Suppression of Capillary Instability of a Polymeric Thread via Parallel Plate Confinement

Bioinspired Anti-Plateau-Rayleigh-Instability on Dual Parallel Fibers

The Plateau-Rayleigh instability (PRI) is a well-known phenomenon where a liquid column always breaks up into droplets to achieve the minimization of surface energy. It normally leads to the non-uniformity of a liquid film, which, however, is unfavorable for the fluid coating process. So far, strategies to overcome this instability rely on either the surfactants, UV/high-temp curing treatments, or specific chemical reactions, which suffer from both limited liquid composition and complicated experimental conditions. Natural mulberry silk, a typical composite fiber, is produced by silkworms through a similar fluidic coating process, but exhibits a remarkably uniform and smooth surface. Drawing inspiration, it is revealed that the unique dual parallel fibers are capable of overcoming the PRI during the fluid coating process. Such anti-PRI ability is attributable to the changes in the Laplace pressure difference caused by the alternative asymmetry of the liquid film, as has been demonstrated by both a force analysis on the irregular liquid film and theoretical simulation according to the stability of the liquid on parallel fibers in the fluid coating process. The strategy is applicable for preparing various smooth functional coatings on fibers, which offers new perspectives for fluid coating and microfluidic technologies.

Nanoscale viscosity of confined polyethylene oxide

Complex fluids near interfaces or confined within nanoscale volumes can exhibit substantial shifts in physical properties compared to bulk, including glass transition temperature, phase separation, and crystallization. Because studies of these effects typically use thin film samples with one dimension of confinement, it is generally unclear how more extreme spatial confinement may influence these properties. In this work, we used x-ray photon correlation spectroscopy and gold nanoprobes to characterize polyethylene oxide confined by nanostructured gratings (<100nm width) and measured the viscosity in this nanoconfinement regime to be ∼500 times the bulk viscosity. This enhanced viscosity occurs even when the scale of confinement is several times the polymer's radius of gyration, consistent with previous reports of polymer viscosity near flat interfaces.

Stability of a jet moving in a rectangular microchannel

We study numerically the basic flow and linear stability of a capillary jet confined in a rectangular microchannel. We consider both the case where the interface does not touch the solid surfaces and that in which the jet adheres to them with a contact angle slightly smaller than 180^{∘}. Given an arbitrary set of values of the governing parameters, the fully developed (parallel) two-dimensional basic flow is calculated and then the growth rate of the dominant perturbation mode is determined as a function of the wave number. The flow is linearly stable if that growth rate is negative for all the wave numbers considered. We show that when the coflowing stream viscosity is sufficiently small in terms of that of the jet, there is an interval of the flow rate ratio Q for which the jet adheres to the walls or not depending on whether the flow is established by decreasing or increasing the value of Q. When the distance between the interface and the channel wall is of the order of the jet radius, the jet is unconditionally unstable. However, for sufficiently small interface-to-wall distances, the viscous stress can dominate the capillary pressure and fully stabilize the flow. Our results suggest that the capillary modes are suppressed and the flow becomes stable when the jet adheres to the channel walls. The combination of the above results indicates that, under certain parametric conditions, stable or unstable jets can be formed depending on whether the experimenter sets the flow rate ratio by decreasing or increasing progressively the jet flow rate while keeping constant that of the outer stream. Our theoretical predictions for the stablity of a coflow in a rectangular channel are consistent with previous experimental results [Humphry et al., Phys. Rev. E 79, 056310 (2009)PLEEE81539-375510.1103/PhysRevE.79.056310].

Mechanisms of spontaneous chain formation and subsequent microstructural evolution in shear-driven strongly confined drop monolayers

