Enhancement of Cell Membrane Invaginations, Vesiculation and Uptake of Macromolecules by Protonation of the Cell Surface

The different pathways of endocytosis share an initial step involving local inward curvature of the cell’s lipid bilayer. It has been shown that to generate membrane curvature, proteins or lipids enforce transversal asymmetry of the plasma membrane. Thus it emerges as a general phenomenon that transversal membrane asymmetry is the common required element for the formation of membrane curvature. The present study demonstrates that elevating proton concentration at the cell surface stimulates the formation of membrane invaginations and vesiculation accompanied by efficient uptake of macromolecules (Dextran-FITC, 70 kD), relative to the constitutive one. The insensitivity of proton induced uptake to inhibiting treatments and agents of the known endocytic pathways suggests the entry of macromolecules to proceeds via a yet undefined route. This is in line with the fact that neither ATP depletion, nor the lowering of temperature, abolishes the uptake process. In addition, fusion mechanism such as associated with low pH uptake of toxins and viral proteins can be disregarded by employing the polysaccharide dextran as the uptake molecule. The proton induced uptake increases linearly in the extracellular pH range of 6.5 to 4.5, and possesses a steep increase at the range of 4> pH>3, reaching a plateau at pH≤3. The kinetics of the uptake implies that the induced vesicles release their content to the cytosol and undergo rapid recycling to the plasma membrane. We suggest that protonation of the cell’s surface induces local charge asymmetries across the cell membrane bilayer, inducing inward curvature of the cell membrane and consequent vesiculation and uptake

Effect of counterions on the Rayleigh-Plateau instability of a charged cylinder

The effect of counterions on the instability of a charged cylinder is investigated. Both axisymmetric and asymmetric perturbations are considered. The analysis shows that the Rayleigh-Plateau instability is modified for a charged cylinder in the presence of counterions. For the axisymmetric instability, the counterions have a stabilizing effect at low values of kappa, the inverse Debye layer thickness. However, the effect is destabilizing at higher values of kappa The asymmetric modes which are stable for an uncharged cylinder are rendered unstable at high values of kappa The analysis should be important in pearling instability of charged cylindrical vesicles. The expression for the correlation time of thermally induced shape fluctuations of charged cylindrical vesicles is also derived

Oscillatory and electrohydrodynamic instabilities in flow over a viscoelastic gel

The stability of oscillatory flows over compliant surfaces is studied analytically and numerically. The type of compliant surfaces studied is the incompressible viscoelastic gel model. The stability is determined using the Floquet analysis, where amplitude of perturbations at time intervals separated by one time period is examined to determine whether perturbations grow or decay. Oscillatory flows pas viscoelastic gels exhibit an instability in the limit of zero Reynolds number, and the transition amplitude of the oscillatory velocity increases with the frequency of oscillations. The transition amplitude has a minimum at a finite wavenumber for the viscoelastic gel model. The instability is found to depend strongly on the gel viscosity eta (g) , and the effect of oscillations on the continuation of viscous modes at intermediate Reynolds number shows a complicated dependence on the oscillation frequency. Experimental studies are carried out on the stability of an oscillatory flow past a viscoelastic gel at zero Reynolds number, and these confirm the theoretical predictions

Time-dependent electrohydrodynamics of a compressible viscoelastic capsule in the small-deformation limit

A theoretical analysis of the time-dependent electrohydrodynamics of a viscoelastic compressible capsule, characterized by the two-dimensional Young's modulus and surface viscosity, is studied in the small-deformation limit. A systematic ac electrohydrodynamics analysis is conducted, and time-independent and time-periodic deformations are related to the electric capillary number and themembrane properties. Additionally, the relaxation of a capsule stretched by a dc electric field is also presented. This necessitates an accurate estimation of the initial strain field in the stretched capsule. Both an oscillatory analysis and an analysis of the relaxation of a stretched capsule are presented for a capsule containing an aqueous phase, modeled as a perfect conductor, and suspended in a perfect dielectric with an infinitesimally thin viscoelastic membrane separating the two. The membrane is assumed to be a perfect dielectric with no electrical contrast with the suspending fluid

Dielectrophoresis and deformation of a liquid drop in a non-uniform, axisymmetric AC electric field

Analytical theory for the dielectrophoresis and deformation of a leaky dielectric drop, suspended in a leaky dielectric medium, subjected to non-uniform, axisymmetric Alternating Current (AC) fields is presented in the small deformation limit. The applied field is assumed to be a combination of a uniform part and a quadrupole component. The analysis shows that the magnitude and the sign of the steady and time-periodic dielectrophoretic velocity depend upon the frequency of the applied voltage. The frequency of oscillatory motion is twice that of the applied frequency and the phase lag is a consequence of charge dynamics. A deformed drop under non-uniform axisymmetric AC fields admits Legendre modes l = 2, 3, 4. The deformation has a frequency-dependent steady and time-periodic parts due to charge and interface dynamics. The steady deformation can be zero at a certain critical frequency in leaky dielectric systems. The time-periodic deformation also has a frequency which is twice the frequency of the applied voltage. In perfect dielectric systems, unlike the steady state deformation which is a balance of Maxwell and curvature stresses, the time-periodic deformation additionally includes viscous stresses associated with the oscillatory shape changes of the drop. A consequence of this effect is a phase lag that is dependent on the charge and interface hydrodynamics and a lag of pi/2 at high frequencies. It also results in vanishing amplitude of the oscillatory deformation at high frequencies. The study should lead to a better understanding of dielectrophoresis under non-uniform axisymmetric AC fields and better electrode design to affect drop breakup

Deformation and breakup of a leaky dielectric drop in a quadrupole electric field

