the disintegration of large clusters of aggregates. Small-angle dynamic light scatt...

Electric field induced phases and dynamical states in suspensions of rods: Association and dissociation of condensed ions

Shear-induced Flow Instabilities: Vortcitiy Banding and Shear-Gradient Concentration Coupling Instability

Hydrophilic- and hydrophobic-clays within an oil droplet immersed in water

Electric-Field Induced Phases and a Glass Transition in Suspensions of Charged Fibrous Viruses (fd): the interplay between helical core-core interactions and electrostatic repulsion

Numerical simulation on SCC (shear-rate gradient concentration coupling) Instability of colloidal spheres

We present here the numerical simulation on SCC (shear-rate gradient concentration coupling) inst... more We present here the numerical simulation on SCC (shear-rate gradient concentration coupling) instability on colloidal sphere in both Cartesian and cylindrical geometries. The flow instability due to coupling between shear-gradients and mass transport has been discussed through the linear stability analysis and shear-induced distortion of the pair-correlation function, which is related to shear rate dependent chemical potential [1-4]. In this poster, behaviors of concentration and shear rate profiles under various concentration, apparent shear rate and chemical potential are calculated to compare the result with the stability analysis discussed in Ref. [4]. In case of a cylindrical couette, the effect of curvature is discussed as a function of stability criterions. References: [1] J.K.G. Dhont, A constitutive relation describing the shear-banding transition, Phys. Rev. E. 60, 4534, 1999. [2] J.K.G. Dhont, W.J. Briels, Gradient and vorticity banding, Rheo. Acta, 47, 257-281, 2008. [3]...

Charged Colloids and Electric Fields

Non-local stresses in highly non-uniformly flowing suspensions: The shear-curvature viscosity

The Journal of Chemical Physics, 2018

For highly non-uniformly flowing fluids, there are contributions to the stress related to spatial... more For highly non-uniformly flowing fluids, there are contributions to the stress related to spatial variations of the shear rate, which are commonly referred to as non-local stresses. The standard expression for the shear stress, which states that the shear stress is proportional to the shear rate, is based on a formal expansion of the stress tensor with respect to spatial gradients in the flow velocity up to leading order. Such a leading order expansion is not able to describe fluids with very rapid spatial variations of the shear rate, like in micro-fluidics devices and in shear-banding suspensions. Spatial derivatives of the shear rate then significantly contribute to the stress. Such non-local stresses have so far been introduced on a phenomenological level. In particular, a formal gradient expansion of the stress tensor beyond the above mentioned leading order contribution leads to a phenomenological formulation of non-local stresses in terms of the so-called "shear-curvature viscosity". We derive an expression for the shear-curvature viscosity for dilute suspensions of spherical colloids and propose an effective-medium approach to extend this result to concentrated suspensions. The validity of the effective-medium prediction is confirmed by Brownian dynamics simulations on highly non-uniformly flowing fluids.

Synchronized oscillations of dimers in biphasic charged fd-virus suspensions

Physical Review E, 2016

Micron-sized colloidal spheres that are dispersed in an isotropic-nematic bi-phasic host suspensi... more Micron-sized colloidal spheres that are dispersed in an isotropic-nematic bi-phasic host suspension of charged rods (fd-virus particles) are shown to spontaneously form dimers, which exhibit a synchronized oscillatory motion. Dimer formation is not observed in mono-phase of isotropic and nematic suspensions. The synchronized oscillations of dimers are connected to the inhomogeneous state of the host suspension of charged rods (fd-viruses) where nematic domains are in coexistence with isotropic regions. The synchronization of oscillations occurs in bulk states, in the absence of an external field. With a low field strength of an applied electric field, the synchronization is rather reduced, but it recovers again when the field is turned off. In this Letter, we report this observation as an example of the strange attractor, occurred in the mixture of PS-dimers in an isotropic-nematic coexistence biphasic fd-virus network. Furthermore, we highlight the synchronization of PS-dimer oscillation is the result of a global bifurcation diagram, driven by a delicate balance between the short-attractive "twisted" interaction of PS-dimers and long ranged electrostatic repulsive interactions of charged fd-rods. The interest is then the local enhancement of "twist-nematic" elasticity in reorienting of the dimer oscillations. The analysis of image-time correlations are provided with the data movies and Fourier transforms of averaged orientations for the synchronized oscillations of dimers in biphasic IN coexistence concentration of charged fd-virus suspensions.