It was experimentally demonstrated by Migler and his collaborators [Phys. Rev. Lett., 2001, 86, 1023; Langmuir, 2003, 19, 8667] that a strongly confined drop monolayer sheared between two parallel plates can spontaneously develop a flow-oriented drop-chain morphology. Here we show that the formation of the chain-like microstructure is driven by far-field Hele-Shaw quadrupolar interactions between drops, and that drop spacing within chains is controlled by the effective drop repulsion associated with the existence of confinement-induced reversing streamlines, i.e., the swapping trajectory effect. Using direct numerical simulations and an accurate quasi-2D model that incorporates quadrupolar and swapping-trajectory contributions, we analyze microstructural evolution in a monodisperse drop monolayer. Consistent with experimental observations, we find that drop spacing within individual chains is usually uniform. Further analysis shows that at low area fractions all chains have the same spacing, but at higher area fractions there is a large spacing variation from chain to chain. These findings are explained in terms of uncompressed and compressed chains. At low area fractions most chains are uncompressed (spacing equals lst, which is the stable separation of an isolated pair). At higher area fractions compressed chains (with tighter spacing) are formed in a process of chain zipping along y-shaped structural defects. We also discuss the relevance of our findings to other shear-driven systems, such as suspensions of spheres in non-Newtonian fluids.

Soft-shear induced phase-separated nanoparticle string-structures in polymer thin films

Application of shear stress has been shown to unidirectionally orient the microstructures of block copolymers and polymer blends. In the present work, we study the phase separation of a novel nanoparticle (NP)-polymer blend thin film system under shear using a soft-shear dynamic zone annealing (DZA-SS) method. The nanoparticles are densely grafted with polymer chains of chemically dissimilar composition from the matrix polymer, which induces phase separation upon thermal annealing into concentrated nanoparticle domains. We systematically examine the influence of DZA-SS translation speed and thus the effective shear rate on nanoparticle domain elongation and compare this with the counterpart binary polymer blend behavior. Unidirectionally aligned nanoparticle string-domains are fabricated in the presence of soft-shear in confined thin film geometry. We expect this DZA-SS method to be applicable to various NP-polymer blends towards unidirectionally aligned nanoparticle structures, which are important to functional nanoparticle structure fabrication.

Capillary rupture of suspended polymer concentric rings

We present the first experimental study on the simultaneous capillary instability amongst viscous concentric rings suspended atop an immiscible medium. The rings ruptured upon annealing, with three types of phase correlation between neighboring rings. In the case of weak substrate confinement, the rings ruptured independently when they were sparsely distanced, but via an out-of-phase mode when packed closer. If the substrate confinement was strong, the rings would rupture via an in-phase mode, resulting in radially aligned droplets. The concentric ring geometry caused a competition between the phase correlation of neighboring rings and the kinetically favorable wavelength, yielding an intriguing, recursive surface pattern. This frustrated pattern formation behavior was accounted for by a scaling analysis.

The science of foaming

The generation of liquid foams is at the heart of numerous natural, technical or scientific processes. Even though the subject of foam generation has a long-standing history, many recent progresses have been made in an attempt to elucidate the fundamental processes at play. We review the subject by providing an overview of the relevant key mechanisms of bubble generation within a coherent hydrodynamic context; and we discuss different foaming techniques which exploit these mechanisms.

Wetting-mediated collective tubulation and pearling in confined vesicular drops of DDAB solutions

Whether driven by external mechanical stresses (shear flow) or induced by membrane-active peptides and/or proteins, the collective growth of tubules in membranous fluids has seldom been reported. The pearling destabilization of these membranous tubules which requires an activation of the shape distortion, often induced by optical tweezers, membrane-active biomolecules or an electrical field, has also rarely been observed under mild experimental conditions. Here we report such events of collective tubulation and pearling destabilization in sessile drops of a didodecyl-dimethylammonium bromide (DDAB) vesicular solution that are confined by a surrounding oil medium. Based on the wetting dynamics and the features of the tubulation process, we show that the growth of the tubules here relies on a mechanism of "pinning-induced pulling" from the retracting drop, rather than the classical hydrodynamic fingering instability. We show that the whole tubulation process is driven by a strong coupling between the bulk properties of the ternary (DAAB/water/oil) system and the dynamics of wetting. Finally, we discuss the pearling destabilization of these tubules under vanishing static interface tension and quite mild tensile force arising from their pulling. We show that under those mild conditions, shape disturbances readily grow, either as pearling waves moving toward the drop-reservoir or as Rayleigh-type peristaltic modulations. Besides revealing singular non-Rayleigh pearling modes, this work also brings new insights into the flow dynamics in membranous tubules anchored to an infinite reservoir.

