Thorsten Auth - Academia.edu (original) (raw)
Papers by Thorsten Auth
Biophysical Journal, 2014
ABSTRACT Background / Purpose: Budding of cell membranes initiates intracellular vesicle transpor... more ABSTRACT Background / Purpose: Budding of cell membranes initiates intracellular vesicle transport and has been studied for a variety of soft matter systems. Using a continuum model, we study the wrapping of a single nano-particle as an interplay of the membrane deformation energy and the adhesion energy between particle and membrane. With the help of numerical energy minimization using triangulated surfaces, we investigate the role of shape and size of the particle as well as of the membrane’s elastic parameters on nano-particle wrapping. Main conclusion: Please find further details in the full abstract: Biophysical Journal, Volume 106, Issue 2 , 576a, (28 January 2014).
Langmuir, 2014
Micron-sized anisotropic particles with homogeneous surface properties at a fluid interface can d... more Micron-sized anisotropic particles with homogeneous surface properties at a fluid interface can deform the interface due to their shape. The particles thereby create excess interfacial area and interact in order to minimize this area, which lowers the total interfacial energy. We present a systematic investigation of the interface deformations around single ellipsoidal particles and cuboidal particles with rounded edges in the near field for various contact angles and particle aspect ratios. The correlation of these deformations with capillary bond energiesthe interaction energies of two particles at contact quantifies the relation between the interactions and the near-field deformations. We characterize the interactions using effective power laws and investigate how anisotropic particles self-assemble by capillary forces. Interface deformations and particle interactions for cuboidal particles are weaker compared with those for ellipsoidal particles with the same aspect ratios. For both particle shapes, the bound state in side-by-side orientation is most stable, while the interaction in tip-to-side orientation is repulsive. Furthermore, we find capillary attraction between spherical and ellipsoidal particles. Our calculations therefore suggest cluster formation of spherical and ellipsoidal particles, which elucidates the role of spherical particles as stoppers for the growth of worm-like chains of ellipsoidal particles. The interaction between spherical and ellipsoidal particles might also explain the suppression of the "coffee-ring effect" that has been observed for evaporating droplets with mixtures of spherical and ellipsoidal particles. In general, our calculations of the near-field interactions complement previous calculations in the far field and help to predict colloidal assembly and rheological properties of particle-laden interfaces.
Towards calculating membrane-wrapping contributions to invasion we sought to identify core parame... more Towards calculating membrane-wrapping contributions to invasion we sought to identify core parameters associated with parasite entry. Four key parameters were selected, adhesive forces, bending energy, target membrane tension and a line tension ).
In this supplementary material, we present numerical data to further characterize the interface d... more In this supplementary material, we present numerical data to further characterize the interface deformation around and the capillary interaction between ellipsoidal and cuboidal particles. For a contact angle θ c = 90 • , a constant contact angle cannot be achieved for a planar interface around an ellipsoidal particle and around a cuboidal particle with rounded edges. For θ c < 90 • , the interface is pulled down at the tips and pulled up at the sides of the ellipsoids, thereby creating excess interface area. shows the deformation of the interface around an ellipsoidal particle in the directions of the long and the short axis.
Biophysical Journal, 2014
The blood stage malaria parasite, the merozoite, has a small window of opportunity during which i... more The blood stage malaria parasite, the merozoite, has a small window of opportunity during which it must successfully target and invade a human erythrocyte. The process of invasion is nonetheless remarkably rapid. To date, mechanistic models of invasion have focused predominantly on the parasite actomyosin motor contribution to the energetics of entry. Here, we have conducted a numerical analysis using dimensions for an archetypal merozoite to predict the respective contributions of the host-parasite interactions to invasion, in particular the role of membrane wrapping. Our theoretical modeling demonstrates that erythrocyte membrane wrapping alone, as a function of merozoite adhesive and shape properties, is sufficient to entirely account for the first key step of the invasion process, that of merozoite reorientation to its apex and tight adhesive linkage between the two cells. Next, parasite-induced reorganization of the erythrocyte cytoskeleton and release of parasite-derived membrane can also account for a considerable energetic portion of actual invasion itself, through membrane wrapping. Thus, contrary to the prevailing dogma, wrapping by the erythrocyte combined with parasite-derived membrane release can markedly reduce the expected contributions of the merozoite actomyosin motor to invasion. We therefore propose that invasion is a balance between parasite and host cell contributions, evolved toward maximal efficient use of biophysical forces between the two cells.