Underlying mechanism of shear-banding in soft glasses of charged colloidal rods with orientational domains

Journal of Rheology, 2022

Soft glasses of colloidal rods ( fd-virus particles) with orientational domains were recently sho... more Soft glasses of colloidal rods ( fd-virus particles) with orientational domains were recently shown to exhibit inhomogeneous flow profiles [Dhont et al., Phys. Rev. Fluids 2, 043301 (2017)]: fracture and accompanied plug flow at small shear rates, which transits to gradient shear-banding on increasing the shear rate, while a uniform flow profile develops at sufficiently high shear rates. These flow profiles coexist with Taylor-vorticity bands. The texture of such glasses under flow conditions consists of domains with varying orientations. The observed gradient shear-banding was solely attributed to the strong shear thinning behavior of the material inside the domains (henceforth abbreviated as domain-interior), without considering the texture stress that is due to interactions between the glassy domains. Here, we present new experiments on the shear-banding transition to assess the role played by the texture stress in comparison to the domain-interior stress. For a large concentrati...

Nonequilbrium Phase Transitions and Equilibrium Textures of Charged Chiral Rods (fd-Viruses)

Both non-equilibrium phase transitions and equilibrium textures of interacting charged chiral rod... more Both non-equilibrium phase transitions and equilibrium textures of interacting charged chiral rods are explored, where thick electric double layers are present in the suspensions of charged chiral fibrous (fd) viruses at a low ionic strength. We first start with the electric phase/state diagram, illustrating dynamic frequency responses at the concentration of isotropic-nematic (I-N) coexistence, leading to various field-induced phases/states. As a low frequency response, two sharp transitions of chiral-nematic phases and dynamical states are induced, while as one transition is found to a homeotropic phases that is stabilized at a high-frequency. The characterizations of field-induced phases/states, and critical slowing down behaviors in the non-equilibrium criticality are discussed, by means of image-time correlation, dynamic light scattering and electric birefringence. For the equilibrium phase behaviors, depolarized optical morphology is studied with their texture dynamics, as an ...

An electric-field induced dynamical state in dispersions of highly charged colloidal rods: comparison of experiment and theory

Colloid and Polymer Science, 2015

Vorticity banding in rodlike virus suspensions

Physical Review E, 2006

Chiral glass of charged DNA rods, cavity loops

Journal of physics communications, Jun 1, 2021

Recently, the collective phase behaviors of charged DNA-viruses (rods) are explored in various lo... more Recently, the collective phase behaviors of charged DNA-viruses (rods) are explored in various low ionic strengths, where much slowing down relaxations are observed in equilibrium. These phases are distinguished by crossing the critical ionic strength, where the salt-dependent effective microscopic dynamics are important with non-monotonic multi-phase boundaries. This is also interested in terms of phase transition and replica symmetry breaking (RSB), realized by the divergence of relaxation time and the exponent power of correlation lengths in dynamic disorderorder transitions. RSB occurs as the result of many-body interactions of randomized orientations. Here, in this paper, one of the chiral mesophases, the X-pattern is highlighted for its long-time metastable state consisting of cavity loops bounded within the planar nematic, as a new type of a glass, noted as a chiral glass. The chiral glass is discussed in terms of the divergences for relaxations in the characteristic time as well the inverse of an optical pitch length, which are analyzed in the long-time-lapsed orientation textures via image-time correlations. The outcome highlights interesting aspects of charge effect in the ‘twist’ elasticity energy for optimizing the long-ranged repulsive electrostatic interactions among charged DNA rods.

Shockwave application enhances the effect of dentin desensitizer

Dental Materials, 2021

Orthopedic Reviews 2010; volume 2:e19

Nonuniform flow in soft glasses of colloidal rods

Physical review fluids, Apr 11, 2017

Despite our reasonably advanced understanding of the dynamics and flow of glasses made of spheric... more Despite our reasonably advanced understanding of the dynamics and flow of glasses made of spherical colloids, the role of shape, i.e., the respective behavior of glasses formed by rodlike, particles is virtually unexplored. Recently, long, thin and highly charged rods (fd-virus particles) were found to vitrify in aqueous suspensions at low ionic strength [Phys. Rev. Lett. 110, 015901 (2013)]. The glass transition of these long-ranged repulsive rods occurs at a concentration far above the isotropic-nematic coexistence region and is characterized by the unique arrest of both the dynamics of domains that constitute the chiral-nematic orientational texture, as well as individual rods inside the domains. Hence, two relevant length scales exist: the domain size of a few hundreds of microns, and the rod-cage size of a few microns, inside the domains. We show that the unique dual dynamic arrest and the existing of two widely separated length scales imparts an unprecedented, highly heterogeneous flow behavior with three distinct signatures. Beyond a weak stress plateau at very small shear rates that characterizes the glass, the kinetic arrest of the domain dynamics gives rise to internal fracture, as a result of domain-domain interactions, as well as wall partial slip. It is shown that, on increasing the shear rate, the fractured plug flow changes to a shear-banded flow profile due to the stress response of the kinetically arrested aligned rods within the domains. Shear-gradient banding occurs due to the strong thinning of the uniform chiral-nematic phase within the domains, i.e., complying with the classic shear-banding scenario, giving rise to a stress plateau in the flow curve. Finally, a linear (uniform) velocity profile is found at the highest shear rates. Vorticity banding is also observed at intermediate and high shear rates. These results point to the crucial role of particle shape in tailoring the flow properties of dense colloidal suspensions. Moreover, they strongly support the argument that the origin of shear banding in soft-particle glasses with long-ranged repulsive interactions is fundamentally different from that of hard-particle glasses with short-ranged repulsive interactions.

