Enhanced Solutal Marangoni Flow Using Ultrasound-Induced Heating for Rapid Digital Microfluidic Mixing

Digital microfluidics based on sessile droplets has emerged as a promising technology for various applications including biochemical assays, clinical diagnostics, and drug screening. Digital microfluidic platforms provide an isolated microenvironment to prevent cross-contamination and require reduced sample volume. Despite these advantages, the droplet-based technology has the inherent limitation of the quiescent flow conditions at low Reynolds number, which causes mixing samples confined within the droplets to be challenging. Recently, solutal Marangoni flows induced by volatile liquids have been utilized for sessile droplet mixing to address the above-mentioned limitation. The volatile liquid vaporized near a sessile droplet induces a surface tension gradient throughout the droplet interface, leading to vortical flows inside a droplet. This Marangoni flow-based droplet mixing method does not require an external energy source and is easy to operate. However, this passive method requires a comparably long time of a few tens of seconds for complete mixing since it depends on the natural evaporation of the volatile liquid. Here, we propose an improved ultrasound-induced heating method based on a nature-inspired ultrasound-absorbing layer and apply it to enhance solutal Marangoni effect. The heater consists of an interdigital transducer deposited on a piezoelectric substrate and a silver nanowire-polydimethylsiloxane composite as an ultrasound-absorbing layer. When the transducer is electrically actuated, surface acoustic waves are produced and immediately absorbed in the composite layer by viscoelastic wave attenuation. The conversion from acoustic to thermal energy occurs, leading to rapid heating. The heating-mediated enhanced vaporization of a volatile liquid accelerates the solutal Marangoni flows and thus enables mixing high-viscosity droplets, which is unachievable by the passive solutal Marangoni effect. We theoretically and experimentally investigated the enhanced Marangoni flow and confirmed that rapid droplet mixing can be achieved within a few seconds. The proposed heater-embedded sessile droplet mixing platform can be fabricated in small size and easily integrated with other digital microfluidic platforms. Therefore, we expect that the proposed sample mixing method can be utilized for various applications in digital microfluidics and contribute to the advancements in the medical and biochemical fields.

1,1,1,2,3,4,4,5,5,5-Decafluoropentane (HFC-4310mee)

HFC-43-10mee is a volatile liquid at room temperature and used as a cleaning agent, aerosol, etc. HFC-43-10mee has low acute inhalation toxicity; 4-hour LC50 in rats of approximately 11,000 ppm. The compound was not a skin or eye irritant and was not a skin sensitizer. A cardiac sensitization response was not observed at 5000 ppm. Inhalation exposure resulted in neurotoxicity consisting of tremors, convulsions, jerking, ataxia, abnormal gait, etc. at exposure concentrations of 2000 ppm and above. Within approximately 30 min of exposure the clinical signs appeared and resolved within 1–2 h during exposure; the rats appeared to adapt such that these clinical signs were no longer observed. The neurotoxicity observed was considered an acute response to HFC-43-10mee. In a 90-day study, rats exposed to 2000 ppm resulted in sporadic clinical signs of neurotoxicity. At 3500 ppm, the clinical signs were evident on most exposure days although as the study progressed the apparent incidence declined likely reflecting adaptation. The NOAEL was 500 ppm. Based on the occurrence of the clinical signs in this and other studies, an acute threshold for neurotoxicity was evident at approximately 2000 ppm and above. No developmental or reproductive toxicity were evident at 2000 ppm, although clinical signs of neurotoxicity occurred in maternal or parental rats at 2000 ppm. No effects on offspring were observed. HFC-43-10mee was not genotoxic in vitro or in vivo. Based on the data, the 8-h TWA WEEL value is 225 ppm (2320 mg/m3). The 15-min STEL is 700 ppm (7217 mg/m3).

Volatile Liquid Detection by Terahertz Technologies

The prospect of being able to move through security without the inconvenience of separating liquids from bags is an exciting one for passengers, and there are important operational benefits for airports as well. Here, two terahertz (THz) systems, 100 GHz sub-THz line scanner and attenuation total reflection-based THz time domain spectroscopy (TDS), have been used to demonstrate the capability of identifying different liquid samples. Liquid samples’ THz complex permittivities are measured and their differences have contributed to the variation of 100 GHz 2D images of volatile liquids with different volumes inside of cannister bottles. The acquired attenuation images at 100 GHz can easily be used to distinguish highly absorbed liquids (Water, Ethanol, Fuel Treatment Chemicals) and low loss liquids (Petrol, Diesel, Kerosene and Universal Bottle Cleaner). The results give a promising feasibility for mm-wave imager and THz spectroscopy to efficiently identify different volatile liquids.

Aerodynamic and impact thresholds for cohesive mixture of sand and non-volatile liquid

The moisture has been recognized to have a significant influence on the initiation movement of sand by wind and consequently on sand transport rates. The pertinent literature regarding these phenomena is sparse and current available theoretical and empirical models exhibit considerable disagreement regarding the magnitude of moisture effects. We believe that these discrepancies comes from the fact that the moisture levels are not well controlled neither properly measured and are susceptible to strongly vary over time due to evaporation. To get rid of the variability of moisture content due to evaporation, we propose a new approach based on the use of non-volatile liquid, namely silicon oil instead of water. This insures a proper control of the liquid content and the production of reliable data concerning the variation of the transport threshold with liquid content.