Two microgel particles are encapsulated in a microdrop having a spherical diameter smaller than the sum of the diameters of the microgels; this causes the microgels to be squeezed together by the oil-water interface of the drop, in turn, making the drop ellipsoidal in shape. By modeling the force applied to the microgels by the drop and equating this to the Hertz contact force of their deformation, we are able to estimate their elastic modulus. By varying the surface tension and shape of the drops, we are able to measure the modulus of the microgels under different loads. This provides a simple technique for quantifying the elasticity of small, deformable objects, including liquid drops, microgels, and cells.
At low capillary number, drop formation in a T-junction is dominated by interfacial effects: as the dispersed fluid flows into the drop maker nozzle, it blocks the path of the continuous fluid; this leads to a pressure rise in the continuous fluid that, in turn, squeezes on the dispersed fluid, inducing pinch-off of a drop. While the resulting drop volume predicted by this "squeezing'' mechanism has been validated for a range of systems, as of yet, the pressure rise responsible for the actual pinch-off has not been observed experimentally. This is due to the challenge of measuring the pressures in a T-junction with the requisite speed, accuracy, and localization. Here, we present an empirical study of the pressures in a T-junction during drop formation. Using Laplace sensors, pressure probes we have developed, we confirm the central ideas of the squeezing mechanism; however, we also uncover other findings, including that the pressure of the dispersed fluid is not constant but rather oscillates in anti-phase with that of the continuous fluid. In addition, even at the highest capillary number for which monodisperse drops can be formed, pressure oscillations persist, indicating that drop formation in confined geometries does not transition to an entirely shear-driven mechanism, but to a mechanism combining squeezing and shearing.
Colloidal gelation has been extensively studied for the case of purely attractive systems, but little is understood about how colloidal gelation is affected by the presence of repulsive interactions. Here we demonstrate the gelation of a binary system of oppositely charged colloids, in which repulsive interactions compete with attractive interactions. We observe that gelation is controlled by varying the total volume fraction, the interaction strength, and the new tuning parameter of the mixing ratio of the two particle types, and present a state diagram of gelation along all these phase-space coordinates. Contrary to commonly studied purely attractive gels, in which weakly quenched gels are more compact and less tenuous, we find that particles in these binary gels form fewer contacts and the gels become more tenuous as we approach the gel point. This suggests that a different mechanism governs gel formation and ultimate structure in binary gelation: particles are unable to form additional favorable contacts through rearrangements, due to the competition of repulsive interactions between similarly charged colloids and attractive interactions between oppositely charged colloids.
We demonstrate the generation of water-in-water ( w/w) jets and emulsions by combining droplet microfluidics and aqueous two-phase systems ( ATPS). The application of ATPS in microfluidics has been hampered by the low interfacial tension between typical aqueous phases. The low tension makes it difficult to form w/w droplets with conventional droplet microfluidic approaches. We show that by mechanically perturbing a stable w/w jet, w/w emulsions can be prepared in a controlled and reproducible fashion. We also characterize the encapsulation ability of w/w emulsions and demonstrate that their encapsulation efficiency can be significantly enhanced by inducing formation of precipitates and gels at the w/w interfaces. Our work suggests a biologically and environmentally friendly platform for droplet microfluidics and establishes the potential of w/w droplet microfluidics for encapsulation-related applications. (C) 2012 American Institute of Physics. [doi:10.1063/1.3670365]
Microfluidic devices can be used to produce single or multiple emulsions with remarkably precise control of both the contents and size of the drops. Since each level of a multiple emulsion is formed by a distinct fluid stream, very efficient encapsulation of materials can be achieved. To obtain high throughput, these devices can be fabricated lithographically, allowing many devices to operate in parallel. However, to form multiple emulsions using a planar microfluidic device, the wettability of its surface must switch from hydrophobic to hydrophilic on the scale of micrometers where the drops are formed; this makes the fabrication of the devices very difficult. To overcome this constraint, we introduce non-planar microfluidic devices with graduated thicknesses; these can make drops even when their wetting properties do not favor drop formation. Nevertheless, the dependence of drop formation on the device geometry, the flow rates and the properties of the fluids, particularly in the case of unfavorable wetting, is very complex, making the successful design of these devices more difficult. Here we show that there exists a critical value of flow of the continuous phase above which drop formation occurs; this value decreases by two orders of magnitude as the wetting to the device wall of the continuous phase improves. We demonstrate how this new understanding can be used to optimize device design for efficient production of double or multiple emulsions.