Shear-banding instabilities

Korea-australia Rheology Journal, 2010

Correction: An electric-field induced dynamical state in dispersions of charged colloidal rods

Soft Matter, 2015

Anisotropic diffusion and phase behaviour of cellulose nanocrystal suspensions

256th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nanoscience, Nanotechnology and Beyond, Date: 2018/08/19 - 2018/08/23, Location: Boston, MA, Aug 1, 2018

Dynamic Frequency Response of Charged Chiral Rods

Various phases and dynamical states are induced by an external AC electric fleld in concentrated ... more Various phases and dynamical states are induced by an external AC electric fleld in concentrated suspensions of charged, flbrous viruses (fd). The observed phases and dynam- ical states are the result of chiral rod-rod interactions between polarized electric double layers, polarized layers of condensed ions and electro-osmotic ∞ow.

Electric-field induced modulation of amorphous protein aggregates: polarization, deformation, and reorientation

Scientific Reports

Proteins in their native state are only marginally stable and tend to aggregate. However, protein... more Proteins in their native state are only marginally stable and tend to aggregate. However, protein misfolding and condensation are often associated with undesired processes, such as pathogenesis, or unwanted properties, such as reduced biological activity, immunogenicity, or uncontrolled materials properties. Therefore, controlling protein aggregation is very important, but still a major challenge in various fields, including medicine, pharmacology, food processing, and materials science. Here, flexible, amorphous, micron-sized protein aggregates composed of lysozyme molecules reduced by dithiothreitol are used as a model system. The preformed amorphous protein aggregates are exposed to a weak alternating current electric field. Their field response is followed in situ by time-resolved polarized optical microscopy, revealing field-induced deformation, reorientation and enhanced polarization as well as the disintegration of large clusters of aggregates. Small-angle dynamic light scatt...

Electric field induced phases and dynamical states in suspensions of rods: Association and dissociation of condensed ions

Shear-induced Flow Instabilities: Vortcitiy Banding and Shear-Gradient Concentration Coupling Instability

Hydrophilic- and hydrophobic-clays within an oil droplet immersed in water

Electric-Field Induced Phases and a Glass Transition in Suspensions of Charged Fibrous Viruses (fd): the interplay between helical core-core interactions and electrostatic repulsion

Numerical simulation on SCC (shear-rate gradient concentration coupling) Instability of colloidal spheres

We present here the numerical simulation on SCC (shear-rate gradient concentration coupling) inst... more We present here the numerical simulation on SCC (shear-rate gradient concentration coupling) instability on colloidal sphere in both Cartesian and cylindrical geometries. The flow instability due to coupling between shear-gradients and mass transport has been discussed through the linear stability analysis and shear-induced distortion of the pair-correlation function, which is related to shear rate dependent chemical potential [1-4]. In this poster, behaviors of concentration and shear rate profiles under various concentration, apparent shear rate and chemical potential are calculated to compare the result with the stability analysis discussed in Ref. [4]. In case of a cylindrical couette, the effect of curvature is discussed as a function of stability criterions. References: [1] J.K.G. Dhont, A constitutive relation describing the shear-banding transition, Phys. Rev. E. 60, 4534, 1999. [2] J.K.G. Dhont, W.J. Briels, Gradient and vorticity banding, Rheo. Acta, 47, 257-281, 2008. [3]...

