Chaotic mixing using source-sink microfluidic flows in a PDMS chip

We present an active fixed-volume mixer based on the creation of multiple source-sink microfluidic flows in a polydimethylsiloxane (PDMS) chip without the need of external or internal pumps. To do so, four different pressure-controlled actuation chambers are arranged on top of the 5μl volume of the mixing chamber. After the mixing volume is sealed/fixed by microfluidic valves made using ‘microplumbing technology', a virtual source-sink pair is created by pressurizing one of the membranes and, at the same time, releasing the pressure of a neighboring one. The pressurized air deforms the thin membrane between the mixing and control chambers and creates microfluidic flows from the squeezed region (source) to the released region (sink) where the PDMS membrane is turned into the initial state. Several schemes of operation of virtual source-sink pairs are studied. In the optimized protocol, mixing is realized in just a sub-second time interval, thanks to the implementation of chaotic advectio

Optical Microscopy in the Nano-World

Scanning near-field optical microscopy (SNOM) is an optical microscopy whose resolution is not bound to the diffraction limit. It provides chemical information based upon spectral, polarization and/or fluorescence contrast images. Details as small as 20 nm can be recognized. Photophysical and photochemical effects can be studied with SNOM on a similar scale. This article reviews a good deal of the experimental and theoretical work on SNOM in Switzerland

Understanding the mixing process in 3D microfluidic nozzle/diffuser systems: simulations and experiments

We characterise computationally and experimentally a three-dimensional (3D) microfluidic passive mixer for various Reynolds numbers ranging from 1 to 100, corresponding to primary flow rates of 10-870 mu l min(-1). The 3D mixing channel is composed of multiple curved segments: circular arcs situated in the substrate plane and curved nozzle/diffuser elements normal to the substrate plane. Numerical simulation provides a detailed understanding of the mixing mechanism resulting from the geometrical topology of the mixer. These Comsol software-based simulations reveal the development of two secondary flows perpendicular to the primary flow: a swirling flow resulting from tangential injection of the flow into the nozzle holes and Dean vortices present in the circular arcs. These phenomena are particularly important at a Reynolds number larger than 30, where mixing occurs by chaotic advection. Experimentally, the 3D mixer is fabricated in a monolithic glass substrate by powder blasting machining, exploiting eroding powder beams at various angles of impact with respect to the substrate plane. Experimental mixing was characterised using two coloured dyes, showing nearly perfect mixing for a microfluidic footprint of the order of a few mm(2), in good agreement with the simulations

Ultra-thick micro-optical components using the PRISM photosensitive flexopolymer

We present the photosensitive flexopolymer PRISM as a new promising material for the realization of thick optical components. The PRISM flexopolymer can be directly polymerized using conventional UV exposure and is simply developed in a water-based solution. A casting method is used to realize flexopolymer layers of a few millimetres thickness in a single application step. Optical components as thick as 2 mm have been fabricated using an exposure time of less than 1 min and a development time below 3 min. No baking process is required, making the process very fast and avoiding any temperature-induced stress problems. Due to its elastomeric nature, the material can be easily applied either on rigid or flexible supports. The good optical transmission of the PRISM flexopolymer in the 400–800 nm spectral range makes it a promising material for optical applications. Refractive index measurements are performed at different wavelengths in the UV–visible range and the flexopolymer refractive index dispersion behaviour is determined. Optical components such as right-angle prisms, penta-prisms and cylindrical lenses with thicknesses up to a few mm have been successfully fabricated