The ability to assemble nanomaterials, such as quantum dots, enables the creation of functional devices that present unique optical and electronic properties. For instance, light-emitting diodes with exceptional color purity can be printed via the evaporative-driven assembly of quantum dots. Nevertheless, current studies of the colloidal deposition of quantum dots have been limited to the surfaces of a planar substrate. Here, we investigate the evaporation-driven assembly of quantum dots inside a confined cylindrical geometry. Specifically, we observe distinct deposition patterns, such as banding structures along the length of a capillary tube. Such coating behavior can be influenced by the evaporation speed as well as the concentration of quantum dots. Understanding the factors governing the coating process can provide a means to control the assembly of quantum dots inside a capillary tube, ultimately enabling the creation of novel photonic devices.

Fusion between suspended lipid vesicles is difficult to achieve without membrane proteins or ions because the vesicles have extremely low equilibrium membrane tension and high poration energy. Nonetheless, vesicle fusion in the absence of mediators can also be achieved by mechanical forcing that is strong enough to induce membrane poration. Here, we employ a strong fluid shear stress to achieve vesicle fusion. By utilizing a unique vortex formation phenomenon in branched channels as a platform for capturing, stressing, and fusing the lipid vesicles, we directly visualize using high-speed imaging the vesicle fusion events, induced solely by shear, on the time scale of submilliseconds. We show that a large vesicle with a size of up to ∼10 μm can be achieved by the fusion of nanoscale vesicles. This technique has the potential to be utilized as a fast and simple way to produce giant unilamellar vesicles and to serve as a platform for visualizing vesicle interactions and fusions in the presence of shear.

An approach based upon sonication–microfluidics is presented to fabricate nanoparticle-coated microbubbles. The gas-in-liquid slug flow formed in a microchannel is subjected to ultrasound, leading to cavitation at the gas–liquid interface. Therefore, microbubbles are formed and then stabilized by the nanoparticles contained in the liquid. Compared to the conventional sonication method, this sonication–microfluidics continuous flow approach has unlimited gas nuclei for cavitation that yields continuous production of foam with shorter residence time. By controlling the flow rate ratios of the gas to the liquid, this method also achieves a higher production volume, smaller bubble size, and less waste of the nanoparticles needed to stabilize the microbubbles.

We study the process of coating the interface of a long gas bubble, which is translating in a horizontal circular capillary tube filled with a colloidal suspension. A typical elongated confined bubble is comprised of three distinct regions: a spherical front cap, a central body that is separated from the tube wall by a thin liquid film, and a spherical cap at the back. These three regions are connected by transitional sections. Particles gradually coat the bubble from the back to the front. We investigate the mechanisms that govern the initial accumulation of the particles and the growth of the particle-coated area on the interface of the bubble. We show that the initial accumulation of particles starts at the stable stagnation ring on the rear cap of the bubble, and the particles will completely coat the spherical cap at the back of the bubble before accumulating on the central body. Armoring the central interface of the bubble with particles thickens the liquid film around the bubble relative to that around the particle-free interface. This effect creates a rather sharp step on the interface of the bubble in the central region, which separates the armored region from the particle-free region. After the bubble is completely coated, the liquid film around the body of the bubble will adjust again to an intermediate thickness. We show that the three distinct thicknesses that the liquid film acquires during the armoring process can be well described analytically.

The Brownian motion of two particles in three dimensions serves as a model for predicting the diffusion-limited reaction rate, as first discussed by von Smoluchowski. Deutch and Felderhof extended the calculation to account for hydrodynamic interactions between the particles and the target, which results in a reduction of the rate coefficient by about half. Many chemical reactions take place in quasi-two-dimensional systems, such as on the membrane or surface of a cell. We perform a Smoluchowski-like calculation in a quasi-two-dimensional geometry, i.e., a membrane surrounded by fluid, and account for hydrodynamic interactions between the particles. We show that rate coefficients are reduced relative to the case of no interactions. The reduction is more pronounced than the three-dimensional case due to the long-range nature of two-dimensional flows.

