Dripping, jetting and tip streaming

Charge transport in electrospinning of polyelectrolyte solutions

This study elucidates the electrical charge transport during electrospinning of weak polyelectrolyte (poly(acrylic acid) (PAA)) solutions by employing current emission measurements. With pH variation, the PAA ionization degree could be controlled from uncharged at low pH to weakly charged at intermediate solution pH. Electrospinning neutral poly(vinylpyrrolidone) (PVP) as a reference polymer solution confirmed established current-flow rate scaling relationships as shown by De La Mora and Loscertales (1994), I ∼ (γKQ)ν, independent of the applied electric field polarity, where ν = 0.5, K is the conductivity, γ is the surface tension, Q is the flow rate, and I is the current. Similarly, the uncharged PAA did not display any polarity dependence, yet ν ≈ 0.8. Negatively charged PAA, however, showed a marked deviation in the current-flow rate behavior, which was affected by the applied electric field polarity. In the case of negative polarity, ν = 0.99, whereas for a positive polarity ν = 0.68. Similarly, the voltage required for stable cone-jet electrospinning of charged PAA was significantly higher in the negative polarity configuration for all tested flow rates (300-1600 μL h-1). As opposed to merely surface charges typically considered when electrospinning leaky dielectric fluids, as suggested by Melcher and Taylor (1969), our results suggest that the measured current is also affected by volumetric charges from charged PAA in the bulk of the jet. The proposed additional charge transport might affect the orientational order within PE-based nanofibers and their diameter.

Microfluidic strategies for engineering oxygen-releasing biomaterials

In the field of tissue engineering, local hypoxia in large-cell structures (larger than 1 mm3) poses a significant challenge. Oxygen-releasing biomaterials supply an innovative solution through oxygen ⁠ delivery in a sustained and controlled manner. Compared to traditional methods such as emulsion, sonication, and agitation, microfluidic technology offers distinct benefits for oxygen-releasing material production, including controllability, flexibility, and applicability. It holds enormous potential in the production of smart oxygen-releasing materials. This review comprehensively covers the fabrication and application of microfluidic-enabled oxygen-releasing biomaterials. To begin with, the physical mechanism of various microfluidic technologies and their differences in oxygen carrier preparation are explained. Then, the distinctions among diverse oxygen-releasing components in regards for oxygen-releasing mechanism, oxygen-carrying capacity, and duration of oxygen release are presented. Finally, the present obstacles and anticipated development trends are examined together with the application outcomes of oxygen-releasing biomaterials based on microfluidic technology in the biomedical area. STATEMENT OF SIGNIFICANCE: Oxygen is essential for sustaining life, and hypoxia (a condition of low oxygen) is a significant challenge in various diseases. Microfluidic-based oxygen-releasing biomaterials offer precise control and outstanding performance, providing unique advantages over traditional approaches for tissue engineering. However, comprehensive reviews on this topic are currently lacking. In this review, we provide a comprehensive analysis of various microfluidic technologies and their applications for developing oxygen-releasing biomaterials. We compare the characteristics of organic and inorganic oxygen-releasing biomaterials and highlight the latest advancements in microfluidic-enabled oxygen-releasing biomaterials for tissue engineering, wound healing, and drug delivery. This review may hold the potential to make a significant contribution to the field, with a profound impact on the scientific community.

Free-Boundary Microfluidic Platform for Advanced Materials Manufacturing and Applications

Microfluidics, with its remarkable capacity to manipulate fluids and droplets at the microscale, has emerged as a powerful platform in numerous fields. In contrast to conventional closed microchannel microfluidic systems, free-boundary microfluidic manufacturing (FBMM) processes continuous precursor fluids into jets or droplets in a relatively spacious environment. FBMM is highly regarded for its superior flexibility, stability, economy, usability, and versatility in the manufacturing of advanced materials and architectures. In this review, a comprehensive overview of recent advancements in FBMM is provided, encompassing technical principles, advanced material manufacturing, and their applications. FBMM is categorized based on the foundational mechanisms, primarily comprising hydrodynamics, interface effects, acoustics, and electrohydrodynamic. The processes and mechanisms of fluid manipulation are thoroughly discussed. Additionally, the manufacturing of advanced materials in various dimensions ranging from zero-dimensional to three-dimensional, as well as their diverse applications in material science, biomedical engineering, and engineering are presented. Finally, current progress is summarized and future challenges are prospected. Overall, this review highlights the significant potential of FBMM as a powerful tool for advanced materials manufacturing and its wide-ranging applications.

