No abstract

We report on a fluorescence microscopy study of the monolayer collapse and shedding behavior due to shell compression during the dissolution of air-filled, lipid-coated microbubbles in degassed media. The monolayer shell was comprised of saturated diacyl phosphatidylcholine (C12:0 to C22:0) and an emulsifier, poly(ethylene glycol)-40 stearate. The morphologies of monolayer collapse structures and shed particles were monitored as a function of phospholipid acyl chain length (n) and temperature. The two components formed a single miscible phase when the phospholipid was near or above its main phase transition temperature, and collapse occurred via suboptical particles to vesicles (both were shed) and tubes as chain length increased. Conversely, two-phase coexistence was observed when the lipid was below its main phase transition temperature. For these bubbles, a transition from primary collapse to secondary collapse was observed. Primary collapse was observed as a loss of expanded phase due to vesiculation. Secondary collapse involved the rapid propagation of monolayer folds and simultaneous deformation. For very rigid monolayers, we observed substantial surface buckling with simultaneous nucleation and growth of folds. The folds merged at a single point or region, providing a conduit for the entire excess lipid to shed in a single event, and the bubble smoothed and became more spherical. These results are discussed in the context of general binary phospholipid collapse behavior, microbubble dissolution behavior, medical applications, and the dissolution behavior of natural microbubbles.

A method is presented that allows novel measurement of the effect of microstructure on the oxygen permeability of highly condensed, polycrystalline phospholipid monolayers. Oxygen permeability of the polycrystalline shell coating a stationary microbubble is measured directly using an apposing microelectrode in the induced transfer mode and modeling oxygen flux through the shell and intervening aqueous medium. Varying cooling rate through the phospholipid main phase transition permits control of shell microstructure by manipulation of crystalline domain size and shape. Domain boundary density, defined as the ratio of the mean domain perimeter to the mean domain area, of the microbubble shell is determined by fluorescence microscopy. Oxygen permeability was shown to increase linearly with domain boundary density at a constant phospholipid acyl chain length and, accordingly, was shown to decrease exponentially with increasing chain length at a constant domain boundary density. Modification of the energy barrier theory to account for microstructural effects, in terms of the domain boundary density, provides a general equation to model passive transport through polycrystalline monolayer films. Results from this method show promise in determining the gas transport kinetics of medical microbubbles and the gas exchange characteristics of biological monolayers.

A thermally stripped acrylic polymer was wet with a series of liquids possessing a broad range of properties. Previously, novel wetting behavior by water was reported for the polymer, which included the formation of a wetting ridge structure substantially larger than those reported elsewhere and the complete halting of the three-phase line. This allows metastable angles ranging from 0 degrees to greater than 150 degrees to be achieved through changes in the sessile drop volume. Greater advancing angles are prevented by the collapse of the drop, producing what has been described as stick-and-break propagation. In Wilhelmy plate experiments for metal plates coated with the polymer, this mechanism produces a quasi-periodic pattern of lines composed of ridge structures. Similar behavior was observed for all liquids tested. Differences were observed in the maximum force measured with a tensiometer (pinning force) and the average distance between ridges for the formed pattern (pinning distance). These quantities are shown to be related to the height of the ridge structures. The kinematic viscosity of the liquids appears to be an important variable for the wetting process. A comparison of pinning quantities at various rates with the master curve of the polymer indicate that its viscoelastic properties govern, to a great extent, the observed rate dependencies; i.e., higher rates produce greater elastic behavior and smaller ridge heights. Also important is the polymer's tendency for creep deformation. The ridge apex is shown to be displaced a significant distance through ridge deformation, which modifies its symmetry.

A film composed of a thermal-stripped, solvent-borne acrylic polymer is shown to completely arrest motion of the three-phase line for water as a result of ridge structure formation. This mechanism produces anomalous wetting behavior including the arbitrary selection of contact angles, formation of quasi-periodic ridge structures on surfaces, and requirement of stick and break motion for wetting line advancement, a novel mechanism reported here. The ridges are retained by the polymer subsequent to wetting, which are 2 scales larger in height than those described previously. This allows for their characterization, which shows significant detail including the hierarchical apex structure where a cutoff area is used in theoretical treatment to avoid a singularity. Results of Wilhelmy plate experiments show a spatial connection between quasi-periodic variation in force-displacement curves and the wetting ridges on plate. These results are consistent with the dominance of the viscoelastic properties of the substrate in determining wetting behavior.