Moving air–liquid interfaces, for example, bubbles, play a significant role in the detachment and transport of colloids and microorganisms in confined systems as well as unsaturated porous media. Moreover, they can effectively prevent and/or postpone the development of mature biofilms on surfaces that are colonized by bacteria. Here we demonstrate the dynamics and quantify the effectiveness of this bubble-driven detachment process for the bacterial strain Staphylococcus aureus. We investigate the effects of interface velocity and geometrical factors through microfluidic experiments that mimic some of the confinement features of pore-scale geometries. Depending on the bubble velocity U, at least three different flow regimes are found. These operating flow regimes not only affect the efficiency of the detachment process but also modify the final distribution of the bacteria on the surface. We organize our results according to the capillary number, , where μ and γ are the viscosity and the surface tension, respectively. Bubbles at very low velocities, corresponding to capillary numbers Ca < 5 × 10–5, exhibit detachment efficiencies of up to 80% at the early stage of bacterial adhesion. In contrast, faster bubbles at capillary numbers Ca > 10–3, have lower detachment efficiencies and cause significant nonuniformities in the final distribution of the cells on the substrate. This effect is associated with the formation of a thin liquid film around the bubble at higher Ca. In general, at higher bubble velocities bacterial cells in the corners of the geometry are less influenced by the bubble passage compared to the central region.

Liquid-infused surfaces are rough or patterned surfaces in which a lubricating fluid, such as oil, is infused, which exhibits various original properties (omniphobicity, biofouling, drag reduction). An outer flow in a confined geometry can entrain the oil trapped between the pattern of the surfaces by shearing the oil–water interface and cause the loss of the omniphobic properties of the interface. Starting from the theoretical analysis of Wexler et al. (Shear-driven failure of liquid-infused surfaces. Phys. Rev. Lett. 2015, 114, 168301), where a pure aqueous solution is the outer phase, we extend the predictions by introducing an extraction efficiency parameter α and by accounting for new dynamical effects induced by surfactants and aqueous foams. For surfactant solutions, decreasing the oil–water interfacial tension (γow) not only enhances oil extraction as expected but also modifies the dynamics of the receding oil–water interface through the variations of the receding contact angle (θ) with the capillary number (Ca), which is the ratio between the viscous and the capillary forces at the oil–water interface. For aqueous foams, the extraction dynamics are also influenced by the foam flow: oil is sheared by the thin film between the bubbles and the lubricating layer, which imposes a stronger interfacial shear compared to pure aqueous solutions. In both surfactant and foam cases, the experimental observations show the existence of nonuniform extraction dynamics related to the surfactant-induced instability of a two-fluid shear flow.

The effect of patterned substrates on the onset and behavior of the Faraday instability is studied experimentally. We show that the onset of Faraday standing waves in a vertically oscillating layer of liquid can be delayed due to the topography of the underlying two-dimensional patterned substrate. The magnitude of this stabilization effect can be predicted by existing linear stability theories, and we provide an additional physical explanation for the behavior. These observations suggest the feasibility of exploiting the Faraday instability in thin liquid layers in practical engineering systems.

We study pressure and flow-rate fluctuations in microchannels, where the flow rate is supplied by a syringe pump. We demonstrate that the pressure fluctuations are induced by the flow-rate fluctuations coming from mechanical oscillations of the pump motor. Also, we provide a mathematical model of the effect of the frequency of the pump on the normalized amplitude of pressure fluctuations and introduce a dimensionless parameter incorporating pump frequency, channel geometry and mechanical properties that can be used to predict the performance of different microfluidic device configurations. The normalized amplitude of pressure fluctuations decreases as the frequency of the pump increases and the elasticity of the channel material decreases. The mathematical model is verified experimentally over a range of typical operating conditions and possible applications are discussed.

Liquid-infused surfaces display advantageous properties that are normally associated with conventional gas-cushioned superhydrophobic surfaces. However, the surfaces can lose their novel properties if the infused liquid drains from the surface. We explore how drainage due to gravity or due to an external flow can be prevented through the use of chemical patterning. A small area of the overall surface is chemically treated to be preferentially wetted by the external fluid rather than the infused liquid. These sacrificial regions disrupt the continuity of the infused liquid, thereby preventing the liquid from draining from the texture. If the regions are patterned with the correct periodicity, drainage can be prevented entirely. The chemical patterns are created using spray-coating or deep-UV exposure, two facile techniques that are scalable to generate large-scale failure-resistant surfaces.