Effective colloidal emulsion droplet regulation in flow-focusing glass capillary microfluidic device via collection tube variation

Colloidal emulsion droplets, created using glass capillary microfluidic devices, have been found in a myriad of applications, serving as subtle microcarriers, delicate templates, etc. To meet the objective requirements under varying circumstances, it is crucial to efficiently control the morphology and dimensions of the droplets on demand. The glass capillary collection tube is a crucial component of the flow-focusing microfluidic system due to its close association with the geometrical confinement of the multiphasic flow. However, there are currently no guidelines for the design of the morphology and dimensions of the glass capillary collection tube, which shall result in a delay in assessing serviceability until after the microfluidic device is prepared, thereby causing a loss of time and effort. Herein, an experimental study was conducted to investigate the effect of the geometrical characteristics of glass capillary collection tubes on the production of colloidal emulsion droplets. After characterizing the generated colloidal emulsion droplets, it was found that the geometrical variations of the glass capillary collection tube resulted in numerical disparities of droplets due to different degrees of flow-focusing effects. The stronger flow-focusing effect produced smaller droplets at a higher frequency, and the dimensional variation of colloidal emulsion droplets was more responsive to varying flow rates. Furthermore, the transformation from colloidal single-core double-emulsion droplets to multi-core double-emulsion droplets also changed with the flow rate due to the glass capillary collection tube morphology-determined varying flow-focusing effect. These experimental findings can offer qualitative guidance for the design of glass capillary microfluidic devices in the preliminary stage, thus facilitating the smooth production of desired colloidal emulsion droplets.

Volatile Droplets on Water are Sculpted by Vigorous Marangoni-Driven Subphase Flow

The shapes of highly volatile oil-on-water droplets become strongly asymmetric when they are out of equilibrium. The unsaturated organic vapor atmosphere causes evaporation and leads to a strong Marangoni flow in the bath, unlike that previously seen in the literature. Inspecting these shapes experimentally on millisecond and submillimeter time and length scales and theoretically by scaling arguments, we confirm that Marangoni-driven convection in the subphase mechanically stresses the droplet edges to an extent that increases for organic droplets of smaller contact angle and accordingly smaller thickness. The viscous stress generated by the subphase overcomes the thermodynamic Laplace pressure. The oil droplets develop copious regularly spaced fingers, and these fingers develop spike-shaped and branched treelike structures. Unlike this behavior for single-component (surfactant-free) oil droplets, droplets composed of two miscible (surfactant-free) organic liquids develop a rim of the less volatile component along the droplet perimeter, from which jets of monodisperse smaller droplets eject periodically due to the Rayleigh-Plateau instability. When evaporation shrinks droplets to μm size, their shapes fluctuate chaotically, and ellipsoidal shapes rupture into smaller daughter droplets when subphase convection flow pulls them in opposite directions. The shape of the evaporating oil droplets is kneaded and sculpted by vigorous flow in the water subphase.

Electrostatic force on a spherical particle confined between two planar surfaces

A charge-free particle in a uniform electric field experiences no net force in an unbounded domain. A boundary, however, breaks the symmetry and the particle can be attracted or repelled to it, depending on the applied field direction [Z. Wang et al., Phys. Rev. E, 2022, 106, 034607]. Here, we investigate the effect of a second boundary because of its common occurrence in practical applications. We consider a spherical particle suspended between two parallel walls and subjected to a uniform electric field, applied in a direction either normal or tangential to the surfaces. All media are modeled as leaky dielectrics, thus allowing for the accumulation of free charge at interfaces, while bulk media remain charge-free. The Laplace equation for the electric potential is solved using a multipole expansion and the boundaries are accounted for by a set of images. The results show that in the case of a normal electric field, which corresponds to a particle between two electrodes, the force is always attractive to the nearer boundary and, in general, weaker that the case of only one wall. Intriguingly, for a given particle-wall separation we find that the force may vary nonmonotonically with confinement and its magnitude may exceed the one-wall value. In the case of tangential electric field, which corresponds to a particle between insulating boundaries, the force follows the same trends but it is always repulsive.

