Biomedical subjects

Th M Fischer

Publications and source records attributed to Th M Fischer.

8 recordsLinked to original sources

PubMed2006

Flow-controlled phase boundaries in Langmuir monolayers.

Flow-controlled fingering of the liquid expanded/liquid condensed phase boundary in a 2-d insoluble monolayer is investigated using a laser-induced thermocapillary pump. Spatially periodic perturbations of the initially smooth monolayer phase boundary between a liquid expanded and liquid condensed phase are shown to lead to the development of steady profile of one-dimensional fingers. The steady-state modulation wave vector and the transient growth rate increase with the flow velocity that drives the instability following power scaling laws consistent with a theory of Bruinsma, Rondelez, and Levine (Bruinsma, R.; Rondelez, F.; Levine, A. Eur. Phys. J. E. 2001, 6, 191) on flow rather than diffusion dominated instabilities in monolayers.

Air↗

PubMed2006

Laser-induced local collapse in a langmuir monolayer.

Heating of a two-dimensional, methyloctadecanoate, Langmuir monolayer with a focused laser induces the local collapse of the monolayer. We observe the growth of a three-dimensional collapse aggregate that is fed by an inward flow of the two-dimensional monolayer surroundings. The experiments are explained with a hydrodynamic model describing the dynamics of the local collapse. From this theory we predict that local collapse can be induced if the collapse pressure of the monolayer decreases faster with temperature than with the surface tension of the pure air/water interface. Such conditions are fulfilled for lung surfactants, and it should therefore be possible to perform time-resolved local studies of the collapse of lung surfactants at those temperatures.

Journal Article↗

PubMed2006

Autonomously moving nanorods at a viscous interface.

We study the autonomous motion of catalytic nanorods in Gibbs monolayers. The catalytic activity of the rods on a hydrogen peroxide aqueous subphase gives rise to anomalous translational and rotational diffusion. The rods perform a Levy-walk superdiffusive motion that can be decomposed into thermal orientation fluctuations and an active motion of the rods with a constant velocity along their long axis. Since interfacial dissipation increases relative to bulk phase dissipation when miniaturizing the size of objects moving in the interface, the autonomous nanorods allow for precise measurements of surface shear viscosities as low as a few nN s/m. The cross over from active motion toward passive diffusion when increasing the surfactant concentration is explained by a loss of friction asymmetry of the rods.

Journal Article↗

PubMed2005

Nonequilibrium bubbles in a flowing langmuir monolayer.

We investigate the nonequilibrium behavior of two-dimensional gas bubbles in Langmuir monolayers. A cavitation bubble is induced in liquid expanded phase by locally heating a Langmuir monolayer with an IR-laser. At low IR-laser power the cavitation bubble is immersed in quiescent liquid expanded monolayer. At higher IR-laser power thermo capillary flow around the laser-induced cavitation bubble sets in. The thermo capillary flow is caused by a temperature dependence of the gas/liquid line tension. The slope of the line tension with temperature is determined by measuring the thermo capillary flow velocity. Thermodynamically stable satellite bubbles are generated by increasing the surface area of the monolayer. Those satellite bubbles collide with the cavitation bubble. Upon collision the satellite bubbles either coalesce with the cavitation bubble or slide past the cavitation bubble. Moreover we show that the satellite bubbles can also be produced by the emission from the laser-induced cavitation bubbles.

Capillary Action↗

PubMed2005

String defects connecting pairs of half-integer disclinations and tilting transition of a langmuir monolayer.

The static and dynamic string defect textures connecting pairs of half-integer disclinations have been observed by Brewster angle microscopy in the solid phase of pentacosadiynoic acid Langmuir monolayers. The static string defect structures have appeared coexisting with two kinds of point disclinations that have four and two black brushes. The use of local laser heating has allowed one to observe kinetics of creation and annihilation of string defects connecting the two-half-integer disclinations in the splitting process of an s = 1 point disclination into fractional disclinations. These kinetics have been analyzed by studying the competition between the orientational elasticity of the molecules and the line tension of the string and the drag force of the disclinations.

Journal Article↗

PubMed2004

Magnetic beads as interfacial nanoprobes.

We use paramagnetic beads to probe strongly localized magnetic fields from one-dimensional nanomagnets. Using a polarization microscope in reflection mode, we find that light reflected from beads exhibits intensity fluctuations which may help us understand Brownian motion near interfaces. We estimate the height fluctuations and femtonewton forces acting on the beads.

Journal Article↗

PubMed2004

Spreading dynamics of 2D dipolar Langmuir monolayer phases.

We study the spreading of a liquid 2D dipolar droplet in a Langmuir monolayer. Interfacial tensions (line tensions) and microscopic contact angles depend on the scale on which they are probed and obey a scaling law. Assuming rapid equilibration of the microscopic contact angle and ideal slippage of the 2D solid/liquid and solid/gas boundary, the driving force of spreading is merely expressed by the shape-dependent long-range interaction integrals. We obtain good agreement between experiment and numerical simulations using this theory.

Computer Simulation↗

PubMed2002

Cavitation of Langmuir monolayers.

Cavitation in liquid expanded and liquid condensed Langmuir monolayers induced by laser heating or microbubble coalescence is studied experimentally using fluorescence and Brewster angle microscopy. The kinetics of hole closure of two-dimensional (2D) gaseous cavitation bubbles exhibits a decelerated dynamics for cavities surrounded by a liquid expanded phase and an accelerated dynamics for cavities in a liquid condensed phase. Most of the cavities in liquid condensed phases possess a nonconvex shape and do not close. The results are compared with theoretical predictions derived for 2D cavitation of liquid monolayers of different surface shear viscosities, and for solid monolayers with diffusive flux of vacancies and interstitials. While part of the theory is in qualitative agreement with the experiment, the experimentally observed hole persistence within the liquid condensed phases and the hole closure within liquid expanded phases remains to be explained. The technique of microbubble coalescence might be particularly useful for the study of the rheological properties of hexatic phases.

Journal Article↗