In this study, the evaporation characteristics and drying patterns of various sessile ferrofluid droplets on certain substrate under horizontal magnetic fields of controlled intensities are reported. The effects of droplet concentration and magnetic field intensity on the duration of each evaporation stage and drying patterns of droplets have been systematically investigated. It turned out that a plateau appears at the initial stage of evaporation in the absence of magnetic field and it was found that the plateau value is positively correlated with the concentration of ferrofluid droplets. Under the external magnetic field, the evaporation time of droplets decreases, the stage of contact line retreat extends, the stage of late pinning mode shortens, and the deposition area of ferrofluid droplet decreases compared to that of without magnetics field. The deposition area increases gradually and becomes more uniform with the increase of magnetic field. The decrease of friction force which is due to the decrease of the number of nanoparticles at the contact line under external magnetic field is the main reason for the observed phenomena. We found that the coffee ring and the uniform deposition inside the droplet will be destroyed when the magnetic field intensity is higher than a critical value. Our work has a significant reference value for the evaporation of sessile magnetic fluid droplets under the applied magnetic field, especially when the drying pattern needs to be precisely controlled, such as in spray or biomedicine.

The inactivation of viruses in aerosol particles (aerosols) and droplets depends on many factors, but the precise mechanisms of inactivation are not known. The system involves complex physical and biochemical interactions. We reviewed the literature to establish current knowledge about these mechanisms and identify knowledge gaps. We identified 168 relevant papers and grouped results by the following factors: virus type and structure, aerosol or droplet size, temperature, relative humidity (RH) and evaporation, chemical composition of the aerosol or droplet, pH and atmospheric composition. These factors influence the dynamic microenvironment surrounding a virion and thus may affect its inactivation. Results indicate that viruses experience biphasic decay as the carrier aerosols or droplets undergo evaporation and equilibrate with the surrounding air, and their final physical state (liquid, semi-solid or solid) depends on RH. Virus stability, RH and temperature are interrelated, but the effects of RH are multifaceted and still not completely understood. Studies on the impact of pH and atmospheric composition on virus stability have raised new questions that require further exploration. The frequent practice of studying virus inactivation in large droplets and culture media may limit our understanding of inactivation mechanisms that are relevant for transmission, so we encourage the use of particles of physiologically relevant size and composition in future research.

Single-cell RNA sequencing supports the isolation of individual cells and barcoding of cDNA, specific to each cell of origin. Subsequent sequencing of the generated library yields both the gene expression sequences and the cellular barcode, allowing distinction of gene expression patterns across individual cells. The 10X Genomics 3\' HT assay uses a droplet-based method to isolate individual cells within oil emulsions, combined with a gel bead coated in uniquely barcoded primers, specific to each bead. The high-throughput, HT, assay is similar to its predecessor (3\' v3.1) in reaction chemistry but utilizes (a) higher numbers of cellular barcodes, (b) a new, proprietary chip designed to target up to 60,000 cells per lane, and (c) captures up to 16 samples per run. The 3\' HT assay supports whole cells and nuclei as input, with an approximate 60% capture rate. Here we describe the methods for sample quality control (QC) assays, loading and operation of the Chromium X instrument for cell capture, and cDNA synthesis and library preparation for downstream Illumina sequencing.

Aerosol transmission remains a major challenge for control of respiratory viruses, particularly those causing recurrent epidemics, like influenza A virus (IAV). These viruses are rarely expelled alone, but instead are embedded in a consortium of microorganisms that populate the respiratory tract. The impact of microbial communities and inter-pathogen interactions upon stability of transmitted viruses is well-characterized for enteric pathogens, but is under-studied in the respiratory niche. Here, we assessed whether the presence of five different species of commensal respiratory bacteria could influence the persistence of IAV within phosphate-buffered saline and artificial saliva droplets deposited on surfaces at typical indoor air humidity, and within airborne aerosol particles. In droplets, presence of individual species or a mixed bacterial community resulted in 10- to 100-fold more infectious IAV remaining after 1 h, due to bacterial-mediated flattening of drying droplets and early efflorescence. Even when no efflorescence occurred at high humidity or the bacteria-induced changes in droplet morphology were abolished by aerosolization instead of deposition on a well plate, the bacteria remained protective. Staphylococcus aureus and Streptococcus pneumoniae were the most stabilizing compared to other commensals at equivalent density, indicating the composition of an individual\'s respiratory microbiota is a previously unconsidered factor influencing expelled virus persistence.IMPORTANCEIt is known that respiratory infections such as coronavirus disease 2019 and influenza are transmitted by release of virus-containing aerosols and larger droplets by an infected host. The survival time of viruses expelled into the environment can vary depending on temperature, room air humidity, UV exposure, air composition, and suspending fluid. However, few studies consider the fact that respiratory viruses are not alone in the respiratory tract-we are constantly colonized by a plethora of bacteria in our noses, mouth, and lower respiratory system. In the gut, enteric viruses are known to be stabilized against inactivation and environmental decay by gut bacteria. Despite the presence of a similarly complex bacterial microbiota in the respiratory tract, few studies have investigated whether viral stabilization could occur in this niche. Here, we address this question by investigating influenza A virus stabilization by a range of commensal bacteria in systems representing respiratory aerosols and droplets.