Physical Review E, 2003
The effect of anchored linear and star polymers in the mushroom regime on the curvature elasticit... more The effect of anchored linear and star polymers in the mushroom regime on the curvature elasticity of membranes is investigated by Monte Carlo simulations and scaling arguments. We describe a simulation method to calculate the free energy of anchored polymers as a function of membrane curvature, which is accurate enough to extract reliable values for the polymer-induced spontaneous curvature ⌬c 0 , bending rigidity ⌬, and saddle-splay modulus ⌬ . For self-avoiding linear and star polymers, the universal amplitudes of the curvature moduli as well as the effects of finite chain lengths are determined, to our knowledge, for the first time. We find that star polymers have the unique property of strongly affecting c 0 and , but leaving essentially unchanged. Furthermore, star polymers are shown to have a much stronger effect on membrane properties than an equivalent number of linear polymers.
Acs Nano, 2014
An in-depth understanding of the interface between cells and nanostructures is one of the key cha... more An in-depth understanding of the interface between cells and nanostructures is one of the key challenges for coupling electrically excitable cells and electronic devices. Recently, various 3D nanostructures have been introduced to stimulate and record electrical signals emanating from inside of the cell. Even though such approaches are highly sensitive and scalable, it remains an open question how cells couple to 3D structures, in particular how the engulfment-like processes of nanostructures work. Here, we present a profound study of the cell interface with two widely used nanostructure types, cylindrical pillars with and without a cap. While basic functionality was shown for these approaches before, a systematic investigation linking experimental data with membrane properties was not presented so far. The combination of electron microscopy investigations with a theoretical membrane deformation model allows us to predict the optimal shape and dimensions of 3D nanostructures for cell-chip coupling.
Epl, 2014
Spontaneous segregation of run-and-tumble particles with different velocities in microchannels is... more Spontaneous segregation of run-and-tumble particles with different velocities in microchannels is investigated by numerical simulations. Self-propelled particles are known to accumulate in the proximity of walls. Here we show how fast particles expel slower ones from the wall leading to a segregated state. The mechanism is understood as a function of particle velocities, particle density, or channel width. In the presence of an external fluid flow, particles with two different velocities segregate due to their different particle fluxes. Promising applications can be found in the development of microfluidic lab-on-a-chip devices for sorting of particles with different motilities.
Advances in Planar Lipid Bilayers and Liposomes, 2011
The Journal of Physical Chemistry B, 2013
It has been established that the addition of amphiphilic diblock copolymers has a boosting effect... more It has been established that the addition of amphiphilic diblock copolymers has a boosting effect in bicontinuous microemulsions by decreasing the minimum amount of surfactant needed to solubilize equal volumes of oil and water. The strength of the polymer effect was found to be about twice larger than the theoretical prediction. This discrepancy is explained by confinement. Previous experimental studies always considered large oil and water domains of size d compared to the typical polymer end-to-end radius, R ee . The ratio of these two parameters R ee /d defines the confinement parameter. We investigated the sensitivity of the polymer influence extending the range of confinement. We combined macroscopic observations of the phase behavior with microscopic measurements of the structure by small-angle neutron scattering (SANS). Both results were compared with computer simulations on the basis of the theoretical concept of Helfrich. The simulations predict an enhanced sensitivity of the polymer at medium confinement and a reversed behavior at larger confinement. The higher sensitivity at medium confinement is only slightly visible experimentally, whereas the reversed behavior (antiboosting) is clearly present. Finally, a comparison with homopolymer addition showed a common high confinement behavior for diblock copolymers and for homopolymers.
