2565943
Dynamic excitations in membranes induced by optical tweezers.
We present the phenomenology of transformations in lipid bilayers that are excited by laser tweezers. A variety of dynamic instabilities and shape transformations are observed, including the pearling instability, expulsion of vesicles, and more exotic ones, such as the formation of passages. Our physical picture of the laser-membrane interaction is based on the generation of tension in the bilayer and loss of surface area. Although tension is the origin of the pearling instability, it does not suffice to explain expulsion of vesicles, where we observe opening of giant pores and creeping motion of bilayers. We present a quantitative theoretical framework to understand most of the observed phenomenology. The main hypothesis is that lipid is pulled into the optical trap by the familiar dielectric effect, is disrupted, and finally is repackaged into an optically unresolvable suspension of colloidal particles. This suspension, in turn, can produce osmotic pressure and depletion forces, driving the observed transformations.
121149902
Undulations, steric interaction and cohesion of fluid membranes
SummaryThe theory of undulations of fluid membranes is reviewed and in some parts extended. The functional dependences of the steric interaction of undulating membranes are derived in a new way from simple physical arguments. Discussing the competition between steric repulsion and van der Waals attraction, one finds that membranes which usually separate (e.g. giant egg lecithin vesicles) should cohere if under lateral tension. The contours of two cohering vesicles observed when egg lecithin was swelling are analysed to show that the net energy of cohesion can be extremely small (≲10−5 erg cm−2).RiassuntoLa teoria delle ondulazioni delle membrane dei fluidi è rivista ed estesa in alcune parti. Si derivano le dipendenze funzionali dell'interazione sterica delle membrane ondulate in un nuovo modo da semplici argomenti fisici. Discutendo la competizione tra repulsione sterica e attrazione di Van der Waals, si trova che membrane che di solito separano (per esempio, grandi vescicole di lecitina dell'uovo) dovrebbero aderire se sottoposte a tensione laterale. I contorni di due vescicole contigue osservate quando la lecitina dell'uovo si sta rigonfiando mostrano che l'energia netta di coesione può essere estremamente piccola (≲10−5 erg cm−2).РезюмеАнализируется теория неровностей жидких мембран. Из простых физических аргументов выводятся функциональные зависимости стерического взаимодействия волнистых мембран. Рассматривая конкуренцию между стерическим отталкиванием и притяжением Ван дер Ваальса, получается, что мембраны, которые обычно разделяют, должны связывать при поперечном напряжении. Анализируются контуры двух связанных пузырьков и показывается, что суммарная энергия сцепления может быть чрезвочайно малой (≲10−5 эрг см−2).
31950817
Entropy-driven tension and bending elasticity in condensed-fluid membranes.
Sensitive micropipet methods have been used to measure the relation between tension and the projected surface area in fluid membranes of vesicles over a 4-order-of-magnitude range in tension (10 −3 -10 dyn/cm). In the low-tension regime (<0.5 dyn/cm), the data confirm the prediction of equilibrium theory that the projected area should increase logarithmically with tension as shape fluctuations become progressively restricted
34338601
The curvature elasticity of fluid membranes : A catalogue of vesicle shapes
2014 Shapes of closed fluid membranes such as those formed by lecithin in water were calculated as a function of enclosed volume, membrane area and spontaneous curvature. As the area can be taken to be constant, the only elasticity controlling the shapes of these vesicles is that of curvature. A large variety of rotationally symmetric shapes are presented, allowing for indentations, cavities and contact of the membrane with itself. LE JOURNAL DE PHYSIQUE TOME 37, NOVEMBRE 1976, Classification .Physics Abstracts 7.130
120876530
Configurations of fluid membranes and vesicles
Abstract Vesicles consisting of a bilayer membrane of amphiphilic lipid molecules are remarkably flexible surfaces that show an amazing variety of shapes of different symmetry and topology. Owing to the fluidity of the membrane, shape transitions such as budding can be induced by temperature changes or the action of optical tweezers. Thermally excited shape fluctuations are both strong and slow enough to be visible by video microscopy. Depending on the physical conditions, vesicles adhere to and unbind from each other or a substrate. This article describes the systematic physical theory developed to understand the static and dynamic aspects of membrane and vesicle configurations. The preferred shapes arise from a competition between curvature energy, which derives from the bending elasticity of the membrane, geometrical constraints such as fixed surface area and fixed enclosed volume, and a signature of the bilayer aspect. These shapes of lowest energy are arranged into phase diagrams, which separate regi...
26421864
The minimum energy of bending as a possible explanation of the biconcave shape of the human red blood cell.