We investigate the equilibrium morphology of a finite volume of liquid placed on two parallel rigid fibers of different radii. As observed for identical radii fibers, the liquid is either in a column morphology or adopts a drop shape depending on the inter-fiber distance. However the cross-sectional area and the critical inter-fiber distance at which the transition occurs are both modified by the polydispersity of the fibers. Using energy considerations, we analytically predict the critical inter-fiber distance corresponding to the transition between the column and the drop morphologies. This distance depends both on the radii of the fibers and on the contact angle of the liquid. We perform experiments using a perfectly wetting liquid on two parallel nylon fibers: the results are in good agreement with our analytical model. The morphology of the capillary bridges between fibers of different radii is relevant to the modeling of large arrays of polydisperse fibers.

We report noncircular, stable liquid propagation patterns in a displacement process in a confined thin patterned porous layer. For constant fluid injection rates, the average front location of the interface r(t) exhibits a power-law behavior r ∝ t1/2; however, when surface tension effects become important, the interface displays noncircular shapes, e.g., square, rectangular, or octagonal, and maintains the same shape during most of the injection process. The interface shape is controlled by the value of a dimensionless group representing the strength of surface tension stresses relative to stresses accompanying injection. Furthermore, we show that the propagation patterns of the interface can be controlled by the relative orientation of the different porous layers.

A multiscale continuum model is constructed for a mechanosensitive (MS) channel gated by tension in a lipid bilayer membrane under stresses due to fluid flows. We illustrate that for typical physiological conditions vesicle hydrodynamics driven by a fluid flow may render the membrane tension sufficiently large to gate a MS channel open. In particular, we focus on the dynamic opening/closing of a MS channel in a vesicle membrane under a planar shear flow and a pressure-driven flow across a constriction channel. Our modeling and numerical simulation results quantify the critical flow strength or flow channel geometry for intracellular transport through a MS channel. In particular, we determine the percentage of MS channels that are open or closed as a function of the relevant measure of flow strength. The modeling and simulation results imply that for fluid flows that are physiologically relevant and realizable in microfluidic configurations stress-induced intracellular transport across the lipid membrane can be achieved by the gating of reconstituted MS channels, which can be useful for designing drug delivery in medical therapy and understanding complicated mechanotransduction.

The incomplete growth of nanowires that are synthesized by template-assisted electrodeposition presents a major challenge for nanowire-based devices targeting energy and electronic applications. In template-assisted electrodeposition, the growth of nanowires in the pores of the template is complex and unstable. Here we show theoretically and experimentally that the dynamics of this process is diffusion-limited, which results in a morphological instability driven by a race among nanowires. Moreover, we use our findings to devise a method to control the growth instability. By introducing a temperature gradient across the porous template, we manipulate ion diffusion in the pores, so that we can reduce the growth instability. This strategy significantly increases the length of nanowires. In addition to shedding light on a key nanotechnology, our results may provide fundamental insights into a variety of interfacial growth processes in materials science such as crystal growth and tissue growth in scaffolds.

We present a microfluidic method to continuously produce multicompartment microfibers, where embedded single or double emulsion droplets are regularly spaced along the length of the fiber. Both hydrophobic and hydrophilic compounds can be encapsulated in different microcompartments of the fiber for storage, selective dissolution, and delivery applications, as well as to provide multifunctionality.

We study the effect of softness of the microchannel on the process of droplet generation in two-phase flows in a T-junction microchannel. One side of the microchannel has a flexible thin PDMS layer, which vibrates naturally while droplets are generated; the deformation frequency coincides with the frequency of droplet formation. Furthermore, we compare the polydispersity of water-in-oil droplets formed with a microchannel with one soft wall with those formed in a conventional rigid microchannel. We show that deformation of the soft wall reduces the polydispersity in the droplet size.

We studied the dissolution dynamics of CO2 gas bubbles in a microfluidic channel, both experimentally and theoretically. In the experiments, spherical CO2 bubbles in a flow of a solution of sodium dodecyl sulfate (SDS) first shrink rapidly before attaining an equilibrium size. In the rapid dissolution regime, the time to obtain a new equilibrium is 30 ms regardless of SDS concentration, and the equilibrium radius achieved varies with the SDS concentration. To explain the lack of complete dissolution, we interpret the results by considering the effects of other gases (O2, N2) that are already dissolved in the aqueous phase, and we develop a multicomponent dissolution model that includes the effect of surface tension and the liquid pressure drop along the channel. Solutions of the model for a stationary gas bubble show good agreement with the experimental results, which lead to our conclusion that the equilibrium regime is obtained by gas exchange between the bubbles and liquid phase. Also, our observations from experiments and model calculations suggest that SDS molecules on the gas–liquid interface form a diffusion barrier, which controls the dissolution behaviour and the eventual equilibrium radius of the bubble.