Soft Matter, 2013
Membrane budding and wrapping of particles, such as viruses and nano-particles, play a key role i... more Membrane budding and wrapping of particles, such as viruses and nano-particles, play a key role in intracellular transport and have been studied for a variety of biological and soft matter systems. We study nano-particle wrapping by numerical minimization of bending, surface tension, and adhesion energies. We calculate deformation and adhesion energies as a function of membrane elastic parameters and adhesion strength to obtain wrapping diagrams. We predict unwrapped, partially-wrapped, and completely-wrapped states for prolate and oblate ellipsoids for various aspect ratios and particle sizes. In contrast to spherical particles, where partially-wrapped states exist only for finite surface tensions, partially-wrapped states for ellipsoids occur already for tensionless membranes. In addition, the partially-wrapped states are long-lived, because of an increased energy cost for wrapping of the highly-curved tips. Our results suggest a lower uptake rate of ellipsoidal particles by cells and thereby a higher virulence of tubular viruses compared with icosahedral viruses, as well as co-operative budding of ellipsoidal particles on membranes. arXiv:1303.5567v1 [cond-mat.soft]
Proceedings of the National Academy of Sciences, 2010
The remarkable deformability of the human red blood cell (RBC) results from the coupled dynamic r... more The remarkable deformability of the human red blood cell (RBC) results from the coupled dynamic response of the phospholipid bilayer and the spectrin molecular network. Here we present quantitative connections between spectrin morphology and membrane fluctuations of human RBCs by using dynamic full-field laser interferometry techniques. We present conclusive evidence that the presence of adenosine 5′-triphosphate (ATP) facilitates nonequilibrium dynamic fluctuations in the RBC membrane that are highly correlated with the biconcave shape of RBCs. Spatial analysis of the fluctuations reveals that these non-equilibrium membrane vibrations are enhanced at the scale of spectrin mesh size.
Physical Review E, 2007
The fluctuation spectra and the intermembrane interaction of two membranes at a fixed average dis... more The fluctuation spectra and the intermembrane interaction of two membranes at a fixed average distance are investigated. Each membrane can either be a fluid or a solid membrane, and in isolation, its fluctuations are described by a bare or a wave-vector-dependent bending modulus, respectively. The membranes interact via their excluded-volume interaction; the average distance is maintained by an external, homogeneous pressure. For strong coupling, the fluctuations can be described by a single, effective membrane that combines the elastic properties. For weak coupling, the fluctuations of the individual, noninteracting membranes are recovered. The case of a composite membrane consisting of one fluid and one solid membrane can serve as a microscopic model for the plasma membrane and cytoskeleton of the red blood cell. We find that, despite the complex microstructure of bilayers and cytoskeletons in a real cell, the fluctuations with wavelengths lambda greater, similar 400 nm are well described by the fluctuations of a single, polymerized membrane (provided that there are no inhomogeneities of the microstructure). The model is applied to the fluctuation data of discocytes (&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;normal&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; red blood cells), a stomatocyte, and an echinocyte. The elastic parameters of the membrane and an effective temperature that quantifies active, metabolically driven fluctuations are extracted from the experiments.
Physical Review E, 2013
The fluctuation pressure of a lipid-bilayer membrane is important for the stability of lamellar p... more The fluctuation pressure of a lipid-bilayer membrane is important for the stability of lamellar phases and the adhesion of membranes to surfaces. In contrast to many theoretical studies, which predict a decrease of the pressure with the cubed inverse distance between the membranes, Freund suggested very recently a linear inverse distance dependence [Proc. Natl. Acad. Sci. U.S. A. 110, 2047 (2013)]. We address this discrepancy by performing Monte Carlo simulations for a membrane model discretized on a square lattice and employ the wall theorem to evaluate the pressure for a single membrane between parallel walls. For distances that are small compared with the lattice constant, the pressure indeed depends on the inverse distance as predicted by Freund. For intermediate distances, the pressure depends on the cubed inverse distance as predicted by Helfrich [Z. Naturforsch. A 33, 305 (1978)]. Here, the crossover length between the two regimes is a molecular length scale. Finally, for distances large compared with the mean squared fluctuations of the membrane, the entire membrane acts as a soft particle and the pressure on the walls again depends linearly on the inverse distance.