The membrane of the erythrocyte is assumed responsible for the cell's curious biconcave shape. Combined theoretical and experimental results explain the biconcave shape by the principle of least total curvature of the membrane. This has been interpreted to mean that the energy of bending is minimized. The red-cell membrane is considered uniform over its surface. To accommodate the known viscoelastic properties of the membrane the expression for the bending energy of an element of area, d A , was modified to: d U ∝ (1/ R 1 2 + 1/ R 2 2 ) d A . R 1 and R 2 are the principal radii of curvature. The classical equation for the ovals of Cassini was altered to provide an expression for generating hypothetical red-cell profiles. Using the Cassini equation and the bending relation above (ignoring other membrane forces), the energy was determined for many surfaces of revolution having the same area and volume. The geometry and energy of the shape with least energy closely approximated the bending energy and geometry of the actual red-cell profile with the same area and volume. In addition to predicting the geometry of 23 cells in an isotonic medium and nine osmotically swollen cells, the bending hypothesis predicted a swelling sequence for a single cell which was similar to the observed series of Rand (1967) . The existence of cup shapes frequently observed in hypotonic media fits within the framework of the bending hypothesis. The concept of bending energy adequately explains the shape of the erythrocyte.
24949930
Elastic Properties of Lipid Bilayers: Theory and Possible Experiments
Abstract A theory of the elasticity of lipid bilayers is proposed. Three types of strain, i. e. stretching, tilt and curvature, are distinguished and the associated stresses are identified. It is argued that in the case of vesicles (= closed bilayer films) the only elasticity controlling nonspherical shapes is that of curvature. Euler-Lagrange equations are derived for the shape in magnetic fields and under excess outside pressure. It is shown that magnetic fields can deform spherical vesicles into ellipsoids of revolution. Under excess outside pressure the spherical shape becomes unstable at a certain threshold pressure. Both effects can be influenced by a spontaneous curvature of the bilayer. Some possible experiments to determine the elastic properties are also discussed
20843122
Bending resistance and chemically induced moments in membrane bilayers.
Pure bending of a membrane bilayer is developed including different properties for each membrane half. Both connected and unconnected bilayer surfaces are treated. The bilayer bending resistance is the resultant of parallel surface compression "resistances." The neutral surface is a function of the upper and lower surface compressibility moduli and does not necessarily coincide with the mid-surface. Alterations in the interfacial chemical free energy density (surface tension) on either face can create induced bending moments and produce curvature; even small changes can have a pronounced curvature effect. Chemically induced moments are considered as a possible mechanism for crenation of red blood cells.
97280519
Frequency spectrum of the flicker phenomenon in erythrocytes
2014 Red blood cells show a remarkable flicker phenomenon under physiological conditions. We have studied experimentally the correlations functions G(R12, 03C9) for the flicker intensities measured at two different points r1 r2 on the cell surface, at various filtering frequencies 03C9. We find that the shape of G is universal and involves only one characteristic length 03BB(03C9) varying like 03C9-n, where 0.12 n 0.19. Measurements of G(0, 03C9) (i.e. at a single point) show an 03C9-m dependence, where 1.30 m 1.45. These results are then interpreted theoretically in term of thermal fluctuations of the cell thickness. In physiological conditions the membrane surface tension vanishes exactly and the resistance to deformation is mainly due to curvature energy. In this approximation : a) the fluctuations are of very large amplitude (a fraction of a micron) as required by the observations ; b) the detailed shape of the correlations is in rather good agreement with the theory ; c) the scaling length 03BB(03C9) is expected to vary like 03C9-1/6; d) the single point spectrum G(0, 03C9) should go like 03C9-4/3. The approximation involves the neglect of some non linear effects related to surface tension and to the Evans elastic energy. We show that the inclusion of non linear terms leads to a problem related to the critical point of a (special) two dimensional magnetic system. Our approximation (equivalent to a mean field theory) essentially ignores the exponent correction ~ introduced by Fisher for phase transitions. Thus, the experimental evidence favours a rather small ~. We conclude that a purely physical interpretation of the flicker effect is sufficient, but that rather stringent physiological conditions are required to maintain the zero surface tension which is crucial to the effect. Tome 36 No 11 Novembre 1975 Classification Physics Abstracts 9.720 (*) Institut Pasteur, rue du Dr Roux, Paris 15e. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0197500360110103500
6015649
Spinodal fluctuations of budding vesicles.
Hans-Gunther Dobereiner,1,2,3 Evan Evans,2 Udo Seifert,3 and Michael Wortis1 1Physics Department, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6 2Department of Physics, University of British Columbia, 6224 Agriculture Road, Vancouver, British Columbia, Canada V6T 2A6 3Max-Planck-Institut fur Kolloidund Grenzflachenforschung, Kantstrasse 55, 14513 Teltow-Seehof, Germany (Received 15 May 1995)
38030534
Buckling microtubules in vesicles.
