‘Electrowetting as a tool for two phase flow microfluidic operations’
In this thesis the focus is on: the design, fabrication and implementation of a microfluidic chip, capable of manipulating small water droplets in oil flow.
‘This choice is motivated by the desire to analyse individual cells,’ says Arjen Pit. Microfluidics enables the high speed production of well-defined droplets, of which some contain a cell to be analysed.’
Using electrode geometries embedded in microchannels the droplets: can be guided along a rail, trapped at a specific location, split in two, merged together, and sorted at high speed (based on content). ‘In this sense my research was application driven,’ Arjen says. ‘Controlling and manipulating the drops always stood first.’
At the Physics of Complex Fluids Group (PCF), led by Professor Frieder Mugele, the physical aspects were considered as the main purpose. Attention to the biological aspects was given at the Medical Cell BioPhysics Group, led by Professor Leon Terstappen. ‘The multidisciplinary spirit is strong at PCF,’ Arjen says. ‘We house various disciplines and research topics.’
In the first stages of his PhD project Arjen focused on various techniques to manipulate oil droplets. He came up with a clear conclusion: ‘The most predictable, controllable and reliable technique capable of performing all the required tasks in a microchannel, is the use of electric potential wells.’
He succeeded in trapping, splitting and sorting 36 guided droplets against the oil flow at arrays of trapping electrodes. ‘A next step was to let the droplets interact with antibodies printed on the substrate,’ Arjen says. ‘This is needed in order to use surface plasmon resonance as a diagnostics tool.’
These patches of antibodies form hydrophilic areas in an otherwise hydrophobic channel. Arjen: ‘So we developed a fabrication process to create a dual-sided microchip, with actuation electrodes in the bottom substrate and hydrophilic gold patches on the hydrophobic top substrate. We were capable of electrically trapping a droplet, long enough to interact. By using a secondary pair of electrodes the droplet could be pulled from the patch.’
High speed sorting
The system was further optimized, enabling the sorting of droplets at really high speeds. The working principle of the sorter was based on a contrast in conductivity, between the drop and the continuous ambient phase, which ensured successful operation even for drops of highly conductive biological media like phosphate buffered saline. Moreover, since the electric field does not penetrate the drop, its content is protected from electrical currents and Joule heating.
‘Scientific research is a gradual process,’ Arjen shares. ‘When I observed the complex chip functioning really well, I felt relieved and very happy. Then we focused on the fundamental aspects more. By going back to the basics of electrostatics. The parameters that dictate the magnitude of electric energy and electric force were described successfully.’
A simple theoretical ‘capacitive model’ allowed a quantitative prediction of the electrostatic forces exerted on the drops.
The maximum achievable sorting rate is determined by a competition between electrostatic and hydrodynamic forces. Sorting speeds up to 1200 per second were demonstrated for conductive drops of 160 picolitres in low viscosity oil. Finally, together with the group of Wilhelm Huck at the Radboud University Nijmegen, experiments were performed to sort droplets containing fluorescently labelled cells.
Arjen’s vision on the techniques used in this PhD is thought-out: ‘The relatively large and predictable forces acting on the droplet are advantageous, but the complex fabrication method is costly. Whether the downsides outweigh the advantages for practical applications remains to be seen.’
Together with his colleagues, Arjen succeeded in publishing many papers in renowned journals including: Lab-on-a-Chip, Biomicrofluidics, Nature Communications, Microfluidics & Nanofluidics, and MicroMachines. He also was a speaker on the Electrowetting Conference in Cincinnati. ‘For those interested: the next Electrowetting Conference will be held in Twente in 2018!,’ Arjen says (info at: electrowetting.org).
‘Also the poster presentation at STW annual congress was a highlight, winning the “STW Leerling” prize together with Ivan Stajonovic,’ he says. ‘And the poster prize I won on the Mesa+ Day (together with Dieter ‘t Mannetje) was really rewarding, your work being appreciated by your near colleagues. In all my writings and presentations, I pay a lot of attention explaining the backgrounds in detail. I feel that in that way knowledge and ideas are spread more widely than to the in-crowd communities only.’
As for his future job, Arjen would like to go and work in industry. ‘I prefer working at a small company,’ he says. ‘At this moment a start-up company in Amsterdam - working to market manipulating droplets techniques on a micro scale, for various applications - especially appeals to me. I hope to contribute with my knowledge, R&D skills and expertise to this very interesting development.'