Influence of substrate confinement on the phase-correlation in the capillary breakup of arrays of patterned polymer stripes

We investigated the influence of substrate confinement on the capillary breakup of parallel nonaxisymmetric polymer stripes suspended on top of, or confined between, another immiscible polymer pattern. When the residual layer thickness of the pattern was reasonably large, the PS (or PMMA) stripes confined within PMMA (or PS) trenches broke up, either nucleated, out-of-phase, or without clear phase correlation depending on the geometry and viscosity ratio between the two polymers. In stark contrast, for the two extreme cases of viscosity ratios we studied, in-phase breakup of confined polymer stripes was always observed when the alternating PS/PMMA stripes were formed, that is, without residual layer, regardless of the specific geometry.

Slow growth of the Rayleigh-Plateau instability in aqueous two phase systems

This paper studies the Rayleigh-Plateau instability for co-flowing immiscible aqueous polymer solutions in a microfluidic channel. Careful vibration-free experiments with controlled actuation of the flow allowed direct measurement of the growth rate of this instability. Experiments for the well-known aqueous two phase system (ATPS, or aqueous biphasic systems) of dextran and polyethylene glycol solutions exhibited a growth rate of 1 s(-1), which was more than an order of magnitude slower than an analogous experiment with two immiscible Newtonian fluids with viscosities and interfacial tension that closely matched the ATPS experiment. Viscoelastic effects and adhesion to the walls were ruled out as explanations for the observed behavior. The results are remarkable because all current theory suggests that such dilute polymer solutions should break up faster, not slower, than the analogous Newtonian case. Microfluidic uses of aqueous two phase systems include separation of labile biomolecules but have hitherto be limited because of the difficulty in making droplets. The results of this work teach how to design devices for biological microfluidic ATPS platforms.

A sliding plate microgap rheometer for the simultaneous measurement of shear stress and first normal stress difference

A new generation of the "flexure-based microgap rheometer" (the N-FMR) has been developed which is also capable of measuring, in addition to the shear stress, the first normal stress difference of micrometer thin fluid films. This microgap rheometer with a translation system based on compound spring flexures measures the rheological properties of microliter samples of complex fluids confined in a plane couette configuration with gap distances of h = 1-400 μm up to shear rates of γ = 3000 s(-1). Feed back loop controlled precise positioning of the shearing surfaces with response times <1 ms enables to control the parallelism within 1.5 μrad and to maintain the gap distance within 20 nm. This precise gap control minimizes squeeze flow effects and allows therefore to measure the first normal stress difference N(1) of the thin film down to a micrometer gap distance, with a lower limit of N(1)/γ = 9.375×10(-11) η/h(2) that depends on the shear viscosity η and the squared inverse gap. Structural development of complex fluids in the confinement can be visualized by using a beam splitter on the shearing surface and a long working distance microscope. In summary, this new instrument allows to investigate the confinement dependent rheological and morphological evolution of micrometer thin films.

Suppression of instabilities in multiphase flow by geometric confinement

We investigate the effect of confinement on drop formation in microfluidic devices. The presence or absence of drop formation is studied for two immiscible coflowing liquids in a microfluidic channel, where the channel width is considerably larger than the channel height. We show that stability of the inner fluid thread depends on the channel geometry: when the width of the inner fluid is comparable to or larger than the channel height, hydrodynamic instabilities are suppressed, and a stable jet that does not break into drops results; otherwise, the inner fluid breaks into drops, in either a dripping or jetting regime. We present a model that accounts for the data and experimentally exploit this effect of geometric confinement to induce the breakup of a jet at a spatially defined location.

Cell sorting in three dimensions: topology, fluctuations, and fluidlike instabilities

Previous 2D and 3D models concluded that cell sorting requires cytoskeletal fluctuations and is stalled by high tension at heterotypic interfaces. New deterministic and stochastic models show that this is not true in 3D. Sorting in 3D involves both topological untangling and domain coalescence. Coalescence requires fluctuations and low tension, but untangling does not. It occurs by a Plateau-Rayleigh instability of cell threads-deterministically driven by high tension. At high minority-cell fractions, untangling dominates and significant partial sorting can occur without fluctuations.