Physical Review E, 2005
The effect of linear homopolymers, diblock copolymers, and star polymers anchored to a membrane o... more The effect of linear homopolymers, diblock copolymers, and star polymers anchored to a membrane on the membrane's fluctuation spectrum is investigated for low grafting densities. Due to the nonlocality of the polymer-membrane interaction, the effective bending rigidity eff ͑q͒ of the composite membrane is found to depend strongly on the wave vector q of the membrane undulations. Analytical calculations for ideal linear chains and simulations for ideal and self-avoiding linear chains as well as for star polymers are presented. The analytical calculations are based on the Green's function approach of Bickel and Marques ͓Eur. Phys. J. E 9, 349 ͑2002͔͒; for the simulations the Monte Carlo method is used. The functional form of eff ͑q͒ differs for end-grafted chains and diblock copolymers. In general, the polymer effect is most pronounced for undulations on length scales larger than or comparable to the polymer size, and decreases rapidly for smaller undulation wavelengths. Anchored linear chains always increase ; anchored star polymers may increase as well as decrease , depending on whether they are anchored symmetrically or asymmetrically to the membrane.
Physical Review E, 2013
Collective behavior of self-propelled particles is observed on a microscale for swimmers such as ... more Collective behavior of self-propelled particles is observed on a microscale for swimmers such as sperm and bacteria as well as for protein filaments in motility assays. The properties of such systems depend both on their dimensionality and the interactions between their particles. We introduce a model for self-propelled rods in two dimensions that interact via a separation-shifted Lennard-Jones potential. Due to the finite potential barrier, the rods are able to cross. This model allows us to efficiently simulate systems of self-propelled rods that effectively move in two dimensions but can occasionally escape to the third dimension in order to pass each other. Our quasi-two-dimensional self-propelled particles describe a class of active systems that encompasses microswimmers close to a wall and filaments propelled on a substrate. Using Monte Carlo simulations, we first determine the isotropic-nematic transition for passive rods. Using Brownian dynamics simulations, we characterize cluster formation of self-propelled rods as a function of propulsion strength, noise, and energy barrier. Contrary to rods with an infinite potential barrier, an increase of the propulsion strength does not only favor alignment but also effectively decreases the potential barrier that prevents crossing of rods. We thus find a clustering window with a maximum cluster size at medium propulsion strengths.
Biophysical Journal, 2014
ABSTRACT Background / Purpose: Budding of cell membranes initiates intracellular vesicle transpor... more ABSTRACT Background / Purpose: Budding of cell membranes initiates intracellular vesicle transport and has been studied for a variety of soft matter systems. Using a continuum model, we study the wrapping of a single nano-particle as an interplay of the membrane deformation energy and the adhesion energy between particle and membrane. With the help of numerical energy minimization using triangulated surfaces, we investigate the role of shape and size of the particle as well as of the membrane’s elastic parameters on nano-particle wrapping. Main conclusion: Please find further details in the full abstract: Biophysical Journal, Volume 106, Issue 2 , 576a, (28 January 2014).
Langmuir, 2014
Micron-sized anisotropic particles with homogeneous surface properties at a fluid interface can d... more Micron-sized anisotropic particles with homogeneous surface properties at a fluid interface can deform the interface due to their shape. The particles thereby create excess interfacial area and interact in order to minimize this area, which lowers the total interfacial energy. We present a systematic investigation of the interface deformations around single ellipsoidal particles and cuboidal particles with rounded edges in the near field for various contact angles and particle aspect ratios. The correlation of these deformations with capillary bond energiesthe interaction energies of two particles at contact quantifies the relation between the interactions and the near-field deformations. We characterize the interactions using effective power laws and investigate how anisotropic particles self-assemble by capillary forces. Interface deformations and particle interactions for cuboidal particles are weaker compared with those for ellipsoidal particles with the same aspect ratios. For both particle shapes, the bound state in side-by-side orientation is most stable, while the interaction in tip-to-side orientation is repulsive. Furthermore, we find capillary attraction between spherical and ellipsoidal particles. Our calculations therefore suggest cluster formation of spherical and ellipsoidal particles, which elucidates the role of spherical particles as stoppers for the growth of worm-like chains of ellipsoidal particles. The interaction between spherical and ellipsoidal particles might also explain the suppression of the "coffee-ring effect" that has been observed for evaporating droplets with mixtures of spherical and ellipsoidal particles. In general, our calculations of the near-field interactions complement previous calculations in the far field and help to predict colloidal assembly and rheological properties of particle-laden interfaces.