The integration of Microbial Fuel Cells (MFCs) in a microfluidic geometry can significantly enhance the power density of these cells, which would have more active bacteria per unit volume. Moreover, microfluidic MFCs can be operated in a continuous mode as opposed to the traditional batch-fed mode. Here we investigate the effect of fluid flow on the performance of microfluidic MFCs. The growth and the structure of the bacterial biofilm depend to a large extent on the shear stress of the flow. We report the existence of a range of flow rates for which MFCs can achieve maximum voltage output. When operated under these optimal conditions, the power density of our microfluidic MFC is about 15 times that of a similar-size batch MFC. Furthermore, this optimum suggests a correlation between the behaviour of bacteria and fluid flow.

A common element in physiological flow networks, as well as most domestic and industrial piping systems, is a T junction that splits the flow into two nearly symmetric streams. It is reasonable to assume that any particles suspended in a fluid that enters the bifurcation will leave it with the fluid. Here we report experimental evidence and a theoretical description of a trapping mechanism for low-density particles in steady and pulsatile flows through T-shaped junctions. This mechanism induces accumulation of particles, which can form stable chains, or give rise to significant growth of bubbles due to coalescence. In particular, low-density material dispersed in the continuous phase fluid interacts with a vortical flow that develops at the T junction. As a result suspended particles can enter the vortices and, for a wide range of common flow conditions, the particles do not leave the bifurcation. Via 3D numerical simulations and a model of the two-phase flow we predict the location of particle accumulation, which is in excellent agreement with experimental data. We identify experimentally, as well as confirm by numerical simulations and a simple force balance, that there is a wide parameter space in which this phenomenon occurs. The trapping effect is expected to be important for the design of particle separation and fractionation devices, as well as used for better understanding of system failures in piping networks relevant to industry and physiology.

An approach based upon sonication–microfluidics is presented to fabricate nanoparticle-coated microbubbles. The gas-in-liquid slug flow formed in a microchannel is subjected to ultrasound, leading to cavitation at the gas–liquid interface. Therefore, microbubbles are formed and then stabilized by the nanoparticles contained in the liquid. Compared to the conventional sonication method, this sonication–microfluidics continuous flow approach has unlimited gas nuclei for cavitation that yields continuous production of foam with shorter residence time. By controlling the flow rate ratios of the gas to the liquid, this method also achieves a higher production volume, smaller bubble size, and less waste of the nanoparticles needed to stabilize the microbubbles.

We describe a microfluidic system for generating a sequence of liquid droplets of multiple concentrations in a single experimental condition. The series of final droplets has the combination of the compositions varying periodically, with polydispersity of the size less than 8%. By utilizing the design of the microchannel geometry and the passive control of three immiscible fluids (oil, water, and air) including generation, breakup, separation and coalescence of droplets, we can change the system to generate diverse sets of combination of materials. The device can be used for testing different concentration of materials in picoliter volumes and developing a new way to deliver dynamic signals of chemicals with microfluidics.

We describe a technique that measures ultralow interfacial tensions using paramagnetic spheres in a co-flow microfluidic device designed with a magnetic section. Our method involves tuning the distance between the co-flowing interface and the magnet's center, and observing the behavior of the spheres as they approach the liquid–liquid interface–the particles either pass through or are trapped by the interface. Using threshold values of the magnet-to-interface distance, we make estimates of the two-fluid interfacial tension. We demonstrate the effectiveness of this technique for measuring very low interfacial tensions, O(10−6–10−5) N m−1, by testing solutions of different surfactant concentrations, and we show that our results are comparable with measurements made using a spinning drop tensiometer.

We describe the results of experiments and mathematical analysis of the deformation of a free surface by an aggregate of magnetic particles. The system we study is differentiated from ferrofluid systems because it contains regions rich with magnetic material as well as regions of negligible magnetic content. In our experiments, the magnetic force from a spherical permanent magnet collects magnetic particles to a liquid–air interface, and deforms the free surface to form a hump. The hump is composed of magnetic and non-magnetic regions due to the particle collection. When the magnet distance falls below a threshold value, we observe the transition of the hump to a jet. The mathematical model we develop, which consists of a numerical solution and an asymptotic approximation, captures the shape of the liquid–air interface during the deformation stage and a scaling prediction for the critical magnet distance for the hump to become a jet.

