Measurement of asphaltenes using optical spectroscopy on a microfluidic platform.

We present a microfluidic apparatus and method for the measurement of asphaltene content in crude-oil samples. The measurement is based on an optical absorption technique, where it was established that asphaltene coloration correlated linearly with asphaltene weight content. The initial absorbance of the oil is measured, and asphaltenes are removed from the oil by the addition of n-alkane, leading to flocculation and subsequent filtration. The absorbance of the deasphalted oil (maltenes) is then measured, and the initial asphaltene content is revealed by the change in absorbance. The asphaltene optical densities correlated linearly with conventional weight measurement results (e.g., ASTM D6560) for 38 crude-oil samples from around the world. Sample measurement repeatability was shown to be within ±2% over several months. Other aspects influencing performance of the system were evaluated, including plug dispersion, flocculation kinetics, membrane degradation, and channel clogging. The microfluidic approach described here permits asphaltene content measurement in less than 30 min as opposed to days required with traditional gravimetric techniques. This many-fold reduction in measurement time will enable more frequent characterization of crude oil samples.

Benzophenone absorption and diffusion in poly(dimethylsiloxane) and its role in graft photo-polymerization for surface modification.

Following the great success of traditional microfluidic devices across many disciplines, a new class of microfluidic systems emerged in recent years, which features finely tuned, localized surface modifications within the microstructures in order to keep up with the demand for devices of ever increasing complexity (lab on chip, assay on chip, etc.). Graft photopolymerization has become a powerful tool for such localized surface modifications particularly in combination with poly(dimethylsiloxane) (PDMS) devices, as it is compatible with many functional monomers and allows for high spatial resolution. However, application within enclosed PDMS microstructures and in particular well-controlled surface-directed polymerization remains challenging. Detailed understanding of the interaction between photoinitiator, benzophenone (BP), and polymer matrix is needed. We have developed a visualization technique, which allows for observation of reacted BP in situ within the PDMS matrix. We present a detailed study on solvent-driven BP diffusion providing results essential to successful surface treatment. We also identified and investigated photoinitiator inhibition by oxygen and provide appropriate mitigation strategies.

Wettability patterning by UV-initiated graft polymerization of poly(acrylic acid) in closed microfluidic systems of complex geometry.

Many microfluidic applications require modified surface wettability of the microchannels. Patterning of wettability within enclosed microfluidic structures at high spatial resolution has been challenging in the past. In this paper, we report an improved method for altering the surface wettability in poly(dimethylsiloxane) (PDMS) microchannels by UV-induced graft polymerization of poly(acrylic acid). Our method presents significant improvements in terms of wettability contrast and spatial resolution of the patterned structures as compared to recent literature and is in particular applicable to complex microfluidic structures with a broad range of channel sizes and aspect ratios. A key part of our work is the clear description of the surface treatment process with the identification of key parameters, some of which have been overlooked, neglected, or misinterpreted in previous works. We have studied these key parameters in detail and provide recommended values for each parameter supported by experimental results. This detailed understanding of the treatment process and the effects of the critical parameters on it allowed us to significantly improve quality and reliability of the treatment process.