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Digital microfluidics, which relies on the movement of drops, is relatively immune to clogging problems, making it suited for micro-reactor applications. Here, graphene oxide paper of 100 μm thickness, fabricated by blade coating sedimented dispersions onto roughened substrates, followed by drying and mechanical exfoliation, was found to be relatively free of cracks and curling. It also exhibited high wettability and elasto-capillary characteristics. Possessing low enough stiffness, it could rapidly and totally self-wrap water drops of 20 μL volume placed 2 mm from its edge when oriented between 0 and 60° to the horizontal. This complete wrapping behavior allowed drops to be translated via movement of the paper over long distances without dislodgement notwithstanding accelerations and decelerations. An amount of 2 drops that were wrapped with separate papers, when collided with each other at speeds up to 0.64 m/s, were found to eschew coalescence. This portends the development of robust digital microfluidic approaches for micro-reactors.

Low-cost analytical solutions built around microcomputers like the Raspberry Pi help to facilitate laboratory investigations in resource limited venues. Here, three camera modules (V1.3 with and without filter, as well as NoIR) that work with this microcomputer were assessed for their suitability in imaging fluorescent DNA following agarose gel electrophoresis. Evaluation of their utility was based on signal-to-noise (SNR) and noise variance metrics that were developed. Experiments conducted with samples were subjected to Polymerase Chain Reaction (PCR), and the amplified products were separated using gel electrophoresis and stained with Midori green. Image analysis revealed the NoIR camera performed the best with SNR and noise variance values of 21.7 and 0.222 respectively. In experiments conducted using UV LED lighting to simulate ethidium bromide (EtBr) excitation, the NoIR and V1.3 with filter removed cameras showed comparable SNR values.

Thermal profiling provides the understanding needed to enhance the reliability of Polymerase Chain Reaction (PCR) systems, but is difficult to perform experimentally when the reagents are housed in capillary tubes. The use of 3D numerical simulation with COMSOL here showed that with capillary tubes attached to a slider that moved over 3 differentially heated blocks without slots, the central axis temperature points of the tube provided poorer estimates of the thermal performance of the scheme by as much as 8.5 °C compared with the average cross-sectional temperature. It also showed that a 2-mm-thick slider would be able to improve the thermal response characteristics of a 16.5-mm-thick slider by up to 12 °C. Despite this, the method provided the best immunity to condensation effects in PCR. With the standard stationary heated block method, the temperature profiles in the capillary tubes were found to be highly affected by the amount of water condensate present in the slots. Since condensation is random, this would result in fluctuating PCR thermal behavior. When the tilt method is conducted with slot-guided capillary tubes placed heated blocks, the condensate heat transfer problem remained with stiction also affecting the movements of the capillary tubes.

The method of applying programmed tilt displacements on Polymerase Chain Reaction (PCR) reactants held in capillary tubes that are displaced to and from closely adjacent holding regions kept at different constant temperatures overcomes many of the issues encountered with other approaches. Investigations here found laser diode line generators to provide sufficient excitation light levels but not electroluminescent (EL) wires for fluorescent emission detection. When optical fiber sensing was used, the levels of excited light from scattering far exceeded the fluorescent emission levels from the reagent. Camera recording however enabled filterless detection of the fluorescent emission as the excitation light levels from the scattering was low enough and fluorescent imaging was facilitated by the point spread function (PSF) axial length conditions being met. The capacity for quantitative PCR was also illustrated using an architecture comprising a heated glass block.

Polymerase Chain Reaction (PCR) amplifies specific deoxyribonucleic acid (DNA) targets whereby the reaction vessels are placed in high thermal inertia media and subjected to multiple cycles of discrete temperatures to effect denaturation, annealing and extension events. Precise temperature control and efficient heat transfer are crucial factors in achieving amplification reproducibility and reliability as well as in reducing the time required for PCR. Understanding the temperature distribution and transfer characteristics during PCR will facilitate system optimization to enhance the efficiency of DNA amplification. We describe a numerical thermal study investigating temperature profiles achieved using the classical method of manual transfer of PCR reaction vessels through a series of temperature-controlled media compared with that of a more recently reported method that displaces reaction vessels spatially by tilting through different temperature zones. It was found that while the classical method allows the reagent to achieve uniform temperature distribution, natural convection effects would necessitate translation speeds in excess of 150 mm/min in order to limit the reagent temperature drop to within 10 °C. With the tilt method, uniform temperature distributions as well as temperature settling times within 5 s could be attained. The simulation results demonstrated here indicate that the tilt method can be developed further to augment extreme PCR requiring less than 2s for each cycle.