In this article high-yield (77%) and high-speed (2700 cells s(-1)) single cell droplet encapsulation is described using a Dean-coupled inertial ordering of cells in a simple curved continuous microchannel. By introducing the Dean force, the particles will order to one equilibrium position after travelling less than 1 cm. We use a planar curved microchannel structure in PDMS to spatially order two types of myeloid leukemic cells (HL60 and K562 cells), enabling deterministic single cell encapsulation in picolitre drops. An efficiency of up to 77% was reached, overcoming the limitations imposed by Poisson statistics for random cell loading, which yields only 37% of drops containing a single cell. Furthermore, we confirm that > 90% of the cells remain viable. The simple planar structure and high throughput provided by this passive microfluidic approach makes it attractive for implementation in lab on a chip (LOC) devices for single cell applications using droplet-based platforms.
Therapeutic ultrasound is widely employed in clinical applications but its mechanism of action remains unclear. Here we report prompt fluidization of a cell and dramatic acceleration of its remodeling dynamics when exposed to low intensity ultrasound. These physical changes are caused by very small strains (10(-5)) at ultrasonic frequencies (10(6) Hz), but are closely analogous to those caused by relatively large strains (10(-1)) at physiological frequencies (10(0) Hz). Moreover, these changes are reminiscent of rejuvenation and aging phenomena that are well-established in certain soft inert materials. As such, we suggest cytoskeletal fluidization together with resulting acceleration of cytoskeletal remodeling events as a mechanism contributing to the salutary effects of low intensity therapeutic ultrasound.
Micrometer-sized hydrogel particles that contain living cells can be fabricated with exquisite control through the use of droplet-based microfluidics and bioinert polymers such as polyethyleneglycol (PEG) and hyperbranched polyglycerol (hPG). However, in existing techniques, the microgel gelation is often achieved through harmful reactions with free radicals. This is detrimental for the viability of the encapsulated cells. To overcome this limitation, we present a technique that combines droplet microfluidic templating with bio-orthogonal thiol-ene click reactions to fabricate monodisperse, cell laden microgel particles. The gelation of these microgels is achieved via the nucleophilic Michael addition of dithiolated PEG macro-cross-linkers to acrylated hPG building blocks and does not require any initiator. We systematically vary the microgel properties through the use of PEG linkers with different molecular weights along with different concentrations of macromonomers to investigate the influence of these parameters on the viability and proliferation of encapsulated yeast cells. We also demonstrate the encapsulation of mammalian cells including fibroblasts and lymphoblasts.
Monodisperse poly(DL-lactic acid) (PLA) particles of diameters between 11 and 121 mu m were fabricated in flow focusing glass microcapillary devices by evaporation of dichloromethane (DCM) from emulsion droplets at room temperature. The dispersed phase was 5% (w/w) PLA in DCM containing 0.1-2 mM Nile Red and the continuous phase was 5% (w/w) poly(vinyl alcohol) in reverse osmosis water. Particle diameter was 2.7 times smaller than the diameter of the emulsion droplet template, indicating very low particle porosity. Monodisperse droplets have only been produced under dripping regime using a wide range of dispersed phase flow rates (0.002-7.2 cm(3).h(-1)), continuous phase flow rates (0.3-30 cm(3).h(-1)), and orifice diameters (50-237 mu m). In the dripping regime, the ratio of droplet diameter to orifice diameter was inversely proportional to the 0.39 power of the ratio of the continuous phase flow rate to dispersed phase flow rate. Highly uniform droplets with a coefficient of variation (CV) below 2% and a ratio of the droplet diameter to orifice diameter of 0.5-1 were obtained at flow rate ratios of 4-25. Under jetting regime, polydisperse droplets (CV > 6%) were formed by detachment from relatively long jets (between 4 and 10 times longer than droplet diameter) and a ratio of the droplet size to orifice size of 2-5.