Hydrogen peroxide (H2O2) plays a crucial role as an oxidizing agent within the tropospheric environment, making a substantial contribution to sulfate formation in hydrated aerosols and cloud and fog droplets. Field observations show that high levels of H2O2 are often observed in heavy haze events and polluted air. However, the source of H2O2 remains unclear. Here, using the droplets formed in situ by the deliquescence of hygroscopic compounds under a high relative humidity (RH), the formation of H2O2 by the photochemistry of imidazole-2-carbaldehyde (2-IC) under ultraviolet irradiation was explored. The results indicate that 2-IC produces IM-C•-OH and IM-C•═O radicals via H transfer itself to its excited triplet state and generates H2O2 and organic peroxides in the presence of O2, which has an evident oxidizing effect on SO2, suggesting the potential involvement of this pathway in the formation of atmospheric sulfate. H2O2 formation is limited in acidic droplets or droplets containing ammonium ions, and no H2O2 is detected in droplets containing nitrate, whereas droplets containing citric acid have an obvious promotion effect on H2O2 formation. These findings provide valuable insights into the behaviors of atmospheric photosensitizers, the source of H2O2, and the formation of sulfate in atmospheric droplets.

A perfluoropolyether (PFPE)-based microfluidic device with cross-junction microchannels was fabricated with the purpose of producing uniform droplets. The microchannels were developed using CO2 laser engraving. PFPE was chosen as the main material because of its excellent solvent resistance. Polyethylene glycol diacrylate (PEGDA) was mixed with PFPE to improve the hydrophilic properties of the inner surface of the microchannels. The microchannels of the polydimethylsiloxane microfluidic device had a blackened and rough surface after laser engraving. By contrast, the inner surface of the microchannels of the PFPE-PEGDA microfluidic device exhibited a smooth surface. The lower power and faster speed of the laser engraving resulted in the development of microchannels with smaller dimensions, less than 30 μm in depth. The PFPE and PFPE-PEGDA microfluidic devices were used to produce uniform water and oil droplets, respectively. We believe that such a PFPE-based microfluidic device with CO2-laser-engraved microchannels can be used as a microfluidic platform for applications in various fields, such as biological and chemical analysis, extraction, and synthesis.

Advances in digital light projection(DLP) based (bio) printers have made printing of intricate structures at high resolution possible using a wide range of photosensitive bioinks. A typical setup of a DLP bioprinter includes a vat or reservoir filled with liquid bioink, which presents challenges in terms of cost associated with bioink synthesis, high waste, and gravity-induced cell settling, contaminations, or variation in bioink viscosity during the printing process. Here, we report a vat-free, low-volume, waste-free droplet bioprinting method capable of rapidly printing 3D soft structures at high resolution using model bioinks and model cells. A multiphase many-body dissipative particle dynamics model was developed to simulate the dynamic process of droplet-based DLP printing and elucidate the roles of surface wettability and bioink viscosity. Process variables such as light intensity, photo-initiator concentration, and bioink formulations were optimized to print 3D soft structures (∼0.4-3 kPa) with a typical layer thickness of 50µm, an XY resolution of 38 ± 1.5μm and Z resolution of 237 ± 5.4µm. To demonstrate its versatility, droplet bioprinting was used to print a range of acellular 3D structures such as a lattice cube, a Mayan pyramid, a heart-shaped structure, and a microfluidic chip with endothelialized channels. Droplet bioprinting, performed using model C3H/10T1/2 cells, exhibited high viability (90%) and cell spreading. Additionally, microfluidic devices with internal channel networks lined with endothelial cells showed robust monolayer formation while osteoblast-laden constructs showed mineral deposition upon osteogenic induction. Overall, droplet bioprinting could be a low-cost, no-waste, easy-to-use, method to make customized bioprinted constructs for a range of biomedical applications.

