Microfluidic detection and analysis by integration of evanescent wave sensing with thermocapillary actuation

An integrated system capable of microfluidic actuation, detection and sensing is described which combines evanescent wave sensing with thermocapillary manipulation. Liquid droplets or streams transported across the beam path of a planar thin film waveguide, which encapsulates the microheater array, induce attenuation of the propagating waveguide modes. The attenuated signal is used to monitor droplet location, dye concentration in aqueous solutions and reaction kinetics for enzymatic hydrolysis of the sugar X-galactose by beta-galactosidase

Hydrodynamic fingering instability of driven wetting films: hindrance by diffusion

Recent experimental and theoretical efforts have revealed the existence of a fingering instability at the moving front of thin liquid films forced to spread under gravitational, rotational or surface shear stresses, as for example by using the Marangoni effect. The authors describe how the presence of a precursor film in front of the spreading macroscopic film, whether it is by prewetting the substrate or by surface diffusion or multilayer absorption, can prevent the development of the instability

Model for the fingering instability of spreading surfactant drops

We show that the Marangoni effect drives the fingering instability observed at the edge of an aqueous surfactant drop spreading on a thin film of water. A calculation of the unperturbed flow profile demonstrates that the spreading of the drop is controlled by the dynamics of a thin layer which develops in front of the drop. The surface-tension gradient in this region leads to the fingering instability via a mechanism mathematically similar to that in Hele-Shaw flow despite the very different underlying physics

Reverse undercompressive shock structures in driven thin film flow

We show experimental evidence of a new structure involving an undercompressive and reverse undercompressive shock for draining films driven by a surface tension gradient against gravity. The reverse undercompressive shock is unstable to transverse perturbations while the leading undercompressive shock is stable. Depending on the pinch-off film thickness, as controlled by the meniscus, either a trailing rarefaction wave or a compressive shock separates from the reverse undercompressive shock

Angelo: 3D Lithography Based on Thermocapillary Sculpting of Nanofilms

Background Many systems in nature caused by flows generated by thermal gradients give rise to beautiful pattern formations, perhaps the most famous of which is the Rayleigh-Bénard instability triggered by buoyancy effects. This instability, which occurs in systems ranging in size from millimeters to kilometers and beyond, generates arrays of fluid cells which undergo circular convection. Evaporative remnants of this instability in multicomponent systems can produce spectacular polygonal salt crust formations as seen in the largest salt flat on Earth called the Salar de Uyuni in Bolivia. Once the dimensions of a fluid system are reduced to the micron scale, however, the surface to volume ratio increases tremendously, whereupon surface forces like thermocapillarity become predominant and trigger instabilities such as Bénard-Marangoni flow. In recent years, digital control over thermocapillary forces along an air/liquid interface has led to the development of planar microfluidic devices capable of numerous functions including droplet positioning and registration, routing, coalescence, scission and even mixing Electrostatic, Phononic or Thermocapillary Driving Force? For over 15 years now, researchers have struggled to explain a phenomenon whereby a flat molten polymeric nanofilm exposed to a nearby cooler substrate undergoes spontaneous formation of nanopillars, rings, spirals, chain-like forms, labyrinth and other structures separated by about 1 -10 microns. Even after solidification, these 3D arrays exhibit ultrasmooth interfaces, which are particularly advantageous for use in optical and photonic systems. Three very different mechanisms have been proposed as the source of instability: (i) electrostatic attraction between the molten film and proximate substrate due to image charge (Chou et al. 2002), (ii) radiation pressure from coherent interface reflections of acoustic phonons (Schäffer et al. 2003), and (iii) enhancement of film fluctuations by thermocapillary forces Fabrication of Microlens Arrays by Angelo Technique To capitalize on this finding, we turn toward nonlinear amplification of film deformation by patterned masks of slender pin arrays placed in close proximity and maintained at a cooler temperature than the nanofilm. The resulting temperature distribution projected onto the film surface produces the necessary thermocapillary stress distribution for sculpting desired liquid shapes in 3D. Numerical studies based on finite element simulations are used to tune the amplitude and pitch of the resulting microlens arrays. Scanning white light interferometry of the solidified shapes also helps optimize array parameters for micro-optical applications such as Shack-Hartmann wavefront sensors Acknowledgements SMT wishes to thank the students and postdocs who contributed to this work over the years, namely A

Fingering instability in thin wetting films

We report on a new hydrodynamic instability which occurs at the spreading edge of a thin wetting film. A drop of aqueous surfactant solution placed on a glass surface moistened with a thin layer of water spreads by propagating fingers, whose velocity and shape depend on the thickness of the ambient water layer and on the surfactant concentration. The two fluids are miscible and show negligible viscosity difference, ruling out a Saffman-Taylor instability. We propose that the Marangoni effect, which is fluid flow induced by gradients in surface tension, drives the instability

The current role of touch imprint cytology in sentinel lymph node intra-operatory examination

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