Microtubules are ordered polymers of the protein tubulin that are part of the flexible, plastic, and even self-effacing skeleton of eukaryotic cells [1,2]. They are hollow cylinders, 25 nm in diameter and tens of microns long [3]. Microtubules have large bending rigidity, with a persistence length of the order of millimeters [4]. The elastic properties of microtubules and their interaction with the cell membrane are essential in defining cell morphology. This Letter is concerned with microtubules within artificial phospholipid vesicles, their shapes, and the shape transformations induced by membrane tension. A single microtubule deforms a vesicle into an oval. Under tension, the microtubule buckles and the vesicle recovers a spherical shape. This allows a measurement of the microtubule bending rigidity. A large number of microtubules organize inside a vesicle into a linear bundle which protrudes without piercing the membrane. The vesicle contains the bundle by extending a pair of narrow tubes about the microtubule extremities while maintaining
122043854
Acceleration and trapping of particles by radiation pressure
Micron-sized particles have been accelerated and trapped in stable optical potential wells using only the force of radiation pressure from a continuous laser. It is hypothesized that similar accelerations and trapping are possible with atoms and molecules using laser light tuned to specific optical transitions. The implications for isotope separation and other applications of physical interest are discussed.
22249730
Applications of laser radiation pressure.
Use of lasers has revolutionized the study and applications of radiation pressure. Light forces have been achieved which strongly affect the dynamics of individual small particles. It is now possible to optically accelerate, slow, stably trap, and manipulate micrometer-sized dielectric particles and atoms. This leads to a diversity of new scientific and practical applications in fields where small particles play a role, such as light scattering, cloud physics, aerosol science, atomic physics, quantum optics, and high-resolution spectroscopy.
8197447
Biological applications of optical forces.
D ESIGN CON SID ERATION S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 Bll/ding a Trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 Beam Steering 254 Trapping Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 T RAPPIN G TH EORY . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .. . . . . . . . . . . . ... . . . . 260 Ray-Optics Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 Electromagnetic Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 F ORCE M EASUREM EN T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 Measurement of Trap Stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 Physics of Trup Stiffness Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 B�ownia,! Motion During Force Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 272 PlcotenslOmeters 273 Other Applications of Picotensiometry .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 Determinants of Trapping Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277
25601593
Quantitative measurements of force and displacement using an optical trap.
We combined a single-beam gradient optical trap with a high-resolution photodiode position detector to show that an optical trap can be used to make quantitative measurements of nanometer displacements and piconewton forces with millisecond resolution. When an external force is applied to a micron-sized bead held by an optical trap, the bead is displaced from the center of the trap by an amount proportional to the applied force. When the applied force is changed rapidly, the rise time of the displacement is on the millisecond time scale, and thus a trapped bead can be used as a force transducer. The performance can be enhanced by a feedback circuit so that the position of the trap moves by means of acousto-optic modulators to exert a force equal and opposite to the external force applied to the bead. In this case the position of the trap can be used to measure the applied force. We consider parameters of the trapped bead such as stiffness and response time as a function of bead diameter and laser beam power and compare the results with recent ray-optic calculations.
122262429
Optical Trapping and Manipulation of Neutral Particles Using Lasers
Use of this sensitive and novel technique of trapping and manipulation has revolutionized experimental studies in the fields of light scattering, atomic physics, and the biological sciences. The ability to control the dynamics of small neutral particles enables many fundamental studies and practical applications.
34997359
Entropic expulsion in vesicles.
We report experimental observations and theoretical analysis of entropy driven expulsion in giant lipid vesicles, produced by the application of optical tweezers. The tweezers induce tension in membranes by attracting lipid material into the optical trap. This suppresses fluctuations in the vesicles and converts them into tense pressurized spheres. After shutting off the laser, overpressurized vesicles spontaneously expel inner vesicles, releasing their inner pressure.
30045045
Instability and "pearling" states produced in tubular membranes by competition of curvature and tension.
We investigate the stability of tubular fluid membranes by perturbing them with optical tweezers. A peristaltic instability appears, with wavelength on the order of the tube circumference, characterized by tautness and suppression of curvature fluctuations in the membrane. We interpret this in terms of a model that includes a surface tension term in the elastic energy, and describes a transition to stable, finite amplitude peristaltic states. At high amplitudes the experiment reveals new dynamic states of "pearls" interconnected via thin tubes along which they travel and aggregate.
46100353
Dynamical Theory of the Pearling Instability in Cylindrical Vesicles.
We give a simple theory for recent experiments of Bar-Ziv and Moses% Phys. Rev. Lett. {\bf73} (1994) 1392, in which tubular vesicles are excited using laser tweezers to a ``peristaltic'' state. Considering the hydrodynamics of a bilayer membrane under tension, we reproduce some of the qualitative behavior seen and find a value for the wavelength of the instability in terms of independently measured material parameters, in rough agreement with the experimental values.