WSCNet: Biomedical Image Recognition for Cell Encapsulated Microfluidic Droplets

Microfluidic droplets accommodating a single cell as independent microreactors are frequently demanded for single-cell analysis of phenotype and genotype. However, challenges exist in identifying and reducing the covalence probability (following Poisson's distribution) of more than two cells encapsulated in one droplet. It is of great significance to monitor and control the quantity of encapsulated content inside each droplet. We demonstrated a microfluidic system embedded with a weakly supervised cell counting network (WSCNet) to generate microfluidic droplets, evaluate their quality, and further recognize the locations of encapsulated cells. Here, we systematically verified our approach using encapsulated droplets from three different microfluidic structures. Quantitative experimental results showed that our approach can not only distinguish droplet encapsulations (F1 score > 0.88) but also locate each cell without any supervised location information (accuracy > 89%). The probability of a "single cell in one droplet" encapsulation is systematically verified under different parameters, which shows good agreement with the distribution of the passive method (Residual Sum of Squares, RSS < 0.5). This study offers a comprehensive platform for the quantitative assessment of encapsulated microfluidic droplets.

Biomimetic Extracellular Scaffolds by Microfluidic Superstructuring of Nanofibers

The extracellular matrix is a dynamic framework bearing chemical and morphological cues that support many cellular functions, and artificial analogs with well-defined chemistry are of great interest for biomedical applications. Herein, we describe hierarchical, extracellular-matrix-mimetic microgels, termed "superbundles" (SBs) composed of peptide amphiphile (PA) supramolecular nanofiber networks created using flow-focusing microfluidic devices. We explore the effects of altered flow rate ratio and PA concentration on the ability to create SBs and develop design rules for producing SBs with both cationic and anionic PA nanofibers and gelators. We demonstrate the morphological similarities of SBs to decellularized extracellular matrices and showcase their ability to encapsulate and retain proteinaceous cargos with a wide variety of isoelectric points. Finally, we demonstrate that the novel SB morphology does not affect the well-established biocompatibility of PA gels.

Microfluidic Methods for Generation of Submicron Droplets: A Review

Submicron droplets are ubiquitous in nature and widely applied in fields such as biomedical diagnosis and therapy, oil recovery and energy conversion, among others. The submicron droplets are kinetically stable, their submicron size endows them with good mobility in highly constricted pathways, and the high surface-to-volume ratio allows effective loading of chemical components at the interface and good heat transfer performance. Conventional generation technology of submicron droplets in bulk involves high energy input, or relies on chemical energy released from the system. Microfluidic methods are widely used to generate highly monodispersed micron-sized or bigger droplets, while downsizing to the order of 100 nm was thought to be challenging because of sophisticated nanofabrication. In this review, we summarize the microfluidic methods that are promising for the generation of submicron droplets, with an emphasize on the device fabrication, operational condition, and resultant droplet size. Microfluidics offer a relatively energy-efficient and versatile tool for the generation of highly monodisperse submicron droplets.

Food aeration: Effect of the surface-active agent type on bubble deformation and break-up in a viscous Newtonian fluid: From single bubble to process-scale

Two continuous whipping devices, a rotor-stator (RS) and a narrow angular gap unit (NAGU), were used to produce aerated food with a 25% (v/v) gas fraction target. The liquid phase was a Newtonian model-solution containing 2% (w/w) of either whey proteins (WPC), sodium caseinate (SCN), or tween 20 (TW20). Strong differences emerged regarding gas incorporation and bubble size as a function of process parameters: namely, rotation speed and residence time. To improve understanding of the results obtained at pilot-scale, a second investigation consisting in the observation of the deformation and break-up of single gas bubbles has been undertaken using successively a Couette device and an impeller close to NAGU. For proteins, the observation of single bubble deformation and break-up showed that bubble break-up occurred by tip-streaming above a well-defined critical Capillary number Ca_c of 0.27 and 0.5 for SCN and WPC, respectively, whereas no break-up was observed with TW20 even though Ca reached 10. The poor foaming ability obtained with TW20 could be explained by a poor break-up mechanism, promoting coalescence and gas plugs at high shear instead of gas incorporation. Conversely, protein promote tip-streaming as the major break-up mechanism at low shear rate, explaining why rotation speed is not a key process parameter. Differences observed between SCN and WPC can be attributed to diffusion limitation for SCN when a much larger surface area is generated during aeration.