We synthesized uniform crimped microfibers with controlled dimensions using a microfluidic approach, whereby a liquid jet flows from a narrow channel into a wider channel. The liquid jet, sheathed by an immiscible non-reacting liquid, undergoes ultraviolet (UV)-initiated gelation upstream of the channel widening. At the channel widening, the reacting jet may buckle due to an axial compressive stress, and the transient buckled morphology is preserved in the structure of the resulting solid fiber as the gelation reaction rapidly goes to completion. We investigated the effects of different experimental conditions, such as flow rate, UV light position, concentration of photoinitiator and UV light intensity on controlling the morphology of the microfibers, and we observed that the degree of crimp in the microfiber is dependent mainly on the extent of reaction.

Gas-in-oil-in-water-in-oil triple emulsions are fabricated with a microfluidic method. The encapsulating layers can be triggered for release by ultrasound, owing to the gas core. Due to the stability in the atmosphere, the emulsions are polymerized by using UV light outside the device to fabricate compound particles with a gas-in-liquid-in-solid structure.

We investigate experimentally the evaporation of liquid accumulated on a pair of parallel fibers, rigid or flexible. The liquid wetting the fibers can adopt two distinct morphologies: a compact drop shape, whose evaporation dynamics is similar to that of an isolated aerosol droplet, or a long liquid column of constant cross-section, whose evaporation dynamics depends upon the aspect ratio of the column. We thus find that the evaporation rate is constant for drops, while it increases strongly for columns as the interfiber distance decreases, and we propose a model to explain this behavior. When the fibers are flexible, the transition from drops to columns can be induced by the deformation of the fibers because of the capillary forces applied by the drop. Thus, we find that the evaporation rate increases with increasing flexibility. Furthermore, complex morphology transitions occur upon drying, which results in spreading of the drop as it evaporates.

We report the preparation of antibubbles by microfluidic methods. More specifically, we demonstrate a two-step approach, wherein a monodisperse water-in-oil-in-water (W/O/W) emulsion of core–shell construction is first generated via microfluidics and freeze-dried thereafter to yield, upon subsequent reconstitution, an aqueous dispersion of antibubbles. Stable antibubbles are attained by stabilization of the air–water interfaces through a combination of adsorbed particles and polymeric surfactant. The antibubbles strongly resemble the double emulsion templates from which they were formed. When triggered to release, antibubbles show complete release of their cores within about 100 ms.

Lubrication plays a crucial role in reducing friction for transporting heavy objects, from moving a 60-ton statue in ancient Egypt to relocating a 15,000-ton building in modern society. Although in China spoked wheels appeared ca. 1500 B.C., in the 15th and 16th centuries sliding sledges were still used in transporting huge stones to the Forbidden City in Beijing. We show that an ice lubrication technique of water-lubricated wood-on-ice sliding was used instead of the common ancient approaches, such as wood-on-wood sliding or the use of log rollers. The technique took full advantage of the natural properties of ice, such as sufficient hardness, flatness, and low friction with a water film. This ice-assisted movement is more efficient for such heavy-load and low-speed transportation necessary for the stones of the Forbidden City. The transportation of the huge stones provides an early example of ice lubrication and complements current studies of the high-speed regime relevant to competitive ice sports.

Biofilms are antibiotic-resistant, sessile bacterial communities that occupy most moist surfaces on Earth and cause chronic and medical device-associated infections. Despite their importance, basic information about biofilm dynamics in common ecological environments is lacking. Here, we demonstrate that flow through soil-like porous materials, industrial filters, and medical stents dramatically modifies the morphology of Pseudomonas aeruginosa biofilms to form 3D streamers, which, over time, bridge the spaces between obstacles and corners in nonuniform environments. We discovered that accumulation of surface-attached biofilm has little effect on flow through such environments, whereas biofilm streamers cause sudden and rapid clogging. We demonstrate that flow-induced shedding of extracellular matrix from surface-attached biofilms generates a sieve-like network that captures cells and other biomass, which add to the existing network, causing exponentially fast clogging independent of growth. These results suggest that biofilm streamers are ubiquitous in nature and strongly affect flow through porous materials in environmental, industrial, and medical systems.

