Marangoni-driven spreading of a droplet on a miscible thin liquid layer

HYPOTHESIS

The coalescence of droplets with liquid-gas interfaces of different surface tensions is common in nature and industrial applications, where the Marangoni-driven film spreading is an essential process. Unlike immiscible fluids governed by triple contact line dynamics, the mixing between two miscible fluids strongly couples with the film spreading process, which are expected to manifest distinct power-law relations for the temporal increase in the film radius.

EXPERIMENTS

We experimentally investigate the Marangoni-driven film spreading phenomenon for a droplet with lower surface tension dropping onto a miscible, thin liquid layer. The temporal growth of the film radius was detected by using a novel deep convolutional neural network, the U2-net method. Scaling analysis was performed to interpret the spreading dynamics of the film.

FINDINGS

We find that the film radius exhibits a three-stage power-law relation over time, with the exponent varying from 1/2 to 1/8, and back to 1/2. The diffusion-affected Marangoni stresses in these three stages were derived, and two estimations of viscous stress were considered. Through estimating and balancing the viscous stress with the Marangoni stress, the three-stage power-law relation was derived and validated.

Magmatic-hydrothermal fluid evolution of the tin-polymetallic metallogenic systems from the Weilasituo ore district, Northeast China

The large Weilasituo Sn-polymetallic deposit is a recent exploration discovery in the southern Great Xing'an Range, northeast China. The ore cluster area shows horizontal mineralization zoning, from the inner granite body outward, consisting of high-T Sn-W-Li mineralization, middle-T Cu-Zn mineralization and peripheral low-T Pb-Zn-Ag mineralization. However, the intrinsic genetic relationship between Sn-W-Li mineralization and peripheral vein-type Pb-Zn-Ag-Cu mineralization, the formation mechanism and the deep geological background are still insufficiently understood. Here, we use fluid inclusions, trace elements concentrations in quartz and sphalerite, and H-O isotope studies to determine the genetic mechanism and establish a metallogenic model. Fluid inclusion microthermometry and Laser Raman spectroscopic analysis results demonstrates that the aqueous ore-forming fluids evolved from low-medium salinity, medium-high temperature to low salinity, low-medium temperature fluids. Laser Raman spectroscopic analysis shows that CH4 is ubiquitous in fluid inclusions of all ore stages. Early ore fluids have δ18OH2O (v-SMOW) values from + 5.5 to + 6.2‰ and δD values of approximately - 67‰, concordant with a magmatic origin. However, the late ore fluids shifted toward lower δ18OH2O (v-SMOW) (as low as 0.3‰) and δD values (~ - 136‰), suggesting mixing between external fluids derived from the wall rocks and a contribution from meteoric water. Ti-in-quartz thermometry indicates a magmatic crystallization temperature of around 700 °C at a pressure of 1.5 kbar for the magmatic ore stage. Cathodoluminescence (CL) imaging and trace element analysis of quartz from a hydrothermal vug highlight at least three growth episodes that relate to different fluid pulses; each episode begins with CL-bright, Al-Li-rich quartz, and ends with CL-dark quartz with low Al and Li contents. Quartz from Episode 1 formed from early Sn-(Zn)-rich fluids which were likely derived from the quartz porphyry. Quartz from episodes 2 and 3 formed from Zn-(Sn)-Cu-rich fluid. The early magmatic fluid is characterized by low fS2. The SO2 produced by magma degassing reacted with heated water to form SO42-, causing the shift from low fS2 to high fS2. The SO42- generated was converted to S2- by mixing with CH4-rich, Fe and Zn-bearing external fluid which led to late-stage alteration and dissolution of micas in vein walls, thus promoting crystallization of pyrrhotite, Fe-rich sphalerite and chalcopyrite and inhibiting the precipitation of anhydrite. This study shows that ore formation encompassed multiple episodes involving steadily evolved fluids, and that the addition of external fluids plays an important role in the formation of the later Cu-Zn and Ag-Pb-Zn mineralization in the Weilasituo ore district.

Controlling pore-scale processes to tame subsurface biomineralization

Microorganisms capable of biomineralization can catalyze mineral precipitation by modifying local physical and chemical conditions. In porous media, such as soil and rock, these microorganisms live and function in highly heterogeneous physical, chemical and ecological microenvironments, with strong local gradients created by both microbial activity and the pore-scale structure of the subsurface. Here, we focus on extracellular bacterial biomineralization, which is sensitive to external heterogeneity, and review the pore-scale processes controlling microbial biomineralization in natural and engineered porous media. We discuss how individual physical, chemical and ecological factors integrate to affect the spatial and temporal control of biomineralization, and how each of these factors contributes to a quantitative understanding of biomineralization in porous media. We find that an improved understanding of microbial behavior in heterogeneous microenvironments would promote understanding of natural systems and output in diverse technological applications, including improved representation and control of fluid mixing from pore to field scales. We suggest a range of directions by which future work can build from existing tools to advance each of these areas to improve understanding and predictability of biomineralization science and technology.

Acoustic droplet vaporization-mediated dissolved oxygen scavenging in blood-mimicking fluids, plasma, and blood

Acoustic droplet vaporization (ADV) has been shown to reduce the partial pressure of oxygen (PO₂) in a fluid. The goals of this study were three-fold: 1) to determine the ADV pressure amplitude threshold in fluids that had physiologically relevant values for surface tension, protein concentration, and viscosity; 2) to assess whether these parameters and fluid mixing affect ADV-mediated PO₂ reduction; and 3) to assess the feasibility of ADV-mediated PO₂ reduction in plasma and whole blood. In vitro ADV experiments were conducted using perfluoropentane droplets (number density: 5 × 10⁶ ± 0.2 × 10⁶/mL) dispersed in fluids (saline, polyvinylpyrrolidone solutions, porcine plasma, or porcine whole blood) that had a physiological range of surface tensions (62-68 mN/m), protein concentrations (0 and 68.7 mg/mL), and viscosities (0.7-4 cP). Droplets were exposed to pulsed ultrasound (5 MHz, 4.25 MPa peak negative pressure) while passing through a 37 °C flow system with inline PO₂ sensors. In select experiments, the fluid also passed through mixing channels after ultrasound exposure. Our results revealed that the ADV pressure thresholds were the same for all fluids. Surface tension and protein concentration had no effect on PO₂ reduction. Increasing viscosity attenuated PO₂ reduction. However, the attenuated effect was absent after fluid mixing. Furthermore, ADV-mediated PO₂ reduction in whole blood (30.8 ± 3.2 mmHg) was less than that in a polyvinylpyrrolidone solution (40.2 ± 2.1 mmHg) with equal viscosity. These findings should be considered when planning clinical studies of ADV-mediated PO₂ reduction and other biomedical applications of ADV.