2247843
Front propagation in the pearling instability of tubular vesicles
Recently Bar-Ziv and Moses discovered a dynamical shape transformation in- duced in cylindrical lipid bilayer vesicles by the action of laser tweezers. We develop a hydrody- namic theory of fluid bilayers in interaction with the surrounding water and argue that the effect of the laser is to induce a sudden tension in the membrane. We refine our previous analysis to account for the fact that the shape transformation is not uniform but propagates outward from the laser trap. Applying the marginal stability criterion to this situation gives us an improved prediction for the selected initial wavelength and a new prediction for the propagation velocity, both in rough agreement with the experimental values. For example, a tubule of initial radius 0.7 ~m has a predicted initial sinusoidal perturbation in its diameter with wavelength S-S /Lm, as observed. The perturbation propagates as a front with the qualitatively correct front velocity a bit less than 100 ~m/s. In particular we show why this velocity is initially constant, as ob- served and so much smaller than the natural scale set by the tension. We also predict that the front velocity should increase linearly with laser power. Finally we introduce an approximate hydrodynamic model applicable to the fully nonlinear regime. This model exhibits propagating fronts as well as fully-developed "pearled" vesicles similar to those seen in the experiments.
6043793
Spontaneous Expulsion of Giant Lipid Vesicles Induced by Laser Tweezers
Irradiation of a giant unilamellar lipid bilayer vesicle with a focused laser spot leads to a tense pressurized state which persists indefinitely after laser shutoff. If the vesicle contains another object it can then be gently and continuously expelled from the tense outer vesicle. Remarkably, the inner object can be almost as large as the parent vesicle; its volume is replaced during the exit process. We offer a qualitative theoretical model to explain these and related phenomena. The main hypothesis is that the laser trap pulls in lipid and ejects it in the form of submicron objects, whose osmotic activity then drives the expulsion.
135632650
Statistical Thermodynamics Of Surfaces, Interfaces, And Membranes
* NOTE: Each Part begins with an Introduction, and ends with Problems and References. Mixtures and Interfaces * Complex Materials and Interfaces * Review of Classical Statistical Mechanics * Phase Separation in Binary Mixtures * Differential Geometry of Surfaces * Review of Hydrodynamics Interfacial Tension * Free Energy of Surfaces and Interfaces * Surface/Interfacial Tension Theory * Surface-Active Agents Fluctuations of Interfaces * Free Energy of a Fluctuating Interface * Thermal Fluctuations of Interfaces * Capillary Instabilities of Interfaces * Roughening Transition of Solid Surfaces Wetting of Interfaces * Equilibrium * Long-Range Interactions: Macroscopic Theory * Fluctuations of the Contact Line * Equilibrium: Microscopic Description * Dynamics of Wetting Interactions of Rigid Interfaces * Molecular Interactions * Van der Waals Interaction Energies * Continuum Theory of van der Waals Forces * Electrostatic Interactions * Solute-Induced Interactions Flexible Interfaces * Fluid Membranes and Surfactants * Curvature Elasticity of Fluid Membranes * Curvature Moduli * Fluctuations of Fluid Membranes * Interactions of Fluid Membranes Colloidal Dispersions * Dispersions of Interacting Particles * Colloid interactions: DLVO Theory * Long-Range Electrostatic Interactions * Steric Interactions: Polymer Adsorption * Structure of Colloidal Aggregates Self-Absorbing Interfaces * Micelles * Vesicles * Microemulsions * Spongelike and Bicontinuous Phases
22397423
Determination by photon correlation spectroscopy of particle size distributions in lipid vesicle suspensions.
A method of determining particle size distributions in lipid vesicle preparations is outlined. A vesicle suspension is modeled as a polydisperse mixture of spherical shells. The distribution of particle sizes in this mixture is approximated by a continuous, piecewise linear function called a first-order spline. Excellent simultaneous fits to photon correlation spectroscopy data gathered at several different angles are presented. An error analysis is included to indicate the resolution of the method.
11758957
Photon correlation spectroscopic study of the size distribution of phospholipid vesicles.
The dependence of phospholipid vesicle size on lipid composition is investigated by photon correlation spectroscopy. For each lipid composition prolonged ultracentrifugation was used to isolate a nearly uniform population of minimum-sized vesicles. The residual size variations in the samples were sufficient to cause polydispersity that made comparisons between samples difficult. Analyses of the data by the method of cumulants and by a method for approximating the particle size distributions directly are presented. The latter method made possible unambiguous comparisons that revealed small but systematic dependences of vesicle size on composition in vesicles containing mixtures of egg phosphatidylcholine and phosphatidylethanolamine, egg phosphatidylcholine and beef brain sphingomyelin, and in single lipid vesicles of egg phosphatidylcholine, dioleylphosphatidylcholine, and beef brain sphingomyelin. These size dependences are quantified within the resolution limits of the technique and their implications are discussed.