Interfacial instabilities in a microfluidic Hele-Shaw cell

This paper describes surfactant-sensitive, dynamic instabilities that occur to aqueous droplets translating in a continuous flow of hexadecane in a microfluidic Hele-Shaw cell (HSC). A very low interfacial tension (on the order of 0.01 mN m-1) between water and hexadecane allowed for deformation of the droplets along the fields of flow and tip-streaming from moving droplets. In the system of water and hexadecane that we investigated, the use of surfactants in both fluids was necessary to achieve interfacial tension sufficiently low for the instabilities to occur. The droplets entering the HSC stretched orthogonally to the main direction of flow into elongated shapes, with aspect ratios greater than ten to one (width to length). These droplets exhibited two types of instabilities. The first included elongation of droplets, and Rayleigh-Plateau instabilities in the stretched droplets. Arrays of these stretched droplets formed three characteristic patterns that depended on the rates of flow of water and hexadecane. The second was driven by the shear stress exerted on the interface between the two fluids by the top and bottom boundaries of the HSC; this instability is named a "shear-driven instability" (SDI). Our observations supported that the SDI-an effect similar to tip-streaming-resulted from a redistribution of surfactants at the interface between the two fluids.

Effect of nanoconfinement on liquid-crystal polymer chains

We apply a Monte Carlo polymerization model for Gay-Berne [J. Chem. Phys. 74, 3316 (1981)] monomers that we have recently introduced [J. Chem. Phys. 121, 9123 (2004)] to investigate with computer simulations the effects of nanoconfinement and anchoring type on the structure of the main-chain liquid-crystal polymers formed in thin films, in the presence of several types of surface alignment: parallel to the interface (random and uniform) or perpendicular to it (homeotropic). We perform first a study of the confined monomers and then we examine the features of the polymer chains obtained from an isotropic or nematic sample. We find a significant effect of the anchoring conditions on the characteristics of the chains and particularly striking differences between planar and homeotropic boundaries. Furthermore, our results indicate that the choice of different anchorings could be used to tune the linearity and degree of polymerization of the chains.

Mechanism for flow-rate controlled breakup in confined geometries: a route to monodisperse emulsions

This Letter describes a quasistationary breakup of an immiscible, inviscid fluid at low capillary numbers. The breakup proceeds in a coflowing, viscous liquid, in a confined geometry of a long and narrow orifice. In contrast to the capillary instability in an unbounded fluid, the collapse proceeds through a series of equilibria, each yielding the minimum interfacial energy of the fluid-fluid interface. The process is slow in comparison to typical relaxation speeds of the interface, and it is reversible. Its quasistatic character of collapse forms the basis for controlled, high-throughput generation of monodisperse fluid dispersions.

Breakup of a fluid thread in a confined geometry: droplet-plug transition, perturbation sensitivity, and kinetic stabilization with confinement

We investigate the influence of geometrical confinement on the breakup of long fluid threads in the absence of imposed flow using a lattice Boltzmann model. Our simulations primarily focus on the case of threads centered coaxially in a tube filled with another Newtonian fluid and subjected to both impulsive and random perturbations. We observe a significant slowing down of the rate of thread breakup ("kinetic stabilization") over a wide range of the confinement, Lambda= R(tube)/R(thread) < or =10 and find that the relative surface energies of the liquid components influence this effect. For Lambda<2.3, there is a transition in the late-stage morphology between spherical droplets and tube "plugs." Unstable distorted droplets ("capsules") form as transient structures for intermediate confinement (Lambda approximately equal 2.1-2.5). Surprisingly, the thread breakup process for more confined threads (Lambda< or =1.9 ) is found to be sensitive to the nature of the initial thread perturbation. Localized impulsive perturbations ("taps") cause a "bulging" of the fluid at the wall, followed by thread breakup through the propagation of a wave-like disturbance ("end-pinch instability") initiating from the thread rupture point. Random impulses along the thread, modeling thermal fluctuations, lead to a complex breakup process involving a competition between the Raleigh and end-pinch instabilities. We also briefly compare our tube simulations to threads confined between parallel plates and to multiple interacting threads under confinement.