用于胶体悬浮液原位 DLS 测量的微芯片的新设计

Stability of dispersed materials remains an important question in a wide variety of fields such as cosmetics,catalysis, food or energy and the environment. As stability is directly linked to the size of the dispersed colloids itis essential to assess the size distribution of colloidal suspensions. Nowadays, microfluidic-based approachesgenerate increasing interest as they represent flexible and fast measurements allowing high throughput experimentations. However, characterization of colloidal dispersions is usually performed by dynamic light scattering(DLS), that requires static measurements as well as significant volumes, that are not compatible with on-lineanalysis and microfluidics. Moreover, due to flow-induced decorrelation terms, DLS measurements in microfluidic channels are only accurate at very low shear rates.

This work aimed at developing an on-line microfluidic device for dispersed materials characterization usingDLS. The main challenges of this project were i) to adapt the microfabrication of the PDMS device in order tocombine microchannels of hundreds of microns with a milli-fluidic cavity to perform the DLS measurements, andii) to downsize the DLS set up. A PDMS microchip, consisting in a millimeter cavity for DLS measurements inparallel with a microchannel, was designed to perform the measurement on the sample without stopping thesuspension flow during the microfluidic experiment. The cavity geometry was then optimized thanks to numericalsimulation to ensure a good sweep efficiency and to downscale as much as possible without impairing the DLSsignal.

Accessing the stability of dispersed materials is an important matterin a wide variety of fields such as cosmetics, catalysis, food, energy or theenvironment. Suspensions’ stability depends on particle size as well asinter-particle interactions.

One well-known technique to characterize colloidal suspensions –containing particles subjected to Brownian motion – is the Dynamic LightScattering (or DLS) [1–3]. This method, based on Brownian motion, allows the determination of particle sizes from tens of nanometers to a fewmicrons. However, this technique requires to sample suspension (a fewmilliliter), that will potentially modify the system (change of temperature, pressure …) and involves having sufficient volumes.

Fig1

The first step of photolithography consists in depositing a uniformphotoresist layer of controlled thickness on a 10 cm diameter and 1 mmthickness silicon wafer (covered with a silicon oxide (SiO2) layer of 500nm thickness) (from Neyco). The negative photoresist resin is spreadevenly over the entire wafer using a spin coater rotating at a speed of2000 rpm, resulting in a layer of resist about 160 μm thick. Note that thelayer thickness is controlled at the end of the microfabrication process byimaging the PDMS microchannel with an optical microscope.