Open sessile droplet viscometer with low sample consumption

An open microfluidic platform is a promising new innovative approach for developing efficient continuous-flow microfluidic devices. In particular, Laplace-induced pumping exhibits great potential for hemorheological applications owing to its ability to maintain a high flow rate while offering increased sample throughput. Unlike capillary-driven surface flow devices and wicking paper-based analytical devices, Laplace-induced systems deliver liquids quantitatively and in larger volumes. However, they are limited to chemical characterization by either depositing the samples directly into the channels or pumping the samples across the immobilized reagents, which is considered unsuitable for developing advanced point-of-care viscometers.

On this account, a team of Canadian researchers at Queen’s University: Matthias Hermann (graduate student), Dr. Kyle Bachus, and Professor Richard Oleschuk together with Dr. Graham Gibson from CMC Microsystems developed a portable droplet viscometer with low sample consumption based on a two-droplet Laplace-induced pumping system. Their aim was to investigate the feasibility of using devices based on Laplace pumping in hemorheological applications for enhanced efficiency, robustness, and reduced costs. Their work is currently published in the research journal, Lab on a Chip.

In their approach, a superhydrophobic coating and laser micromachining techniques were used to fabricate the open microfluidic chip component. The proposed device measures the viscosity by determining the time required to completely pump one droplet into another droplet. Generally, the pumping behavior was governed by Laplace and Hagen-Poiseuille relations, in which the flow rate is proportional to the kinematic viscosity of the liquid. To measure the pumping progress, the authors used a laser to track the change in curvature of the droplets. The measurements of the dynamic viscosities were carried out in a 500 µm wide and 20 mm long channel. Lastly, the impact of both device-specific and sample-specific parameters was investigated.

The authors reported a portable, cost-effective, and robust viscometer that required less than 10 µL of sample for measurement and less than 15 USD for materials. The flow rate and pumping time were found to depend on three main factors: viscosity of the liquid, the volume of the deposited liquid, and the channel geometry. By tuning the channel geometry and the volume of the deposited liquid, pumping time in the range 10 s – 5 min was obtained for viscosities in the range of 1.03 – 5.35 mPa s. Moreover, the open design enabled the rinsing of the surface, thus reducing the complexity associated with cleaning. The biocompatibility of the used coating was validated by measuring the viscosity of red blood cell emulsions, which was determined in the range of 1.0 to 2.0 mPa with a sample volume of 7.5 µL and sensitivity of 0.07 mPa s.

In summary, the study reported the development of a portable viscometer based on the Laplace-induced pump system. The biocompatibility of the device was successfully validated by measuring the viscosity of red blood cell emulsions. In a statement to Advances in Engineering, Professor Richard Oleschuk said their new device exhibited advantages that make it a promising technology for hemorheological applications.