The deformation and breakup of a leaky dielectric drop suspended in a leaky dielectric medium subjected to a quadrupole electric field are studied. Analytical (linear and nonlinear asymptotic expansions in the electric capillary number, Ca-Q, a ratio of electric to capillary stress) and numerical (boundary element) methods are used. A complete phase diagram for the drop deformation in the R-Q plane is presented, where R and Q are the non-dimensional ratios of the resistivities and dielectric constants, respectively, of the drop and the medium phase. The prolate and oblate deformations are mapped in the phase diagram, and the flow contours are also shown. The large deformation and breakup of a drop at higher Ca-Q are analysed using the boundary element method. Several non-trivial shapes are observed at the onset of breakup of a drop. A prolate drop always breaks above a certain critical value of Ca-Q. In the oblate deformation cases, breakup as well as steady shapes are observed at a higher value of Ca-Q. A detailed study of prolate and oblate deformation tendencies due to the normal and tangential electric stresses and the countervailing role of viscous stresses is presented. The circulation inside a drop is found to be more intense for a quadrupole field as compared with a uniform electric field. More intense internal circulations can lead to enhanced mixing characteristics and will have implications in microfluidic devices

Deformation, breakup and motion of a perfect dielectric drop in a quadrupole electric field

A detailed nonlinear analysis of the deformation and breakup of a perfect dielectric (PD) drop, suspended in another perfect dielectric fluid, in the presence of a quadrupole electric field is presented using analytical (asymptotic) and numerical (boundary integral) methods. The quadrupole field is the simplest kind of an axisymmetric non-uniform electric field. A drop, when placed at the center of such a field, does not translate, thus allowing systematic investigation of the effect of non-uniformity of the electric field. The deformation of a drop under a quadrupole field for PD-PD systems exhibits several novel features as compared to that of a drop under a uniform electric field. The first order analysis predicts oblate deformation for a PD-PD system when the dielectric constant of the suspending medium is larger than that of the drop (Q = epsilon(i)/epsilon(e) 1, and the deformation is larger than that for uniform fields for similar electric capillary numbers. The steady state shapes are defined by higher harmonics as compared to the uniform field. At large capillary numbers, prolate deformations (Q > 1) show breakup whereas oblate deformations (Q < 1) do not. Positive and negative dielectrophoresis is observed when the drop is placed off center, and its translation and simultaneous deformation under quadrupole fields is also investigated. The electro-hydrostatics is unaffected by the viscosity ratio. However, the breakup of the drop and the dielectrophoretic motion and deformation strongly depend upon the viscosity ratio. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3691655

Electrohydrodynamics of a compound vesicle under an AC electric field

Compound vesicles are relevant as simplified models for biological cells as well as in technological applications such as drug delivery. Characterization of these compound vesicles, especially the inner vesicle, remains a challenge. Similarly their response to electric field assumes importance in light of biomedical applications such as electroporation. Fields lower than that required for electroporation cause electrodeformation in vesicles and can be used to characterize their mechanical and electrical properties. A theoretical analysis of the electrohydrodynamics of a compound vesicle with outer vesicle of radius R-0 and an inner vesicle of radius lambda R-0, is presented. A phase diagram for the compound vesicle is presented and elucidated using detailed plots of electric fields, free charges and electric stresses. The electrohydrodynamics of the outer vesicle in a compound vesicle shows a prolate-sphere and prolate-oblate-sphere shape transitions when the conductivity of the annular fluid is greater than the outer fluid, and vice-versa respectively, akin to single vesicle electrohydrodynamics reported in the literature. The inner vesicle in contrast shows sphere-prolate-sphere and sphere-prolate-oblate-sphere transitions when the inner fluid conductivity is greater and smaller than the annular fluid, respectively. Equations and methodology are provided to determine the bending modulus and capacitance of the outer as well as the inner membrane, thereby providing an easy way to characterize compound vesicles and possibly biological cells

Electrohydrodynamic instabilities at interfaces subjected to alternating electric field

Instabilities at the interface of two immiscible fluids, either perfect or leaky dielectrics, subjected to alternating electric fields, is studied using a linear stability analysis in the limit of the electrode spacing being large compared to the wavelength of the perturbation. The Floquet analysis of the stability of this system indicates a significant effect of the frequency on the value of s(max), the growth rate of the fastest growing instabilities and E(Taylor), the minimum field required to excite an instability. It is seen that alternating fields act to damp the system instabilities compared to the direct current (dc) case. Moreover, the growth rate of the instabilities can be tuned from that of leaky dielectric fluids subjected to dc fields, in the low frequency limit, to that of perfect dielectrics in the high frequency limit. It is also observed that for a leaky dielectric-leaky dielectric interface, the alternating current (ac) fields can induce instabilities in a system which is stable at zero frequency, by increasing the frequency of the applied voltage. (C) 2010 [doi:10.1063/1.3431043

Weakly nonlinear analysis of the electrohydrodynamic instability of a charged membrane

The effect of nonlinear interactions on the linear instability of shape fluctuations of a flat charged membrane immersed in a fluid is analyzed using a weakly nonlinear stability analysis. There is a linear instability when the surface tension reduces below a critical value for a given charge density, because a displacement of the membrane surface causes a fluctuation in the counterion density at the surface, resulting in an additional Maxwell normal stress at the surface which is opposite in direction to the stress caused by surface tension. The nonlinear analysis shows that at low surface charge densities, the nonlinear interactions saturate the growth of perturbations resulting in a new steady state with a fluctuation amplitude determined by the balance between the destabilizing electrodynamic force and surface tension. As the surface charge density is increased, the nonlinear terms destabilize the perturbations, and the bifurcation is subcritical. There is also a significant difference in the predictions of the approximate Debye-Huckel and more exact Poisson-Boltzmann equations at high charge densities, with the former erroneously predicting that the bifurcation is supercritical at all charge densities