97045826
Intermolecular and surface forces
The Forces between Atoms and Molecules. Principles and Concepts. Historical Perspective. Some Thermodynamic Aspects of Intermolecular Forces. Strong Intermolecular Forces: Covalent and Coulomb Interactions. Interactions Involving Polar Molecules. Interactions Involving the Polarization of Molecules. van der Waals Forces. Repulsive Forces, Total Intermolecular Pair Potentials, and Liquid Structure. Special Interactions. Hydrogen-Bonding, Hydrophobic, and Hydrophilic Interactions. The Forces between Particles and Surfaces. Some Unifying Concepts in Intermolecular and Interparticle Forces. Contrasts between Intermolecular, Interparticle, and Intersurface Forces. van der Waals Forces between Surfaces. Electrostatic Forces between Surfaces in Liquids. Solvation, Structural and Hydration Forces. Steric and Fluctuation Forces. Adhesion. Fluid-Like Structures and Self-Assembling Systems. Micelles, Bilayers, and Biological Membranes. Thermodynamic Principles of Self-Assembly. Aggregation of Amphiphilic Molecules into Micelles, Bilayers, Vesicles, and Biological Membranes. The Interactions between Lipid Bilayers and Biological Membranes. References. Index.
92885296
The budding transition of phospholipid vesicles : a quantitative study via phase contrast microscopy
In an aqueous environment and under appropriate conditions, amphiphilic molecules spontaneously self-assemble to form closed membrane capsules. The morphology of these fluid lipid vesicles is governed by the bending elastic energy of the membrane. Fluid vesicles exist in a variety of shapes, which may be considered phases in analogy to thermodynamics. This thesis describes experiments which explore the phase diagram of micron-scale phospholipid (SOPC) vesicles, concentrating on the region of prolate shapes. We have especially studied thermally induced parameter changes leading to budding, i.e., the spontaneous formation of a satellite from a prolate vesicle. The observations are carried out via quantitative phase-contrast microscopy. A computerbased protocol is developed for analyzing the two-dimensional contour shapes of the video images. Reconstruction of non-spherical 3D shapes from 2D contour data is discussed in detail. We apply the general procedure to prolate vesicles near the budding transition. Experimentally measured shapes and shape trajectories are compared to theory. There are two main contributions to the elastic energy for single component vesicles: First, an unconstrained bilayer resists bending elastically. Secondly, constraint on monolayer areas connected to vesicle closure leads to an overall stretching/compression of one monolayer relative to the other, which gives rise to an area-difference elastic contribution. Both contributions are found to be important for a quantitative theoretical understanding of the experimental data. They have been incorporated into the area-difference-elasticity (ADE) model, which forms the basis for the interpretation of vesicle shapes and shape transitions in this work. Our main results are as follows: Fourier amplitudes describing the equilibrium prolate shapes exhibit a hierarchical structure dominated by a few low-order amplitudes, largely determined by volume and area constraints.
44995322
Dynamics of Rayleigh-like Instability Induced by Laser Tweezers in Tubular Vesicles of Self-Assembled Membranes
We present a theoretical study of the Rayleigh-like, pearling, instability recently observed in tubular vesicles of bilayer membranes under the action of optical tweezers. The tweezers are argued to pull the membrane into their operation zone, thereby inducing a tension in it. Consequently, the membrane can respond in a Rayleigh-like instability, in which its area decreases. Our approach is based on a linear stability analysis of the deformations. However, because of the non-linear coupling existing between the suction process and the growth of the unstable modes, the instability develops different characteristics. At long times, the growth of the instability is controlled by the suction rate of the tweezers, assumed constant in time. The wavelength λ* of the most unstable mode is time-dependent: at short times it decreases as ∼ t -1/2 ; at long times it is either constant, of the order of the tube radius R, or increases as ∼ t 1/4 . The amplitude of the most unstable mode increases (at long times) as ∼ t 1/2 . We delineate the different regimes under which these behaviours are predicted, and discuss their possible implications on experiment. We also briefly discuss the possibility for other non-linear effects, such as an induced spontaneous curvature in the membrane, which may be responsible for the appearance of very narrow tubes in the late stages.
98058299
Instability and Front Propagation in Laser-Tweezed Lipid Bilayer Tubules
We study the mechanism of the “pearling" instability seen recently in experiments on lipid tubules under a local applied laser intensity. We argue that the correct boundary conditions are fixed chemical potentials, or surface tensions Σ, at the laser spot and the reservoir in contact with the tubule. We support this with a microscopic picture which includes the intensity profile of the laser beam, and shows how this leads to a steady-state flow of lipid along the surface and gradients in the local lipid concentration and surface tension (or chemical potential). This leads to a natural explanation for front propagation and makes several predictions based on the tubule length. While most of the qualitative conclusions of previous studies remain the same, the “ramped" control parameter (surface tension) implies several new qualitative results. We also explore some of the consequences of front propagation into a noisy (due to pre-existing thermal fluctuations) unstable medium.