In this article, we demonstrate a novel microfluidic flow chamber driven by surface acoustic waves. Our device is a closed loop channel with an integrated acoustic micropump without external fluidic connections that allows for the investigation of small fluid samples in a continuous flow. The fabrication of the channels is particularly simple and uses standard milling and PDMS molding. The micropump consists of gold electrodes deposited on a piezoelectric substrate employing photolithography. We show that the pump generates a pressure-driven Poiseuille flow, investigate the acoustic actuation mechanism, characterize the flow profile for different channel geometries, and evaluate the driving pressure, efficiency and response time of the acoustic micropump. The fast response time of our pump permits the generation of non-stationary flows. To demonstrate the versatility of the device, we have pumped a red blood cell suspension at a physiological rate of 60 beats/min.
A direct consequence of the finite compressibility of a swollen microgel is that it can shrink and deform in response to an external perturbation. As a result, concentrated suspensions of these particles exhibit relaxation dynamics and rheological properties which can be very different with respect to those of a hard sphere suspension or an emulsion. We study the reduction in size of ionic microgels in response to increasing number of particles to show that particle shrinkage originates primarily from steric compression, and that the effect of ion-induced de-swelling of the polymer network is negligible. With increasing particle concentration, the single particle dynamics switch from those typical of a liquid to those of a super-cooled liquid and finally to those of a glass. However, the transitions occur at volume fractions much higher than those characterizing a hard sphere system. In the supercooled state, the distribution of displacements is non-Gaussian and the dependence of the structural relaxation time on volume fraction is describable by a Volger-Fulcher-Tammann function. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3697762]
Many bacteria organize themselves into structurally complex communities known as biofilms in which the cells are held together by an extracellular matrix. In general, the amount of extracellular matrix is related to the robustness of the biofilm. Yet, the specific signals that regulate the synthesis of matrix remain poorly understood. Here we show that the matrix itself can be a cue that regulates the expression of the genes involved in matrix synthesis in Bacillus subtilis. The presence of the exopolysaccharide component of the matrix causes an increase in osmotic pressure that leads to an inhibition of matrix gene expression. We further show that non-specific changes in osmotic pressure also inhibit matrix gene expression and do so by activating the histidine kinase KinD. KinD, in turn, directs the phosphorylation of the master regulatory protein Spo0A, which at high levels represses matrix gene expression. Sensing a physical cue such as osmotic pressure, in addition to chemical cues, could be a strategy to non-specifically co-ordinate the behaviour of cells in communities composed of many different species.
Bacterial biofilms are organized communities of cells living in association with surfaces. The hallmark of biofilm formation is the secretion of a polymeric matrix rich in sugars and proteins in the extracellular space. In Bacillus subtilis, secretion of the exopolysaccharide (EPS) component of the extracellular matrix is genetically coupled to the inhibition of flagella-mediated motility. The onset of this switch results in slow expansion of the biofilm on a substrate. Different strains have radically different capabilities in surface colonization: Flagella-null strains spread at the same rate as wild type, while both are dramatically faster than EPS mutants. Multiple functions have been attributed to the EPS, but none of these provides a physical mechanism for generating spreading. We propose that the secretion of EPS drives surface motility by generating osmotic pressure gradients in the extracellular space. A simple mathematical model based on the physics of polymer solutions shows quantitative agreement with experimental measurements of biofilm growth, thickening, and spreading. We discuss the implications of this osmotically driven type of surface motility for nutrient uptake that may elucidate the reduced fitness of the matrix-deficient mutant strains.