Uniform polymeric microfibers of prescribed lengths were synthesized in microfluidic devices using two different approaches—valve actuation and pulses of ultraviolet (UV) light. The more versatile valve approach was employed to demonstrate control of the length of the microfiber as a function of the frequency of valve actuation.

Ice formation on surfaces and structures produces damage and inefficiencies that negatively impact all manners of activities. Not surprisingly, for a long time, an unmet challenge has been to design materials capable of minimizing or even eliminating the formation of ice on the surface of the material. In recent years, there were significant efforts to develop such ice-phobic surfaces by building on the advances made with superhydrophobic materials since these, by definition, tend to repel water. However, a robust response includes the ability to deter the formation of ice when a substrate colder than the freezing temperature is exposed either to impacting water droplets or water vapor (i.e., frost formation). In the latter case, superhydrophobic surfaces in high humidity conditions were shown to allow significant ice accumulation. Consequently, a new design idea was needed. In this issue of ACS Nano, it is shown how a liquid-infiltrated porous solid, where the liquid strongly wets and is retained within the material, has many of the properties desired for an ice-phobic substrate. The composite material exhibits low contact angle hysteresis so only small forces are needed to provoke droplets to slide off of a cold substrate. This new slippery surface shows many characteristics required for ice-phobicity, and a method is demonstrated for applying this kind of material as a coating on aluminum. Ice may have met its match.

Microfluidic approaches for controlled generation of colloidal clusters, for example, via encapsulation of colloidal particles in droplets, have been used for the synthesis of functional materials including drug delivery carriers. Most of the studies, however, use a low concentration of an original colloidal suspension (<10 wt %). Here we demonstrate microfluidic approaches for directly making droplets with moderate (10–25 wt %) and high (>60 wt %) particle concentrations. Three types of microfluidic devices, PDMS flow-focusing, PDMS T-junction, and microcapillary devices, are investigated for direct encapsulation of a high concentration of polystyrene (PS) nanoparticles in droplets. In particular, it is shown that PDMS devices fabricated by soft lithography can generate droplets from a 25 wt % PS suspension, whereas microcapillary devices made from glass capillary tubes are able to produce droplets from a 67 wt % PS nanoparticle suspension. When the PS concentration is between 0.6 and 25 wt %, the size of the droplets is found to change with the oil-to-water flow rate ratio and is independent of the concentration of particles in the initial suspensions. Drop sizes from ∼12 to 40 μm are made using flow rate ratios Qoil/Qwater from 20 to 1, respectively, with either of the PDMS devices. However, clogging occurs in PDMS devices at high PS concentrations (>25 wt %) arising from interactions between the PS colloids and the surface of PDMS devices. Glass microcapillary devices, on the other hand, are resistant to clogging and can produce droplets continuously even when the concentration of PS nanoparticles reaches 67 wt %. We believe that our findings indicate useful approaches and guidelines for the controlled generation of emulsions filled with a high loading of nanoparticles, which are useful for drug delivery applications.

Although the process of chemosensing by individual cells is intrisically stochastic, multicellular organisms exhibit highly regulated responses to external stimulations. Two key elements to understand the deterministic features of chemosensing are intercellular communications and the role of pacemaker cells. To characterize the collective behavior induced by these two factors, we study the spatial-temporal calcium dynamics of fibroblast cells in response to ATP stimulation. We find that closely packed cell colonies exhibit faster, more synchronized, and highly correlated responses compared to isolated cells. In addition, we demonstrate for chemosensing the existence of pacemaker cells and how the presence of gap junctions impact the first step of the collective response. By further comparing these results with the calcium dynamics of cells embedded in thin hydrogel films, where intercellular communication is only possible via diffusing molecules, we conclude that gap junctions are required for synchronized and highly correlated responses among cells in high density colonies. In addition, in high density cell colonies, both communication channels lead to calcium oscillations following the stimulation by external ATP. While the calcium oscillations associated with cells directly exposed to external flows were transient, the oscillations of hydrogel trapped cells can persist with a fundamental frequency and higher harmonics. Our observations and measurements highlight the crucial role of intercellular signaling for generating regulated spatial and temporal dynamics in cell colonies and tissues.