Towards calculating membrane-wrapping contributions to invasion we sought to identify core parame... more Towards calculating membrane-wrapping contributions to invasion we sought to identify core parameters associated with parasite entry. Four key parameters were selected, adhesive forces, bending energy, target membrane tension and a line tension ).
In this supplementary material, we present numerical data to further characterize the interface d... more In this supplementary material, we present numerical data to further characterize the interface deformation around and the capillary interaction between ellipsoidal and cuboidal particles. For a contact angle θ c = 90 • , a constant contact angle cannot be achieved for a planar interface around an ellipsoidal particle and around a cuboidal particle with rounded edges. For θ c < 90 • , the interface is pulled down at the tips and pulled up at the sides of the ellipsoids, thereby creating excess interface area. shows the deformation of the interface around an ellipsoidal particle in the directions of the long and the short axis.
Biophysical Journal, 2014
The blood stage malaria parasite, the merozoite, has a small window of opportunity during which i... more The blood stage malaria parasite, the merozoite, has a small window of opportunity during which it must successfully target and invade a human erythrocyte. The process of invasion is nonetheless remarkably rapid. To date, mechanistic models of invasion have focused predominantly on the parasite actomyosin motor contribution to the energetics of entry. Here, we have conducted a numerical analysis using dimensions for an archetypal merozoite to predict the respective contributions of the host-parasite interactions to invasion, in particular the role of membrane wrapping. Our theoretical modeling demonstrates that erythrocyte membrane wrapping alone, as a function of merozoite adhesive and shape properties, is sufficient to entirely account for the first key step of the invasion process, that of merozoite reorientation to its apex and tight adhesive linkage between the two cells. Next, parasite-induced reorganization of the erythrocyte cytoskeleton and release of parasite-derived membrane can also account for a considerable energetic portion of actual invasion itself, through membrane wrapping. Thus, contrary to the prevailing dogma, wrapping by the erythrocyte combined with parasite-derived membrane release can markedly reduce the expected contributions of the merozoite actomyosin motor to invasion. We therefore propose that invasion is a balance between parasite and host cell contributions, evolved toward maximal efficient use of biophysical forces between the two cells.
Physical Review E, 2003
The effect of anchored linear and star polymers in the mushroom regime on the curvature elasticit... more The effect of anchored linear and star polymers in the mushroom regime on the curvature elasticity of membranes is investigated by Monte Carlo simulations and scaling arguments. We describe a simulation method to calculate the free energy of anchored polymers as a function of membrane curvature, which is accurate enough to extract reliable values for the polymer-induced spontaneous curvature ⌬c 0 , bending rigidity ⌬, and saddle-splay modulus ⌬ . For self-avoiding linear and star polymers, the universal amplitudes of the curvature moduli as well as the effects of finite chain lengths are determined, to our knowledge, for the first time. We find that star polymers have the unique property of strongly affecting c 0 and , but leaving essentially unchanged. Furthermore, star polymers are shown to have a much stronger effect on membrane properties than an equivalent number of linear polymers.
Acs Nano, 2014
An in-depth understanding of the interface between cells and nanostructures is one of the key cha... more An in-depth understanding of the interface between cells and nanostructures is one of the key challenges for coupling electrically excitable cells and electronic devices. Recently, various 3D nanostructures have been introduced to stimulate and record electrical signals emanating from inside of the cell. Even though such approaches are highly sensitive and scalable, it remains an open question how cells couple to 3D structures, in particular how the engulfment-like processes of nanostructures work. Here, we present a profound study of the cell interface with two widely used nanostructure types, cylindrical pillars with and without a cap. While basic functionality was shown for these approaches before, a systematic investigation linking experimental data with membrane properties was not presented so far. The combination of electron microscopy investigations with a theoretical membrane deformation model allows us to predict the optimal shape and dimensions of 3D nanostructures for cell-chip coupling.