54204781
CRITICAL DYNAMICS IN THE PEARLING INSTABILITY OF MEMBRANES
We report quantitative measurements above threshold in the pearling instability of cylindrical membranes. Induced by optical tweezers, the instability propagates outward from the laser trap with well defined wavelength, and velocity. All measured quantities scale with the reduced tension control parameter e ; sS 2 ScdySc. A critical slowing down for initiation of the instability is observed. The values of the velocity, wavelength, and delay time agree with marginal stability and linear analysis. Measurements very close to threshold are strongly masked by thermal fluctuations. [S0031-9007(97)03733-2]
121848896
Propagating pattern selection
Pattern selection is discussed in regard to a situation where a stable, nonuniform state of a nonlinear dissipative system propagates into an initially unstable, homogeneous region. The velocity of the propagating front and the wavelength of the pattern formed behind the front are determined by a marginal-stability criterion. The special system studied here has a Lyapunov functional, but the periodic state which propagates is not the one which minimizes the functional.
45979801
Mutual adhesion of lecithin membranes at ultralow tensions
We present an extensive light-microscopic study of mutual adhesion of egg (and dimyristoyl) lecithin bilayers in highly swollen samples, including adhesion brought about by osmotic inflation of vesicles. The lateral tensions associated with adhesion, if below ca. 3 x 10-4 dyn cm-1 , could be read from a rounding of the membrane next to the contact area. The contact angle of symmetric adhesion, i.e. of two membranes under equal tension, had a maximum of 45°. While showing some scatter, it did not seem to depend on tension down to 3 x 10-6 dyn cm-1, the lowest tension observed. The maximum contact angle of single membranes adhering to bundles of membranes was near 70°. The data indicate that mutual adhesion of lecithin membranes is always induced by lateral tension. The constancy of the contact angles appears to be compatible with undulation theory, but their magnitude compels us to postulate an unknown, optically unresolvable roughness of lecithin membranes which serves as an area reservoir.
42333243
Adhesion of membranes : a theoretical perspective
We theoretically study (i) a large membrane segment and (ii) a closed membrane surface or vesicle that adhere to another surface. The membrane segment can undergo unbinding transitions as a result of thermally excited shape fluctuations. These transitions are studied by renormalization group methods and by Monte Carlo simulations. The shape of a bound vesicle is determined by the interplay of bending and adhesion energies. This interplay leads to adhesion transitions from a free to a bound vesicle state even in the absence of shape fluctuations. Our theory helps to clarify the notion of a contact angle for membranes.
133708386
Laser-Induced Cell Fusion
In recent years a number of different techniques have been developed to fuse cells of various origin in order to obtain hybrids with qualities different from each of the original cells. In many cases, one wants to fuse large numbers of cells (e.g., to generate hybridomas), and fusion occurs more or less randomly (e.g., Kohler and Milstein, 1975). With other methods, individual cells have been fused under controlled conditions (e.g., McGrath and Solter, 1984). So far a prerequisite for fusing two preselected cells has been that they must be isolated away from other cells.
15388261
Dynamics of osmotic fluid flow
The classical thermodynamic treatment of osmotic pressure is quite sufficient to compute most quantities of interest without reference to any molecular model (see, for example, Finkelstein, 1987). However, one’s intuition is always aided by molecular models, and a microscopic picture can be quite useful when thinking about osmotic flow in unfamiliar situations. Therefore, we offer here a surprisingly simple and intuitive molecular model for describing solvent flow driven by osmotically generated pressure gradients. We will show that osmotic pressure arises from the external force on solute particles which is transmitted to the solute by viscous drag. This simple picture enables us to propose a simulation method that can be used to compute the osmotic flows associated with quite complex situations arising in polymer and gel swelling.
8776076
Osmotic pressure induced pores in phospholipid vesicles.
We report a comparative study of the leadage of hydrophilic molecules from vesicles of egg lecithin (EL) and of dipalmitoyllecithin (DPL). The effect of osmotic pressure differences a leakage is consistent with a model for statistical pore nucleation process. The major difference in osmotic pressure induced leakage from DPL and EL is that the number of pore creation sites is much greater in DPL. We suggest that the difference in number of these sites also accounts for other differences in the properties of DPL and EL, namely for differences in vesicle fusion and apparent rate of "flip-flop".