Although the process of chemosensing by individual cells is intrisically stochastic, multicellular organisms exhibit highly regulated responses to external stimulations. Two key elements to understand the deterministic features of chemosensing are intercellular communications and the role of pacemaker cells. To characterize the collective behavior induced by these two factors, we study the spatial-temporal calcium dynamics of fibroblast cells in response to ATP stimulation. We find that closely packed cell colonies exhibit faster, more synchronized, and highly correlated responses compared to isolated cells. In addition, we demonstrate for chemosensing the existence of pacemaker cells and how the presence of gap junctions impact the first step of the collective response. By further comparing these results with the calcium dynamics of cells embedded in thin hydrogel films, where intercellular communication is only possible via diffusing molecules, we conclude that gap junctions are required for synchronized and highly correlated responses among cells in high density colonies. In addition, in high density cell colonies, both communication channels lead to calcium oscillations following the stimulation by external ATP. While the calcium oscillations associated with cells directly exposed to external flows were transient, the oscillations of hydrogel trapped cells can persist with a fundamental frequency and higher harmonics. Our observations and measurements highlight the crucial role of intercellular signaling for generating regulated spatial and temporal dynamics in cell colonies and tissues.

We describe a microfluidic system that uses a magnetic field to sort paramagnetic beads by deflecting them in the direction normal to the flow. In the experiments we systematically study the dependence of the beads' deflection on bead size and susceptibility, magnet strength, fluid speed and viscosity, and device geometry. We also develop a design parameter that can aid in the design of microfluidic devices for immunomagnetic multi-target sorting.

We demonstrate a microfluidic method to first generate double emulsion droplets containing two different inner drops, and to then control the internal coalescence of the encapsulated drops. The advantages of the core-coalescence method are illustrated by fabricating high viscosity particles and by demonstrating the dissolution of cell membranes.

We report a simple route to form robust, inorganic, semi-permeable compartments composed of montmorillonite, a natural plate-like clay mineral that occurs widely in the environment. Mechanical forces due to shear in a narrow gap assemble clay nanoplates from an aqueous suspension onto air bubbles. Translucent vesicles suspended in a single-phase liquid are produced when the clay-covered air bubbles are exposed to a variety of water-miscible organic liquids and water-soluble surfactants. These vesicles of clay are mechanically robust and are stable in water and other liquids. We find that the wetting of organic liquids on clay explains the formation of clay vesicles from clay armored bubbles. Clay vesicles are microporous, exhibit size-selective permeability, and support spontaneous compartmentalization of self-assembling molecules in aqueous environments. The results we report here expand our understanding of potential paths to micro-compartmentalization in natural settings and are of relevance to theories of colloidal aggregation, mineral cycles, and the origins of life.

The mechanisms of red blood cell (RBC) deformation under both static and dynamic, i.e., flow, conditions have been studied extensively since the mid 1960s. Deformation-induced biochemical reactions and possible signaling in RBCs, however, were proposed only fifteen years ago. Therefore, the fundamental relationship between RBC deformation and cellular signaling dynamics i.e., mechanotransduction, remains incompletely understood. Quantitative understanding of the mechanotransductive pathways in RBCs requires integrative studies of physical models of RBC deformation and cellular biochemical reactions. In this article we review the physical models of RBC deformation, spanning from continuum membrane mechanics to cellular skeleton dynamics under both static and flow conditions, and elaborate the mechanistic links involved in deformation-induced ATP release.

Cells are wrapped in inelastic membranes, yet they can sustain large mechanical strains by regulating their area. The area regulation in cells is achieved either by membrane folding or by membrane exo- and endocytosis. These processes involve complex morphological transformations of the cell membrane, i.e., invagination, vesicle fusion, and fission, whose precise mechanisms are still under debate. Here we provide mechanistic insights into the area regulation of cell membranes, based on the previously neglected role of membrane confinement, as well as on the strain-induced membrane tension. Commonly, the membranes of mammalian and plant cells are not isolated, but rather they are adhered to an extracellular matrix, the cytoskeleton, and to other cell membranes. Using a lipid bilayer, coupled to an elastic sheet, we are able to demonstrate that, upon straining, the confined membrane is able to regulate passively its area. In particular, by stretching the elastic support, the bilayer laterally expands without rupture by fusing adhered lipid vesicles; upon compression, lipid tubes grow out of the membrane plane, thus reducing its area. These transformations are reversible, as we show using cycles of expansion and compression, and closely reproduce membrane processes found in cells during area regulation. Moreover, we demonstrate a new mechanism for the formation of lipid tubes in cells, which is driven by the membrane lateral compression and may therefore explain the various membrane tubules observed in shrinking cells.

