Accelerated spreading of inviscid droplets prompted by the yielding of strongly elastic interfacial films

Microfluidic jet impact: Spreading, splashing, soft substrate deformation and injection

HYPOTHESIS

Needle-free injections using microfluidic jets could be optimized by reducing splashing and controlling injection depth. However, this is impeded by an incomplete understanding on how jet characteristics influence impact outcome. We hypothesise that exploring the relation between microfluidic jet characteristics and substrate shear modulus on impact behavior will assist in predicting and giving insights on the impact outcome on skin and injection endpoints.

EXPERIMENTS

To do so, a setup using microfluidic chips, at varying laser powers and stand-off distances, was used to create thermocavitation generated microfluidic jets with ranging characteristics (velocity: 7-77 m/s, diameter: 35-120 μm, Weber-number: 40-4000), which were impacted on substrates with different shear modulus.

FINDINGS

Seven impact regimes were found, depending on jet Weber-number and substrate shear modulus, and we identified three thresholds: i) spreading/splashing threshold, ii) dimple formation threshold, and iii) plastic/elastic deformation threshold. The regimes show similarity to skin impact, although the opacity of skin complicated determining the threshold values. Additionally, we found that jet velocity has a higher predictive value for injection depth compared to the Weber-number, and consequently, the jet-diameter. Our findings provide fundamental knowledge on the interaction between microfluidic jets and substrates, and are relevant for optimizing needle-free injections.

The effect of cationic surfactants on improving natural clinoptilolite for the flotation of cesium

Flotation using cationic surfactants has been investigated as a rapid separation technique to dewater clinoptilolite ion exchange resins, for the decontamination of radioactive cesium ions (Cs⁺) from nuclear waste effluent. Initial kinetic and equilibrium adsorption studies of cesium, suggested the large surface area to volume ratio of the fine zeolite contributed to fast adsorption kinetics and high capacities (q_c = 158.3 mg/g). Adsorption of ethylhexadecyldimethylammonium bromide (EHDa-Br) and cetylpyridinium chloride (CPC) surfactant collectors onto both clean and 5 ppm Cs⁺ contaminated clinoptilolite was then measured, where distribution coefficients (K_d) as high as 10,000 mL/g were evident with moderate concentrations CPC. Measurements of particle sizes confirmed that adsorption of surfactant monolayers did not lead to significant aggregation of the clinoptilolite, while < 8% of the 5 ppm contaminated cesium was remobilised. Importantly for flotation, both the recovery efficiency and dewatering ratios were measured across various surfactant concentrations. Optimum conditions were found with 0.5 mM of CPC and addition of 30 μL of MIBC frother, giving a recovery of ∼90% and a water reduction ratio > 4, highlighting the great viability of flotation to separate and concentrate the contaminated powder in the froth phase.