The use of superhydrophilic surfaces for superwetting applications can be hampered by their wetting degradation over time. In this work, superhydrophilic surfaces were fabricated by graphene oxide (GO) deposition on roughened polyvinyl chloride (PVC) substrates. These surfaces were found to exhibit zero contact angle when enhanced green fluorescent protein (EGFP) was deposited on the surface immediately after GO was applied. However, when the GO-coated substrate was stored for 3 h, the surface displayed a contact angle of 54°. The main contact line spreading of the EGFP drop was more easily discernible using normal (500 lx, low glare and shadow free) lighting than with blue LED lighting in the dark. The average radius of the spreading main contact line was found to follow a power-law relationship that increased at a faster rate with higher relative humidity. Confocal imaging revealed rings of EGFP deposition following superwetting that is attributable to pinning around the islands of GO accumulation. Substrate superhydrophilic and superwetting properties can be preserved by adequate control of relative humidity. Relative humidity of 80 % was easily maintained for 150 min by incorporating a 20 μL water drop in a sealed 240-mL container.

A drop-based approach to facilitate the remediation of methylene blue (MB) solution via catalytic degradation with silver nanowires on a superhydrophobic substrate with a 2 mm hole without fouling is presented here. Studies on the dynamical response of drop removal via pressure rate perturbation, showed greater elapsed times at the 7.61 μL/s air flowrate with 25 μL and 30 μL volume drops than at higher air flowrates used. These results are accounted for by the rate of air flow through the plastron, used to equilibrate the lowering of pressure in the transparent receptacle that tends to prevent drop dislodgement, being only able to operate up to a specific upper limit. A method that calculates the absorbance values based on the intensity spectrum of the nanowire solution as reference was found to reveal the time progress degradation of MB solution by spectrometry. It showed that a 25 μL drop of the MB solution could be degraded within 15 s using 0.25 mg of the silver nanowire.

The use of an inverted burette arrangement that allowed volume and time-course tracking of bubble movement via camera imaging is shown here to improve on the classical inverted jar (or beaker) method for measuring gases generated in small volumes and at slow rates (below 1 mL/s). In tests involving discrete gas volume measurements delivered at 0.57 mL/s, comparison with pre-set air volume delivered by a syringe pump showed high correspondence, linearity, and repeatability. Tests with continuous gas flows at 0.2, 0.32 and 0.57 mL/s revealed average bubble count versus volume trends that displayed high linearity to indicate that the bubble volumes (at any flowrate) were constant. The average bubble count versus time also showed linear trends and repeatability indicating that the time intervals between the appearance of any two bubbles (at a particular flowrate) were uniform. The analytical and simulated results helped to explain why bubble size determination by high speed camera recording for measuring gas volume and flowrate is fraught with inherent problems. An analysis using light ray tracing showed that flowrate determination using such an approach would necessitate complicated image correction when two cameras are used for simultaneous recording due to distortion effects resulting from refraction. A virtual gate approach involving the binarization of images recorded with a standard camera is shown to be more feasible in establishing the flowrate using bubble counting or finding the time interval between bubbles.

Liquid marbles to separate wetting (silver nanowires) from non-wetting (graphite and talcum) particles is described here. Two types of liquid marbles are formed by coating the dry mixture test material on pendant or sessile drops. The silver nanowire particles are separated from the graphite or talcum shell by entering into the liquid core of the marbles. The liquid core is extracted into capillary tubes where the presence of silver nanoparticles was confirmed by a blue to colourless change arising from silver-mediated catalytic reduction of methylene blue and sodium borohydride solution. The use of an inclined rotating surface to coat the pendant drop was found to create liquid marbles in a more controllable manner. A platform tilt angle of 15o followed by rotation was found to provide more uniform albeit only partially coating the bottom of the pendant drop.