When double emulsion droplets flow through a tapered nozzle, the droplets may break up and cause the core to be released. We model the system on the basis of the capillary instability and show that a droplet will not break up when the tilt angle of the nozzle is larger than 9°. For smaller tilt angles, whether the droplet breaks up also depends on the diameter ratio of the core of the droplet to the orifice of the nozzle. We verified this mechanism by experiments. The ideas are useful for the design of nozzles not only to break droplets for controlled release but also to prevent the droplet from rupturing in applications requiring the reinjection of an emulsion.

We present a microfluidic approach for the generation of water droplets containing a high volume fraction of gas bubbles and we provide a design principle for microbubble-based pressure sensing inside channels.

Bacteria inhabit a wide variety of environments in which fluid flow is present, including healthcare and food processing settings and the vasculature of animals and plants. The motility of bacteria on surfaces in the presence of flow has not been well characterized. Here we focus on Pseudomonas aeruginosa, an opportunistic human pathogen that thrives in flow conditions such as in catheters and respiratory tracts. We investigate the effects of flow on P. aeruginosa cells and describe a mechanism in which surface shear stress orients surface-attached P. aeruginosa cells along the flow direction, causing cells to migrate against the flow direction while pivoting in a zig-zag motion. This upstream movement is due to the retraction of type IV pili by the ATPase motors PilT and PilU and results from the effects of flow on the polar localization of type IV pili. This directed upstream motility could be beneficial in environments where flow is present, allowing bacteria to colonize environments that cannot be reached by other surface-attached bacteria.

Although ubiquitous, the processes by which bacteria colonize surfaces remain poorly understood. Here we report results for the influence of the wall shear stress on the early-stage adhesion of Pseudomonas aeruginosa PA14 on glass and polydimethylsiloxane surfaces. We use image analysis to measure the residence time of each adhering bacterium under flow. Our main finding is that, on either surface, the characteristic residence time of bacteria increases approximately linearly as the shear stress increases (∼0–3.5 Pa). To investigate this phenomenon, we used mutant strains defective in surface organelles (type I pili, type IV pili, or the flagellum) or extracellular matrix production. Our results show that, although these bacterial surface features influence the frequency of adhesion events and the early-stage detachment probability, none of them is responsible for the trend in the shear-enhanced adhesion time. These observations bring what we believe are new insights into the mechanism of bacterial attachment in shear flows, and suggest a role for other intrinsic features of the cell surface, or a dynamic cell response to shear stress.

In most environments, such as natural aquatic systems, bacteria are found predominantly in self-organized sessile communities known as biofilms. In the presence of a significant flow, mature multispecies biofilms often develop into long filamentous structures called streamers, which can greatly influence ecosystem processes by increasing transient storage and cycling of nutrients. However, the interplay between hydrodynamic stresses and streamer formation is still unclear. Here, we show that suspended thread-like biofilms steadily develop in zigzag microchannels with different radii of curvature. Numerical simulations of a low-Reynolds-number flow around these corners indicate the presence of a secondary vortical motion whose intensity is related to the bending angle of the turn. We demonstrate that the formation of streamers is directly proportional to the intensity of the secondary flow around the corners. In addition, we show that a model of an elastic filament in a two-dimensional corner flow is able to explain how the streamers can cross fluid streamlines and connect corners located at the opposite sides of the channel.

The integration of Microbial Fuel Cells (MFCs) in a microfluidic geometry can significantly enhance the power density of these cells, which would have more active bacteria per unit volume. Moreover, microfluidic MFCs can be operated in a continuous mode as opposed to the traditional batch-fed mode. Here we investigate the effect of fluid flow on the performance of microfluidic MFCs. The growth and the structure of the bacterial biofilm depend to a large extent on the shear stress of the flow. We report the existence of a range of flow rates for which MFCs can achieve maximum voltage output. When operated under these optimal conditions, the power density of our microfluidic MFC is about 15 times that of a similar-size batch MFC. Furthermore, this optimum suggests a correlation between the behaviour of bacteria and fluid flow.

We present a microfluidic method to continuously produce multicompartment microfibers, where embedded single or double emulsion droplets are regularly spaced along the length of the fiber. Both hydrophobic and hydrophilic compounds can be encapsulated in different microcompartments of the fiber for storage, selective dissolution, and delivery applications, as well as to provide multifunctionality.