Rheology study of polymer film/lipid complex on the air/water interface using optical tweezer

Abstract

An interface shear rheology is widely studied for the relation on a stability of two- dimensional interface between liquid/liquid or gas/liquid like a lipid bilayer, a foam or an emulsion. Polymers were used to increases a stability of the interface. For example, biopolymers such as spectrin, actin laments, collagen, etc near a cell membrane help a cell maintaining a shape and a structure. And synthetic polymers have been used to stabilize an emulsion in companies. We studied the effects of an adsorbed polymer on a lipid monolayer at the air/water interface using the interface shear micro-rheometer (ISMR) with an optically driven probe particle as a model system of a cell membrane with adsorbed biopolymers. The composite of a polyelectolyte and an insoluble lipid monolayer at the air/water interface has signicantly low interface shear modulus which cannot be measured with conventional instruments like a rotational rheometer with a circular knife-edge probe [53, 17] or an linear shear rheometer that uses a magnetic needle as a probe(ISR) [6, 49]. For the reaseon, we built up new interface shear micro-rheometer using an oscillating optical tweezer. It has high sensitivity to study the composite at the air/water interface which is higher sensitivity than one of conventional instruments. The lm was made up of a negative charged polymer such as poly(styrene sulfonate) (PSS) or deoxyribonucleic acid (DNA) and a positive charged lipid monolayer such as dioctadecyldimethylammonium chloride (DODAC) which is insoluble into an aqueous solution. The statistics of the system was done by Ahrens et al. with X-ray reflectivity. We studied 1) the lipid density dependency, 2) the PSS length dependency, 3) the salt (NaCl) concentration dependency, and 4) the stiffness dependency on a rheology of a composite of a polyelectrolyte and an insoluble lipid monolayer at the air/water interface.