Digital PCR (dPCR) has become indispensable in nucleic acid (NA) detection across various fields, including viral diagnostics and mutant detection. However, misclassification of partitions in dPCR can significantly impact accuracy. Despite existing methods to minimize misclassification bias, accurate classification remains elusive, especially for nonamplified target partitions. To address these challenges, this study introduces an innovative microdroplet-based competitive PCR platform for nucleic acid quantification in microfluidic devices independent of Poisson statistics. In this approach, the target concentration (T) is determined from the concentration of competitor DNA (C) at the equivalence point (E.P.), where C/T is 1. Competitive PCR ensures that the ratio of target to competitor DNA remains constant during amplification, reflected in the resultant fluorescence intensity, allowing the quantification of target DNA concentration at the equivalence point. The unique amplification technique eliminates Poisson distribution, addressing misclassification challenges. Additionally, our approach reduces the need for post-PCR procedures and shortens analytical time. We envision this platform as versatile, reproducible, and easily adaptable for driving significant progress in molecular biology and diagnostics.

BACKGROUND: Single-cell droplet microfluidics is an important platform for high-throughput analyses and screening because it provides an independent and compartmentalized microenvironment for reaction or cultivation by coencapsulating individual cells with various molecules in monodisperse microdroplets. In combination with microbial biosensors, this technology becomes a potent tool for the screening of mutant strains. In this study, we demonstrated that a genetically engineered yeast strain that can fluorescently sense agonist ligands via the heterologous expression of a human G-protein-coupled receptor (GPCR) and concurrently secrete candidate peptides is highly compatible with single-cell droplet microfluidic technology for the high-throughput screening of new agonistically active peptides.
RESULTS: The water-in-oil microdroplets were generated using a flow-focusing microfluidic chip to encapsulate engineered yeast cells coexpressing a human GPCR [i.e., angiotensin II receptor type 1 (AGTR1)] and a secretory agonistic peptide [i.e., angiotensin II (Ang II)]. The single yeast cells cultured in the droplets were then observed under a microscope and analyzed using image processing incorporating machine learning techniques. The AGTR1-mediated signal transduction elicited by the self-secreted Ang II peptide was successfully detected via the expression of a fluorescent reporter in single-cell yeast droplet cultures. The system could also distinguish Ang II analog peptides with different agonistic activities. Notably, we further demonstrated that the microenvironment of the single-cell droplet culture enabled the detection of rarely existing positive (Ang II-secreting) yeast cells in the model mixed cell library, whereas the conventional batch-culture environment using a shake flask failed to do so. Thus, our approach provided compartmentalized microculture environments, which can prevent the diffusion, dilution, and cross-contamination of peptides secreted from individual single yeast cells for the easy identification of GPCR agonists.
CONCLUSIONS: We established a droplet-based microfluidic platform that integrated an engineered yeast biosensor strain that concurrently expressed GPCR and self-secreted the agonistic peptides. This offers individually isolated microenvironments that allow the culture of single yeast cells secreting these peptides and gaging their signaling activities, for the high-throughput screening of agonistic peptides. Our platform base on yeast GPCR biosensors and droplet microfluidics will be widely applicable to metabolic engineering, environmental engineering, and drug discovery.

The droplet microfluidic device has become a widely used tool in fields such as physics, chemistry, and biology, but its complexity has limited its widespread application. This report introduces a modular and cost-effective droplet microfluidic device for the controlled production of complex emulsions, including oil and aqueous single emulsions, and double emulsions with varying numbers of encapsulated droplets. The droplet sizes can be precisely controlled by easily replacing flat needles and adjusting the needle position within an axially accelerated co-flow field. This modular device not only allows for easy repair and maintenance in case of device clogging or damage but can also be readily expanded to produce complex emulsions. The low-cost and user-friendly nature of the device greatly facilitates the widespread adoption and utilization of droplet microfluidics.