208609947
Water movement through lipid bilayers, pores, and plasma membranes : theory and reality
THEORY: Osmotic Equilibrium Osmotic Transport (Osmosis) Induced by an Impermeant Solute Tracer Diffusion of Water Single-file Transport Osmotic Transport (Osmosis) LIPID BILAYER MEMBRANES: The Unmodified Membrane Nystatin an Amphotericin B. Gramicidin A PLASMA MEMBRANES: General Considerations The Red Cell Membrane Epithelia: Antidiuretic Hormone (ADH)-Induced Water Permeability Index.
91827102
Pressure as a Probe of Vesicle Fusion
Fusion between two lipid membranes is superficially easy to understand: Two lipid bilayers come into contact and meld into one seamless membrane. But even at this basic level, one must distinguish whether fusion simply means the merging of the two membranes or whether the internal contents (of the liposomes or vesicles) must also mix. Certainly, for the case of vesicles, the requirement that “true” fusion involves the mixing of the internal volumes as well as lipid mixing is a reasonable and practical way of defining fusion. There are other situations, however, where the above definition is inadequate. For instance, in modeling exocytosis, where the internal contents of a liposome is excreted outward, there is no mixing per se of two internal volumes. In this case, the essence of membrane fusion has to be defined slightly differently. Leaving aside such interesting considerations concerning the definition of fusion, there is also much unknown about the process of fusion. Certainly there are many pathways for initiating vesicle membrane fusion; a partial listing would include induction by cations (Papahadjopoulos et al., 1977), by pH (Ellens et al., 1984), by proteins (Morero et al., 1985), by temperature (Verkleij, 1980), by electric field (Zimmermann, 1982; Sowers, 1984), or even spontaneous fusion between vesicles (Suurkuusk et al., 1976; Schullery et al., 1980; Chang et al., 1982).
97128233
Hydration forces between mica surfaces in aqueous electrolyte solutions
Surface forces between molecularly smooth mica sheets were measured in Na+ and K+ aqueous salt solutions (at ∼10−3 M) both at 21°C and, for the case of Na+, at 65°C. Additional short-range repulsive forces were found to be the same at both temperatures. In very low concentrations of electrolyte solution (∼5 × 10−6M) and in hydrochloric acid solutions (up to 1.2 × 10−3M) these forces were completely absent-giving force curves in very good agreement with DLVO theory. Analysis of these force-interaction results appears to prove conclusively the existence of counterion hydration forces on mica. These forces apparently also prevent both bubble coalescence and coagulation of latex colloids at high concentrations of electrolyte. The total interaction between charged colloidal surfaces should involve, as apparently in the case of mica, both short-range repulsive forces due to bound partially hydrated ions and a longer range repulsion due to hydrated counterions in the compressed double-layer. These “hydration” forces should be present in many colloidal systems and be dominant in those with high negative charge densities and little possibility of hydrogen bonding to adjacent water layers. Surfaces consisting of ionic species (e.g., Zwitterionic lecithin bilayers) should give rise to repulsive forces due to the surface ion hydration effects only.
27211866
Instability and deformation of a spherical vesicle by pressure.
The infinitesimal stability of a spherical vesicle (closed membrane) is studied as a function of the pressure difference between the outer and inner media. It is found that above some threshold pressure the spherical vesicle can be deformed into a shape associated with $l\mathrm{th}$-order spherical harmonics. The comparison with numerical examples calculated previously by Deuling and Helfrich shows good agreement. Some applications to red blood cells are discussed.
36679698
Unbinding transition of a biological model membrane.
We have discovered a temperature-driven unbinding transition of fluid membranes in samples of digalactosyldiacylglycerol (DGDG) swollen in 0. 1M NaC1 aqueous solution. Single bilayers were observed by phase contrast light microscopy. The transition is reversible and without apparent hysteresis. Its temperature is only a few C lower for the single membranes of two vesicles than for multilayer systems, but it varies strongly among samples. The bending rigidity of DGDG bilayers was also measured and found to be (0. 12-0.21) x10 ' erg.
20370582
Local unbinding of pinched membranes.