Epl, 2014
Spontaneous segregation of run-and-tumble particles with different velocities in microchannels is... more Spontaneous segregation of run-and-tumble particles with different velocities in microchannels is investigated by numerical simulations. Self-propelled particles are known to accumulate in the proximity of walls. Here we show how fast particles expel slower ones from the wall leading to a segregated state. The mechanism is understood as a function of particle velocities, particle density, or channel width. In the presence of an external fluid flow, particles with two different velocities segregate due to their different particle fluxes. Promising applications can be found in the development of microfluidic lab-on-a-chip devices for sorting of particles with different motilities.
Advances in Planar Lipid Bilayers and Liposomes, 2011
The Journal of Physical Chemistry B, 2013
It has been established that the addition of amphiphilic diblock copolymers has a boosting effect... more It has been established that the addition of amphiphilic diblock copolymers has a boosting effect in bicontinuous microemulsions by decreasing the minimum amount of surfactant needed to solubilize equal volumes of oil and water. The strength of the polymer effect was found to be about twice larger than the theoretical prediction. This discrepancy is explained by confinement. Previous experimental studies always considered large oil and water domains of size d compared to the typical polymer end-to-end radius, R ee . The ratio of these two parameters R ee /d defines the confinement parameter. We investigated the sensitivity of the polymer influence extending the range of confinement. We combined macroscopic observations of the phase behavior with microscopic measurements of the structure by small-angle neutron scattering (SANS). Both results were compared with computer simulations on the basis of the theoretical concept of Helfrich. The simulations predict an enhanced sensitivity of the polymer at medium confinement and a reversed behavior at larger confinement. The higher sensitivity at medium confinement is only slightly visible experimentally, whereas the reversed behavior (antiboosting) is clearly present. Finally, a comparison with homopolymer addition showed a common high confinement behavior for diblock copolymers and for homopolymers.
Soft Matter, 2013
Membrane budding and wrapping of particles, such as viruses and nano-particles, play a key role i... more Membrane budding and wrapping of particles, such as viruses and nano-particles, play a key role in intracellular transport and have been studied for a variety of biological and soft matter systems. We study nano-particle wrapping by numerical minimization of bending, surface tension, and adhesion energies. We calculate deformation and adhesion energies as a function of membrane elastic parameters and adhesion strength to obtain wrapping diagrams. We predict unwrapped, partially-wrapped, and completely-wrapped states for prolate and oblate ellipsoids for various aspect ratios and particle sizes. In contrast to spherical particles, where partially-wrapped states exist only for finite surface tensions, partially-wrapped states for ellipsoids occur already for tensionless membranes. In addition, the partially-wrapped states are long-lived, because of an increased energy cost for wrapping of the highly-curved tips. Our results suggest a lower uptake rate of ellipsoidal particles by cells and thereby a higher virulence of tubular viruses compared with icosahedral viruses, as well as co-operative budding of ellipsoidal particles on membranes. arXiv:1303.5567v1 [cond-mat.soft]
Proceedings of the National Academy of Sciences, 2010
The remarkable deformability of the human red blood cell (RBC) results from the coupled dynamic r... more The remarkable deformability of the human red blood cell (RBC) results from the coupled dynamic response of the phospholipid bilayer and the spectrin molecular network. Here we present quantitative connections between spectrin morphology and membrane fluctuations of human RBCs by using dynamic full-field laser interferometry techniques. We present conclusive evidence that the presence of adenosine 5′-triphosphate (ATP) facilitates nonequilibrium dynamic fluctuations in the RBC membrane that are highly correlated with the biconcave shape of RBCs. Spatial analysis of the fluctuations reveals that these non-equilibrium membrane vibrations are enhanced at the scale of spectrin mesh size.