Numerical Simulation of Motion and Distribution Characteristics for Electrospray Droplets

Electrospray is a typical technology to prepare large amounts of droplets at micro/nano scale. Establishing the relationship between the processing parameters and the motion and distribution characteristics for electrospray droplets is an effective approach to guide the uniform deposition of the electrospray membrane. In this paper, a dynamic model of electrospray droplets based on the fully resolved direct numerical simulation (FR-DNS) method was constructed, and the spatial motion behaviors of charged droplets were simulated. The coupling effect of electric field force, the charge repulsive force, and the gravity on the motion and distribution of electrospray droplets was studied, and the relationship between processing parameters including the applied voltage and distance from the nozzle to the collecting plate and the spatial distribution of charged droplets was clarified in a direct way. The simulation model provided a good approach for the quantitative description of the motion and distribution behaviors for electrospray droplets, which would help to guide the control of the electrospray jet ejection process.

Fire-Shaped Nozzles to Produce a Stress Peak for Deformability Studies

Fire-shaped nozzles can be used to study the deformability of microcapsules, particles, or cells traveling in a flow. Though their geometry depends on the dimensions of the original glass capillary and the heating conditions, they all produce a strain rate peak approximately at the section where the diameter is 1.5 times the minimum. The intensity of this peak and the time from its position to the neck can be easily estimated from the flow rate and three geometrical parameters, without the need for any simulation. In the convergent region of these nozzles, it is possible to observe the evolution of the deformation. It is necessary to use a sufficiently long nozzle to produce the maximum deformation before the neck.

The onset of heterogeneity in the pinch-off of suspension drops

The pinch-off of a liquid drop extruded from a nozzle is a canonical situation that involves a series of self-similar regimes ending in a finite-time singularity. This configuration allows for exploring capillary flows over a large range of scales. In the case of suspension drops, the presence of particles breaks the self-similarity by introducing a length scale that can be much larger than the particle diameter. This length scale is a signature of the heterogeneities and delimitates a regime, in which a continuum approach of a suspension can be used from a regime where the discrete nature of the particles is involved.

At large scales, particulate suspensions flow like homogeneous viscous liquids, but at the particle scale, the role of the local heterogeneity brought by the particles cannot be neglected. The volume fraction also matters; in dense suspensions, particulate effects can be felt across distances much larger than the particle diameter. Therefore, whether a suspension should behave as a homogeneous or heterogeneous fluid is a matter of scale. Here, we consider the canonical situation of the pinch-off of suspension drops to study the behavior of suspensions at different scales. Initially, the filament of suspension thins down like a homogeneous liquid until reaching a critical thickness at which the thinning accelerates. Eventually, a region devoid of particles appears, and the breakup occurs similarly to a homogeneous viscous liquid. Although this problem have been studied for almost 20 y, the role of heterogeneity in the acceleration of the pinch-off is still not understood. We show that the onset of heterogeneity corresponds to the dislocation of the suspensions where local fluctuations in particle concentration increase. We derive scaling laws for the dynamics in the heterogeneous regime and develop a model to predict the coherence length at which the discrete nature of the particles appears, and we demonstrate that this length depends both on the particle size and on the volume fraction of the suspension. We extend this approach to polydisperse suspensions. Our work sheds light on the mesoscopic scale below which starts the heterogeneous regime and a continuum approach is not valid anymore.

One-step electrospinning PMMA-SPO with hierarchical architectures as a multi-functional transparent screen window

A fascinating multifunctional screen window containing air filtration, rain-flow transportation and photochromic functions.

