Capillary‐based microfluidics is a great technique to produce monodisperse and complex emulsions and particulate suspensions. In this review, the current understanding of drop and jet formation in capillary‐based microfluidic devices for two primary flow configurations, coflow and flow‐focusing is summarized. The experimental and theoretical description of fluid instabilities is discussed and conditions for controlled drop breakup in different modes of drop generation are provided. Current challenges in drop breakup with low interfacial tension systems and recent progress in overcoming drop size limitations using electro‐coflow are addressed. In each scenario, the physical mechanisms for drop breakup are revisited, and simple scaling arguments proposed in the literature are introduced.
In regenerative medicine, natural protein-based polymers offer enhanced endogenous bioactivity and potential for seamless integration with tissue, yet form weak hydrogels that lack the physical robustness required for surgical manipulation, making them difficult to apply in practice. The use of higher concentrations of protein, exogenous cross-linkers, and blending synthetic polymers has all been applied to form more mechanically robust networks. Each relies on generating a smaller network mesh size, which increases the elastic modulus and robustness, but critically inhibits cell spreading and migration, hampering tissue regeneration. Here we report two unique observations; first, that colloidal suspensions, at sufficiently high volume fraction (ϕ), dynamically assemble into a fully percolated 3D network within high-concentration protein polymers. Second, cells appear capable of leveraging these unique domains for highly efficient cell migration throughout the composite construct. In contrast to porogens, the particles in our system remain embedded within the bulk polymer, creating a network of particle-filled tunnels. Whereas this would normally physically restrict cell motility, when the particulate network is created using ultralow cross-linked microgels, the colloidal suspension displays viscous behavior on the same timescale as cell spreading and migration and thus enables efficient cell infiltration of the construct through the colloidal-filled tunnels.fibrin | microgels | colloidal assemblies | porosity | cell migration D ecoupling stiffness, pore size, and cell infiltration is a critical hurdle in biomaterials design and has been previously addressed in synthetic hydrogels by enabling cell-mediated degradation via protease-specific peptide cross-linkers (1-4). However, even in these highly engineered systems, compared with native extracellular matrices (ECMs), the mesh size remains a critical limiting factor in host integration; this is because cells are incapable of nonproteolytic, i.e., degradation-independent, migration through such small mesh sizes (5), as illustrated in Fig. 1A.Protein-based biomaterials derived from native ECM represent an attractive alternative to synthetic hydrogels, offering significant benefits through their enhanced endogenous bioactivity. The native ECM and its derivatives act as growth factor/cytokine depots, thus providing a multivalent endogenous binding site for growth factor delivery (6). A classic example of this type of biomaterial is fibrin, the endogenous provisional matrix formed at sites of vascular injury as a result of blood coagulation (7,8). Clinically, to reach desirable mechanical properties for sealing tissues/wounds, fibrin is used at supraphysiological concentrations typically containing ∼10× more fibrinogen than physiological concentrations (9-12). When used at these artificially high concentrations, the characteristic mesh size of the network is unfortunately similar to that of synthetic PEG polymers, which is on the order of tens of nanometers, making i...
Toroidal droplets are inherently unstable due to surface tension. They can break up, similar to cylindrical jets, but also exhibit a shrinking instability, which is inherent to the toroidal shape. We investigate the evolution of shrinking toroidal droplets using particle image velocimetry. We obtain the flow field inside the droplets and show that as the torus evolves, its cross-section significantly deviates from circular. We then use the experimentally obtained velocities at the torus interface to theoretically reconstruct the internal flow field. Our calculation correctly describes the experimental results and elucidates the role of those modes that, among the many possible ones, are required to capture all of the relevant experimental features.he impact of drops with superhydrophobic surfaces (1), the corona splash that results after a drop hits a liquid bath (2), and the behavior of falling rain drops (3) all involve formation of transient toroidal droplets. These types of droplets have also been generated and studied via the Leidenfrost mechanism (4). Quite generally, a nonspherical droplet that is shaped as a torus is unstable and transforms into spherical droplets (5-8). For thin tori, this transformation happens via the Rayleigh-Plateau instability (Movie S1). In contrast, for thick-enough tori, there is no breakup and the toroidal droplet "shrinks" until it collapses onto itself to form a single spherical droplet (Movie S2). In the process, the tube radius grows until, eventually, the handle* of the torus disappears. Note that the spherical shape minimizes the surface area for a given volume. Hence, toroidal droplets always shrink to minimize surface area. The origin of this behavior can be understood from the variation of the mean curvature, H , and hence of the Laplace pressure, ∆p = 2γH , with γ the interfacial tension, around the circular cross-section of the torus. Because H , and hence ∆p, are larger on the outside of the torus than on its inside, the corresponding pressure difference causes the shrinking of the toroidal droplet (9). Assuming that the cross-section of the torus remains circular during the process and that the velocity at the interface is radial, in the reference frame of the circular cross-section, calculations of the shrinking speed were consistent with experimental observations (9). However, recent simulations have found that the cross-section does not remain circular but that it rather deforms significantly as the torus shrinks (10). Despite this difference with the theory, which brings about other additional differences in the flow fields, the simulated shrinking speed was also consistent with the experimental results. Overall, the underlying assumptions of the theory and the discrepancies with the simulation reflect that the shrinking instability of toroidal droplets is still not fully understood.In this paper, we experimentally determine the flow field inside shrinking toroidal drops. We find that the droplet changes shape as it shrinks and that the velocity at the interfac...
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