Physical Review E, 2007
The fluctuation spectra and the intermembrane interaction of two membranes at a fixed average dis... more The fluctuation spectra and the intermembrane interaction of two membranes at a fixed average distance are investigated. Each membrane can either be a fluid or a solid membrane, and in isolation, its fluctuations are described by a bare or a wave-vector-dependent bending modulus, respectively. The membranes interact via their excluded-volume interaction; the average distance is maintained by an external, homogeneous pressure. For strong coupling, the fluctuations can be described by a single, effective membrane that combines the elastic properties. For weak coupling, the fluctuations of the individual, noninteracting membranes are recovered. The case of a composite membrane consisting of one fluid and one solid membrane can serve as a microscopic model for the plasma membrane and cytoskeleton of the red blood cell. We find that, despite the complex microstructure of bilayers and cytoskeletons in a real cell, the fluctuations with wavelengths lambda greater, similar 400 nm are well described by the fluctuations of a single, polymerized membrane (provided that there are no inhomogeneities of the microstructure). The model is applied to the fluctuation data of discocytes (&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;normal&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; red blood cells), a stomatocyte, and an echinocyte. The elastic parameters of the membrane and an effective temperature that quantifies active, metabolically driven fluctuations are extracted from the experiments.
Physical Review E, 2013
The fluctuation pressure of a lipid-bilayer membrane is important for the stability of lamellar p... more The fluctuation pressure of a lipid-bilayer membrane is important for the stability of lamellar phases and the adhesion of membranes to surfaces. In contrast to many theoretical studies, which predict a decrease of the pressure with the cubed inverse distance between the membranes, Freund suggested very recently a linear inverse distance dependence [Proc. Natl. Acad. Sci. U.S. A. 110, 2047 (2013)]. We address this discrepancy by performing Monte Carlo simulations for a membrane model discretized on a square lattice and employ the wall theorem to evaluate the pressure for a single membrane between parallel walls. For distances that are small compared with the lattice constant, the pressure indeed depends on the inverse distance as predicted by Freund. For intermediate distances, the pressure depends on the cubed inverse distance as predicted by Helfrich [Z. Naturforsch. A 33, 305 (1978)]. Here, the crossover length between the two regimes is a molecular length scale. Finally, for distances large compared with the mean squared fluctuations of the membrane, the entire membrane acts as a soft particle and the pressure on the walls again depends linearly on the inverse distance.
Physical Review E, 2005
The effect of linear homopolymers, diblock copolymers, and star polymers anchored to a membrane o... more The effect of linear homopolymers, diblock copolymers, and star polymers anchored to a membrane on the membrane's fluctuation spectrum is investigated for low grafting densities. Due to the nonlocality of the polymer-membrane interaction, the effective bending rigidity eff ͑q͒ of the composite membrane is found to depend strongly on the wave vector q of the membrane undulations. Analytical calculations for ideal linear chains and simulations for ideal and self-avoiding linear chains as well as for star polymers are presented. The analytical calculations are based on the Green's function approach of Bickel and Marques ͓Eur. Phys. J. E 9, 349 ͑2002͔͒; for the simulations the Monte Carlo method is used. The functional form of eff ͑q͒ differs for end-grafted chains and diblock copolymers. In general, the polymer effect is most pronounced for undulations on length scales larger than or comparable to the polymer size, and decreases rapidly for smaller undulation wavelengths. Anchored linear chains always increase ; anchored star polymers may increase as well as decrease , depending on whether they are anchored symmetrically or asymmetrically to the membrane.
Physical Review E, 2013
Collective behavior of self-propelled particles is observed on a microscale for swimmers such as ... more Collective behavior of self-propelled particles is observed on a microscale for swimmers such as sperm and bacteria as well as for protein filaments in motility assays. The properties of such systems depend both on their dimensionality and the interactions between their particles. We introduce a model for self-propelled rods in two dimensions that interact via a separation-shifted Lennard-Jones potential. Due to the finite potential barrier, the rods are able to cross. This model allows us to efficiently simulate systems of self-propelled rods that effectively move in two dimensions but can occasionally escape to the third dimension in order to pass each other. Our quasi-two-dimensional self-propelled particles describe a class of active systems that encompasses microswimmers close to a wall and filaments propelled on a substrate. Using Monte Carlo simulations, we first determine the isotropic-nematic transition for passive rods. Using Brownian dynamics simulations, we characterize cluster formation of self-propelled rods as a function of propulsion strength, noise, and energy barrier. Contrary to rods with an infinite potential barrier, an increase of the propulsion strength does not only favor alignment but also effectively decreases the potential barrier that prevents crossing of rods. We thus find a clustering window with a maximum cluster size at medium propulsion strengths.