Microfluidics-Enabled Soft Manufacture of Materials with Tailorable Wettability

Microfluidics and wettability are interrelated and mutually reinforcing fields, experiencing synergistic growth. Surface wettability is paramount in regulating microfluidic flows for processing and manipulating fluids at the microscale. Microfluidics, in turn, has emerged as a versatile platform for tailoring the wettability of materials. We present a critical review on the microfluidics-enabled soft manufacture (MESM) of materials with well-controlled wettability and their multidisciplinary applications. Microfluidics provides a variety of liquid templates for engineering materials with exquisite composition and morphology, laying the foundation for precisely controlling the wettability. Depending on the degree of ordering, liquid templates are divided into individual droplets, one-dimensional (1D) arrays, and two-dimensional (2D) or three-dimensional (3D) assemblies for the modular fabrication of microparticles, microfibers, and monolithic porous materials, respectively. Future exploration of MESM will enrich the diversity of chemical composition and physical structure for wettability control and thus markedly broaden the application horizons across engineering, physics, chemistry, biology, and medicine. This review aims to systematize this emerging yet robust technology, with the hope of aiding the realization of its full potential.

Co-Axial Gyro-Spinning of PCL/PVA/HA Core-Sheath Fibrous Scaffolds for Bone Tissue Engineering

The present study aspires towards fabricating core-sheath fibrous scaffolds by state-of-the-art pressurized gyration for bone tissue engineering applications. The core-sheath fibers comprising dual-phase poly-ε-caprolactone (PCL) core and polyvinyl alcohol (PVA) sheath are fabricated using a novel "co-axial" pressurized gyration method. Hydroxyapatite (HA) nanocrystals are embedded in the sheath of the fabricated scaffolds to improve the performance for application as a bone tissue regeneration material. The diameter of the fabricated fiber is 3.97 ± 1.31 µm for PCL-PVA/3%HA while pure PCL-PVA with no HA loading gives 3.03 ± 0.45 µm. Bead-free fiber morphology is ascertained for all sample groups. The chemistry, water contact angle and swelling behavior measurements of the fabricated core-sheath fibrous scaffolds indicate the suitability of the structures in cellular activities. Saos-2 bone osteosarcoma cells are employed to determine the biocompatibility of the scaffolds, wherein none of the scaffolds possess any cytotoxicity effect, while cell proliferation of 94% is obtained for PCL-PVA/5%HA fibers. The alkaline phosphatase activity results suggest the osteogenic activities on the scaffolds begin earlier than day 7. Overall, adaptations of co-axial pressurized gyration provides the flexibility to embed or encapsulate bioactive substances in core-sheath fiber assemblies and is a promising strategy for bone healing.

Prediction of outflow parameters for a shower head

Bathing or showering is one of the main pathways of water consumption. The shower head, as an important part of our bathing facilities, is directly related to water saving and bathing satisfaction. To design a water-saving and comfortable shower, it is extremely important to know its likely outflow parameters .In the resent research, the shower head structure and the water jet characteristics, and the Lagrangian control volume (CV) element integration method is used to establish a theoretical model describing the motion of the jet from the spray-holes of a shower head. The model also includes a method of calculation of the spraying force, considering the velocity distribution over the jet cross-section. The model can be used to predict jet parameters, spraying force, water temperature drop, and sprayed water volume distribution. To validate the predictions,experiments are conducted to measure the spraying force, temperature drop, and sprayed water volume distribution for the shower head. Comparisons between experiments and predictions show that the prediction accuracy of the model is satisfactory and the model can be adopted to evaluate the performance of the shower head. In addition, using the model, the salient jet parameters for the spraying hole are also simulated, and the influences of the flow rate and the size of the spraying hole on the motion of the jet can be assessed. These results guide research and development into water-conservation, energy-conservation, and comfort when showering.

The Natural Breakup Length of a Steady Capillary Jet: Application to Serial Femtosecond Crystallography

One of the most successful ways to introduce samples in Serial Femtosecond Crystallography has been the use of microscopic capillary liquid jets produced by gas flow focusing, whose length-to-diameter ratio and velocity are essential to fulfill the requirements of the high pulse rates of current XFELs. In this work, we demonstrate the validity of a classical scaling law with two universal constants to calculate that length as a function of the liquid properties and operating conditions. These constants are determined by fitting the scaling law to a large set of experimental and numerical measurements, including previously published data. Both the experimental and numerical jet lengths conform remarkably well to the proposed scaling law. We show that, while a capillary jet is a globally unstable system to linear perturbations above a critical length, its actual and shorter long-term average intact length is determined by the nonlinear perturbations coming from the jet breakup itself. Therefore, this length is determined solely by the properties of the liquid, the average velocity of the liquid and the flow rate expelled. This confirms the very early observations from Smith and Moss 1917, Proc R Soc Lond A Math Phys Eng, 93, 373, to McCarthy and Molloy 1974, Chem Eng J, 7, 1, among others, while it contrasts with the classical conception of temporal stability that attributes the natural breakup length to the jet birth conditions in the ejector or small interactions with the environment.

