Rotation and alignment of anisotropic particles on nonplanar interfaces.

We study the alignment of micron-scale particles at air-water interfaces with unequal principle radii of curvature by optical microscopy. The fluid interface bends to satisfy the wetting conditions at the three phase contact line where the interface intersects the particle, creating deflections that increase the area of the interface. These deflections decay far from the particle. The far field interface shape has differing principle radii of curvature over length scales large compared to the particle. The deflections create excess area which depends on the angle of the particle with respect to the principle axes of the interface. We show that when particles create surface deflections with quadrupolar modes, the particles rotate to preferred orientations to minimize the free energy. In experiment, we focus on uniform surface energy particles, for which quadrupolar modes are forced by the particle shape. Analytical expressions for the torque and stable states are derived in agreement with experiment and confirmed computationally.

Tip streaming from a drop in the presence of surfactants.

Drop breakup in a linear extensional flow is simulated numerically using a nonlinear model for the surface tension that accounts for maximum packing at the interface. Surface convection sweeps surfactant to the drop poles, where it accumulates and drives the surface tension to near zero. The drop assumes a transient shape with highly pointed tips. From these tips, thin liquid threads are pulled. Subsequently, small, surfactant-rich droplets are emitted from the termini of these threads. The scale of the shed drops depends on the initial surfactant coverage. Dilute initial coverage leads to tip streaming, while high initial coverage leads to the tip dropping breakup mode.

The effects of gramicidin on electroporation of lipid bilayers.

The effects of the channel-forming peptide gramicidin D (gD) on the conductance and electroporation thresholds of planar bilayer lipid membranes, made of the synthetic lipid 1-palmitoyl 2-oleoyl phosphatidylcholine (POPC), was studied. High-amplitude ( approximately 200-900 mV) rectangular voltage pulses of 15 ms duration were used to perturb the bilayers and monitor the transmembrane conductance. Electroporation voltage thresholds were found, and conductance was recorded before and after electroporation. Gramicidin was added to the system in peptide/lipid ratios of 1:10, 000, 1:500, and 1:15. The addition of gD in a ratio of 1:10,000 had no effect on electroporation, but ratios of 1:500 and 1:15 significantly increased the thresholds by 16% (p < 0.0001) and 40% (p < 0.0001), respectively. Membrane conductance before electroporation was measurable only after the addition of gD and increased monotonically as the peptide/lipid ratio increased. The effect of gD on the membrane area expansivity modulus (K) was tested using giant unilamellar vesicles (GUVs). When gD was incorporated into the vesicles in a 1:15 ratio, K increased by 110%, consistent with the increase in thresholds predicted by an electromechanical model. These findings suggest that the presence of membrane proteins may affect the electroporation of lipid bilayers by changing their mechanical properties.

Changes in electroporation thresholds of lipid membranes by surfactants and peptides.

This article reviews recent work from our laboratory that explores how chemical additives may alter the threshold of electroporation of synthetic lipid bilayers. The addition of the nonionic block copolymeric surfactant, poloxamer 188 (P188), at a concentration of 1 mM increased the electroporation thresholds of planar lipid bilayer membranes made of azolectin. For a 10-microsecond rectangular pulse, P188-treated membranes were found to have a statistically higher threshold voltage, longer latency time to rupture, and lower postpulse conductance. Addition of the nonionic surfactant, octaethyleneglycol-mono-n-dodecyl-ether (C12E8), decreased the electroporation threshold of bilayer membranes made of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) by 10-40% for 10-microsecond- to 10-s-duration pulses, in a concentration-dependent manner over concentrations ranging from 0.1 to 10 mM. Postpulse membrane conductance also increased. The opposite effects of the two surfactants on electroporation thresholds may result from their very different structures, which would encourage different modes of surfactant-lipid interactions. To examine protein-lipid interactions and their effects on the electroporation threshold, the effects of a channel-forming polypeptide, gramicidin D (gD), was studied on membrane conductance and electroporation threshold. Electroporation thresholds for 15-ms pulses were unaffected by addition of gramicidin to POPC at a peptide:lipid concentration estimated to be 1:10,000, but increased significantly at ratios of 1:500 and 1:15, while membrane conductance increased monotonically with peptide concentration. A micropipette aspiration technique was applied to giant unilamellar POPC vesicles to measure changes in the membrane physical properties. When gD was added to give an estimated peptide:lipid ratio of 1:15, the membrane area expansivity modulus increased, indicating that the increase in electroporation threshold is correlated with a change in membrane stiffness. Thus, these findings demonstrate that surfactants or peptides can mediate the electroporation threshold of lipid bilayers.

The reduction in electroporation voltages by the addition of a surfactant to planar lipid bilayers.

The effects of a nonionic surfactant, octaethyleneglycol mono n-dodecyl ether (C12E8), on the electroporation of planar bilayer lipid membranes made of the synthetic lipid 1-pamitoyl 2-oleoyl phosphatidylcholine (POPC), was studied. High-amplitude ( approximately 100-450 mV) rectangular voltage pulses were used to electroporate the bilayers, followed by a prolonged, low-amplitude ( approximately 65 mV) voltage clamp to monitor the ensuing changes in transmembrane conductance. The electroporation thresholds of the membranes were found for rectangular voltage pulses of given durations. The strength-duration relationship was determined over a range from 10 micros to 10 s. The addition of C12E8 at concentrations of 0.1, 1, and 10 microM to the bath surrounding the membranes decreased the electroporation threshold monotonically with concentration for all durations (p < 0.0001). The decrease from control values ranged from 10% to 40%, depending on surfactant concentration and pulse duration. For a 10-micros pulse, the transmembrane conductance 150 micros after electroporation (G150) increased monotonically with the surfactant concentration (p = 0.007 for 10 microM C12E8). These findings suggest that C12E8 incorporates into POPC bilayers, allowing electroporation at lower intensities and/or shorter durations, and demonstrate that surfactants can be used to manipulate the electroporation threshold of lipid bilayers.