Charged Colloids and Electric Fields

Non-local stresses in highly non-uniformly flowing suspensions: The shear-curvature viscosity

The Journal of Chemical Physics, 2018

For highly non-uniformly flowing fluids, there are contributions to the stress related to spatial... more For highly non-uniformly flowing fluids, there are contributions to the stress related to spatial variations of the shear rate, which are commonly referred to as non-local stresses. The standard expression for the shear stress, which states that the shear stress is proportional to the shear rate, is based on a formal expansion of the stress tensor with respect to spatial gradients in the flow velocity up to leading order. Such a leading order expansion is not able to describe fluids with very rapid spatial variations of the shear rate, like in micro-fluidics devices and in shear-banding suspensions. Spatial derivatives of the shear rate then significantly contribute to the stress. Such non-local stresses have so far been introduced on a phenomenological level. In particular, a formal gradient expansion of the stress tensor beyond the above mentioned leading order contribution leads to a phenomenological formulation of non-local stresses in terms of the so-called "shear-curvature viscosity". We derive an expression for the shear-curvature viscosity for dilute suspensions of spherical colloids and propose an effective-medium approach to extend this result to concentrated suspensions. The validity of the effective-medium prediction is confirmed by Brownian dynamics simulations on highly non-uniformly flowing fluids.

Synchronized oscillations of dimers in biphasic charged fd-virus suspensions

Physical Review E, 2016

Micron-sized colloidal spheres that are dispersed in an isotropic-nematic bi-phasic host suspensi... more Micron-sized colloidal spheres that are dispersed in an isotropic-nematic bi-phasic host suspension of charged rods (fd-virus particles) are shown to spontaneously form dimers, which exhibit a synchronized oscillatory motion. Dimer formation is not observed in mono-phase of isotropic and nematic suspensions. The synchronized oscillations of dimers are connected to the inhomogeneous state of the host suspension of charged rods (fd-viruses) where nematic domains are in coexistence with isotropic regions. The synchronization of oscillations occurs in bulk states, in the absence of an external field. With a low field strength of an applied electric field, the synchronization is rather reduced, but it recovers again when the field is turned off. In this Letter, we report this observation as an example of the strange attractor, occurred in the mixture of PS-dimers in an isotropic-nematic coexistence biphasic fd-virus network. Furthermore, we highlight the synchronization of PS-dimer oscillation is the result of a global bifurcation diagram, driven by a delicate balance between the short-attractive "twisted" interaction of PS-dimers and long ranged electrostatic repulsive interactions of charged fd-rods. The interest is then the local enhancement of "twist-nematic" elasticity in reorienting of the dimer oscillations. The analysis of image-time correlations are provided with the data movies and Fourier transforms of averaged orientations for the synchronized oscillations of dimers in biphasic IN coexistence concentration of charged fd-virus suspensions.

Underlying mechanism of shear-banding in soft glasses of charged colloidal rods with orientational domains

Journal of Rheology, 2022

Soft glasses of colloidal rods ( fd-virus particles) with orientational domains were recently sho... more Soft glasses of colloidal rods ( fd-virus particles) with orientational domains were recently shown to exhibit inhomogeneous flow profiles [Dhont et al., Phys. Rev. Fluids 2, 043301 (2017)]: fracture and accompanied plug flow at small shear rates, which transits to gradient shear-banding on increasing the shear rate, while a uniform flow profile develops at sufficiently high shear rates. These flow profiles coexist with Taylor-vorticity bands. The texture of such glasses under flow conditions consists of domains with varying orientations. The observed gradient shear-banding was solely attributed to the strong shear thinning behavior of the material inside the domains (henceforth abbreviated as domain-interior), without considering the texture stress that is due to interactions between the glassy domains. Here, we present new experiments on the shear-banding transition to assess the role played by the texture stress in comparison to the domain-interior stress. For a large concentrati...

Nonequilbrium Phase Transitions and Equilibrium Textures of Charged Chiral Rods (fd-Viruses)

Both non-equilibrium phase transitions and equilibrium textures of interacting charged chiral rod... more Both non-equilibrium phase transitions and equilibrium textures of interacting charged chiral rods are explored, where thick electric double layers are present in the suspensions of charged chiral fibrous (fd) viruses at a low ionic strength. We first start with the electric phase/state diagram, illustrating dynamic frequency responses at the concentration of isotropic-nematic (I-N) coexistence, leading to various field-induced phases/states. As a low frequency response, two sharp transitions of chiral-nematic phases and dynamical states are induced, while as one transition is found to a homeotropic phases that is stabilized at a high-frequency. The characterizations of field-induced phases/states, and critical slowing down behaviors in the non-equilibrium criticality are discussed, by means of image-time correlation, dynamic light scattering and electric birefringence. For the equilibrium phase behaviors, depolarized optical morphology is studied with their texture dynamics, as an ...

An electric-field induced dynamical state in dispersions of highly charged colloidal rods: comparison of experiment and theory

Colloid and Polymer Science, 2015

Vorticity banding in rodlike virus suspensions

Physical Review E, 2006