Comparison of surfactant mass transfer with drop formation times from dynamic interfacial tension measurements in microchannels

Dynamic interfacial tension was studied experimentally during drop formation in a flow-focusing microchannel. A low viscosity silicone oil (4.6 mPa s) was the continuous phase and a mixture of 48% w/w water and 52% w/w glycerol was the dispersed phase. An anionic (sodium dodecylsulfate, SDS), a cationic (dodecyltrimethylammonium bromide, DTAB) and a non-ionic (Triton™ X-100, TX100) surfactant were added in the dispersed phase, at concentrations below and above the critical micelle concentration (CMC). For SDS and DTAB the drop size against continuous phase flowrate curves initially decreased with surfactant concentration and then collapsed to a single curve at concentrations above CMC. For TX100 the curves only collapsed at surfactant concentrations 8.6 times the CMC. From the collapsed curves a correlation of drop size with capillary number was derived, which was used to calculate the dynamic interfacial tension at times as low as 3 ms. The comparison of the surfactant mass transport and adsorption times to the interface against the drop formation times indicated that surfactant adsorption also contributes to the time required to reach equilibrium interfacial tension. Criteria were proposed for drop formation times to ensure that equilibrium interfacial tension has been reached and does not affect the drop formation.

Role of Interfacial Tension on Viscous Multiphase Flows in Coaxial Microfluidic Channels

We experimentally investigate the influence of interfacial tension on liquid/liquid microflows for fluids having large viscosity contrasts. A coaxial microdevice is employed to examine the situation where a less-viscous fluid is injected in a sheath of a more-viscous fluid using both immiscible and miscible fluid pairs. Data obtained from high-speed imaging reveal a variety of regular flow regimes, including dripping, jetting, wavy, core-annular, diffusive jet, mist, and inverted thread flow patterns. Flow maps are delineated over a wide range of injection flow rates, and an original methodology based on periodic pattern analysis is developed to clarify relationships between interfacial dynamics and fluid properties of multiphase materials. Specifically, we show the smooth evolution of droplet size and spacing at the transition between dripping and jetting flows and develop scaling relationships based on capillary numbers to predict droplet flow morphologies. For similar flow conditions, reducing interfacial tension leads to a significant decrease in droplet size. For miscible fluid pairs, diffusive jets are observed at low Péclet numbers, whereas wavy core-annular flows are obtained at moderate Reynolds numbers for both immiscible and miscible fluids. This work provides a unifying description of the influence of interfacial properties on viscous microflow phenomena.

Blood Particulate Analogue Fluids: A Review

Microfluidics has proven to be an extraordinary working platform to mimic and study blood flow phenomena and the dynamics of components of the human microcirculatory system. However, the use of real blood increases the complexity to perform these kinds of in vitro blood experiments due to diverse problems such as coagulation, sample storage, and handling problems. For this reason, interest in the development of fluids with rheological properties similar to those of real blood has grown over the last years. The inclusion of microparticles in blood analogue fluids is essential to reproduce multiphase effects taking place in a microcirculatory system, such as the cell-free layer (CFL) and Fähraeus-Lindqvist effect. In this review, we summarize the progress made in the last twenty years. Size, shape, mechanical properties, and even biological functionalities of microparticles produced/used to mimic red blood cells (RBCs) are critically exposed and analyzed. The methods developed to fabricate these RBC templates are also shown. The dynamic flow/rheology of blood particulate analogue fluids proposed in the literature (with different particle concentrations, in most of the cases, relatively low) is shown and discussed in-depth. Although there have been many advances, the development of a reliable blood particulate analogue fluid, with around 45% by volume of microparticles, continues to be a big challenge.