Loosely bound membranes exhibit an unusual elastic response when pinched together by optical tweezers, locally unbinding to a large intermembrane distance. Tweezing a stack of many bound membranes produces extreme local swelling in the vicinity of the tweezing point. We introduce a model that incorporates bending elasticity, fluctuations, and intermembrane interactions to calculate the membrane profiles subject to a local pinch. Theoretically, we find strongly overshooting profiles in agreement with experiment. We predict scaling behavior of the overshoot with the pinch strength and size. PACS numbers: 82.65.Dp, 68. 10.Et, 87.22.Bt Adhesion of biological membranes involves both the attraction of two homogeneous surfaces [1] and the interactions of specific molecular binding sites [2,3]. Experiments suggest that binding sites are dominant locally. However, the overall binding of two membranes with localized "chemical stickers ' is strongly influenced by the elastic response and fluctuations of the embedding bilayer [4]. To study the overall membrane response to a single binding site, we study a simplified model system by locally pinching together two membranes with optical tweezers [5,6]. This system can also model "local collisions, " which are important in the study of interacting, flexible membranes and the understanding of their unbinding transition [7,8]. Previously, we showed that laser tweezers remove area from the membrane, inducing tension in membranes with limited area and thus driving shape transitions. In the present work the tweezers are used primarily as a mechanical tool, clamping at a point two large planar membranes, which effectively have unlimited area and allow only a negligible buildup of tension. We studied bilayers composed of dimyristoyl-phosphatidylcholine (DMPC) (Sigma) produced by a standard technique [9]. Our experimental setup is described elsewhere [10,11]. Typical working conditions were 30 C and laser intensities from 30 up to 150 mW. We selected sets of two or more nearly Oat membranes, which were sections either of huge vesicles or of lamellar sheets that extended from the bottom of the cell to its top. We chose sections of membranes that appeared to be bound or loosely bound, at a distance of -1 p, m. The membranes are pinched by the laser tweezers in the — 0.5 p, m region of the trap. Regardless of the
96380828
Membrane-induced interactions between inclusions
We investigate theoretically the effect of embedded inclusions on membrane structure, and the corresponding membrane-induced interactions between inclusions. We find that the membrane thickness, which is perturbed from its equilibrium value by the coupling to the embedded inclusions, decays non-monotonically with distance from the inclusion boundary. As a result, the membrane-induced interactions between inclusions vary non monotonically with spacing. The periodicity of the perturbation profile, as well as the strength and range of the induced interactions, are proportional to the ratio of the amphiphile bending modulus and compressibility
204015044
Anchor ring-vesicle membranes.
A family of the exact and analytical solutions of the equilibrium shape equation of vesicle membranes is found. They are anchor rings with generating circles of radii in the ratio 1/ \ensuremath{\surd}2 . It is shown that these ring vesicles are stable for some negative values of their spontaneous curvatures, such that experimental construction of such vesicles seems possible. A discussion shows that a positive Gauss-curvature elastic modulus favors the formation of these special vesicles.
45729712
Conformal degeneracy and conformal diffusion of vesicles.
The shape of vesicles with genus g=2, i.e., with two holes or two handles, is studied in the framework of curvature models. These vesicles exhibit a new phase which also persists for higher genus g>2. In this phase, the ground state of the vesicle is conformally degenerate even when the volume, the area, and the total mean curvature of the vesicle are kept constant. It is predicted that such vesicles undergo a new type of diffusive motion, termed conformal diffusion, which should be observable in experiments as pronounced shape fluctuations.
31440180
Shape Transformations of Giant Vesicles: Extreme Sensitivity to Bilayer Asymmetry
Shape transformations of vesicles of lecithin (DMPC) in water are induced by changing the temperature which effectively changes the volume-to-area ratio. Three different routes are found which include i) symmetric-asymmetric re-entrant transitions from a dumbbell to a pear-shaped state, ii) the expulsion of a smaller vesicle (budding), and iii) discocyte–stomatocyte transitions. All of these shape transformations are explained within a model for the bending energy of the bilayer which assumes i) that the two monolayers do not exchange lipid molecules, and ii) that the adjacent monolayers exhibit a small difference in their thermal expansivities which is easily produced, e.g., by residual impurities.
20240034
Shape transitions and shape stability of giant phospholipid vesicles in pure water induced by area-to-volume changes.
Shape transformations of vesicles of dimyristoylphosphatidylcholine (= DMPC) and palmitoyloleylphosphatidylcholine (= POPC) in ion-free water were induced by changing the area-to-volume ratio via temperature variations. Depending on the pretreatment we find several types of shape changes for DMPC (in pure water) at increasing area-to-volume ratio: (a) budding transitions leading to the formation of a chain of vesicles at further increase of the area-to-volume ratio, (b) discocyte-stomatocyte transitions, (c) reentrant dumbbell-pear-dumbbell transitions, and (d) spontaneous blebbing and/or tether formation of spherical vesicles. Beside these transitions a more exotic dumbbell-discocyte transition (e) was found which proceeded via local instabilities. Pears, discocytes, and stomatocytes are stable with respect to small temperature variations unless the excess area is close to values corresponding to limiting shapes of budded vesicles where temperature variations of less than or equal to 0.1 degree C lead to spontaneous budding to the inside or the outside. For POPC we observed only budding transitions to the inside leading either to chains of vesicles or to distributions of equally sized daughter vesicles protruding to the inside of the vesicle. Preliminary experiments concerning the effect of solutes are also reported. The first three types of shape transitions can be explained in terms of the bilayer coupling model assuming small differences in thermal expansivities of the two monolayers. This does not hold for the observed instabilities close to the limiting shapes.