Electrical Conductivity of a Stretching Viscoelastic Filament

Long polymeric chains highly stretched and aligned with the flow confer a strong mechanical anisotropy on a viscoelastic solution. The electrically-driven transport of free ions under such conditions is far from being understood. In this paper, we determine experimentally whether the above-mentioned deviation from isotropy affects the electric charge transport across the liquid. To this end, we measure the electrical conductivity in the flow (stretching) direction of the cylindrical liquid filament formed in the elasto-capillary thinning that arises during the breakup of a viscoelastic liquid bridge. First, we examine the behavior of monodisperse solutions of polyethylene oxide (PEO) in a mixture of glycerine and water. For all the concentrations and molecular weights considered, the filament conductivity remains practically the same as the isotropic conductivity measured under hydrostatic conditions. However, we observe a decrease in the electric current at the end of elasto-capillary regime which may partially be attributed to the reduction of the liquid conductivity. Then, we measure the conductivity of bidisperse solutions of PEO with very different molecular weights. In this case, a significant decrease in conductivity is observed as the filament radius decreases. This constitutes the first experimental evidence of ion mobility reduction in stretching viscoelastic filaments, a relevant effect in applications such as electrospinning.

Aerosol agitation: Quantifying the hydrodynamic stressors on particulates encapsulated in small droplets

Lower respiratory tract infections originate from multiple aerosol sources, varying from droplets erupting from bursting bubbles in a toilet or those produced by human speech. A key component of the aerosol-based infection pathway-from source to potential host-is the survival of the pathogen during aerosolization. Due to their finite-time instability, pinch-off processes occurring during aerosolization have the potential to rapidly accelerate the fluid into focused regions of these droplets, stress objects therein, and if powerful enough, disrupt biological life. However, the extent that a pathogen will be exposed to damaging hydrodynamic stressors during the aerosolization process is unknown. Here we compute the probability that particulates will be exposed to a hydrodynamic stressor during the generation of droplets that range in size from one to 100 microns. For example, particulates in water droplets less than 5 μm have a 50% chance of being subjected to an energy dissipation rate in excess of 10¹¹ W/m³, hydrodynamic stresses in excess of 10⁴ Pa, and strain rates in excess of 10⁷ s^-1, values known to damage certain biological cells. Using a combination of numerical simulations and self-similar dynamics, we show how the exposure within a droplet can be generally predicted from its size, surface tension, and density, even across different aerosolization mechanisms. Collectively, these results introduce aerosol agitation as a potential factor in pathogen transmission and implicate the pinch-off singularity flow as setting the distribution of hydrodynamic stressors experienced within the droplet.

Fluid interfaces with very sharp tips in viscous flow

When a fluid interface is subjected to a strong viscous flow, it tends to develop near-conical ends with pointed tips so sharp that their radius of curvature is undetectable. In microfluidic applications, tips can be made to eject fine jets, from which micrometer-sized drops can be produced. Here we show theoretically that the opening angle of the conical interface varies on a logarithmic scale as a function of the distance from the tip, owing to nonlocal coupling between the tip and the external flow. Using this insight we are able to show that the tip curvature grows like the exponential of the square of the strength of the external flow and to calculate the universal shape of the interface near the tip. Our experiments confirm the scaling of the tip curvature as well as of the interface's universal shape. Our analytical technique, based on an integral over the surface, may also have far wider applications, for example treating problems with electric fields, such as electrosprays.

Capabilities and Limitations of Fire-Shaping to Produce Glass Nozzles

Microfluidic devices for drop and emulsion production are often built using fire-shaped (or fire-polished) glass nozzles. These are usually fabricated manually with inexpensive equipment. The shape limitations and poor reproducibility are pointed as the main drawbacks. Here, we evaluate the capabilities of a new fire-shaping approach which fabricates the nozzle by heating a vertical rotating capillary at the Bottom of a Lateral Flame (BLF). We analyze the effect of the heating conditions, and the capillary size and tolerances. The shape reproducibility is excellent for nozzles of the same size produced with the same conditions. However, the size reproducibility is limited and does not seem to be significantly affected by the heating conditions. Specifically, the minimum neck diameter standard deviation is 3%. Different shapes can be obtained by changing the heating position or the capillary dimensions, though, for a given diameter reduction, there is a minimum nozzle length due to the overturning of the surface. The use of thinner (wall or inner diameter) capillaries allows producing much shorter nozzles but hinders the size reproducibility. Finally, we showed an example of how the performance of a microfluidic device is affected by the nozzle shape: a Gas Dynamic Virtual Nozzle (GDVN) built with a higher convergent rate nozzle works over a wider parametric range without whipping.