Delayed capillary break-up of viscoelastic filaments presents scientific and technical challenges relevant for drop formation, dispensing, and adhesion in industrial and biological applications. The ...flow kinematics are primarily dictated by the viscoelastic stresses contributed by the polymers that are stretched and oriented in a strong extensional flow field resulting from the streamwise gradients created by the capillarity-driven squeeze flow. After an initial inertiocapillary (IC) or viscocapillary (VC) regime, where elastic effects seem to play no role, the interplay of capillarity and viscoelasticity can lead to an elastocapillary (EC) response characterized by exponentially-slow thinning of neck radius (extensional relaxation time is determined from the delay constant). Less frequently, a terminal visco-elastocapillary (TVEC) response with linear decay in radius can be observed and used for measuring terminal, steady extensional viscosity. However, both IC/VC–EC and EC–TVEC transitions are inaccessible in devices that create stretched necks by applying a step strain to a liquid bridge (e.g., capillary breakup extensional rheometer). In this study, we use dripping-onto-substrate rheometry to obtain radius evolution data for unentangled polymer solutions. We deduce that the plots of transient extensional viscosity vs. Hencky strain (scaled by the respective values at the EC–TVEC transition) emulate the functional form of the birefringence–macromolecular strain relationship based on Peterlin’s theory. We quantify the duration and strain between the IC/VC–EC and the EC–TVEC transitions using measures we term elastocapillary span and elastocapillary strain increment and find both measures show values directly correlated with the corresponding variation in extensional relaxation time.
We elucidate the influence of chemical structure on macromolecular hydrodynamics, rheological response, and pinching dynamics underlying drop formation/liquid transfer using polyethylene oxide (PEO) ...and 2-hydroxyethyl cellulose (HEC) as two polymers with distinct Kuhn length and matched overlap concentrations. We contrast the filament pinching dynamics and extensional rheology response using dripping-onto-substrate rheometry protocols. Even though dilute aqueous solutions of both polymers at matched concentrations display comparable shear viscosity, the PEO solutions exhibit distinctively higher values of extensional relaxation time, extent of strain hardening, and transient extensional viscosity, as well as an overall delay in pinch-off. We critically analyze the radius evolution for a pinching filament to posit that the solutions of flexible PEO macromolecules exhibit signatures of underlying coil-stretch transition manifested as a discontinuous, nonmonotonic variation in the extensional rate. In contrast, the solutions of semiflexible HEC show a monotonic increase in extensional rate in response to rising interfacial stress in the pinching filament, implying that the macromolecules undergo progressive stretching and orientation without undergoing coil-stretch transition. We show that the chemistry-dependent contrast in macromolecular dynamics and extensional rheology response can be characterized a priori in terms of three ratios: contour length to Kuhn length (flexibility), contour length to unperturbed coil size (extensibility), and packing length to Kuhn length (a parameter we termed as segmental dissymmetry). We identify the influence of the three ratios − flexibility, extensibility, and segmental dissymmetry − on the critical minimum concentration below which elastocapillary response and extensional relaxation time cannot be measured, the critical concentration above which the influence of concentration fluctuations disappears, and also define a stretched overlap concentration below which the extensional relaxation time becomes concentration-independent.
Quantitative studies of capillarity-driven pinching and extensional rheology of aqueous solutions of polysaccharides like hydroxyethyl cellulose (HEC) are beyond the measurable range of most ...extensional rheometers, and are relatively rare, even though polysaccharides are widely used as rheology modifiers. In this study, we utilize dripping-onto-substrate (DoS) rheometry protocols that we recently developed to characterize the pinching dynamics and extensional rheology response of aqueous HEC solutions. We find that the radius evolution data from the pinching necks show an elastocapillary regime that can be fit to determine the extensional relaxation time even for unentangled HEC solutions that show neither rate-dependent regime in shear viscosity nor measurable elasticity in the shear rheology response measured using torsional rheometers. Furthermore, the radius evolution data for the entangled HEC solutions display a power law regime, previously reported only for multicomponent complex fluids containing particles, bubbles, drops, and lamellar gel networks described with the generalized Newtonian fluid models. However, the entangled HEC solutions that exhibit pronounced shear thinning and measurable elastic moduli also reveal that the power law in the radius evolution data is modulated in the late stage by viscoelastic effects, allowing the measurement of both extensional relaxation time and steady, terminal extensional viscosity. Finally, we show that the pinching dynamics underlying drop formation/liquid transfer of HEC solutions can be determined fairly accurately by measuring shear rheology response, unlike the solutions of flexible polymers that display a stark contrast in response to shear and extensional flows.
Liquid transfer and drop formation/deposition processes involve complex free-surface flows including the formation of columnar necks that undergo spontaneous capillary-driven instability, thinning ...and pinch-off. For simple (Newtonian and inelastic) fluids, a complex interplay of capillary, inertial and viscous stresses determines the nonlinear dynamics underlying finite-time singularity as well as self-similar capillary thinning and pinch-off dynamics. In rheologically complex fluids, extra elastic stresses as well as non-Newtonian shear and extensional viscosities dramatically alter the nonlinear dynamics. Stream-wise velocity gradients that arise within the thinning columnar neck create an extensional flow field, and many complex fluids exhibit a much larger resistance to elongational flows than Newtonian fluids with similar shear viscosity. Characterization of pinch-off dynamics and the response to both shear and extensional flows that influence drop formation/deposition in microfluidic and printing applications requires bespoke instrumentation not available, or easily replicated, in most laboratories. Here we show that dripping-onto-substrate (DoS) rheometry protocols that involve visualization and analysis of capillary-driven thinning and pinch-off dynamics of a columnar neck formed between a nozzle and a sessile drop can be used for measuring shear viscosity, power law index, extensional viscosity, relaxation time and the most relevant processing timescale for printing. We showcase the versatility of DoS rheometry by characterizing and contrasting the pinch-off dynamics of a wide spectrum of simple and complex fluids: water, printing inks, semi-dilute polymer solutions, yield stress fluids, food materials and cosmetics. We show that DoS rheometry enables characterization of low viscosity printing inks and polymer solutions that are beyond the measurable range of commercially-available capillary break-up extensional rheometer (CaBER). We show that for high viscosity fluids, DoS rheometry can be implemented relatively inexpensively using an off-the-shelf digital camera, and for many complex fluids, similar power law scaling exponent describes both neck thinning dynamics and the shear thinning response.
Quantitative studies of capillary-driven thinning and pinch-off dynamics of semidilute polyelectrolyte solutions, and their response to extensional flows, typically encountered in drop formation ...applications, are relatively rare and are the focus of this contribution. Here the pinch-off dynamics and extensional rheology of two model polyelectrolytespoly(sodium 4-styrenesulfonate) (NaPSS) and poly(acrylic acid) (PAA)in two different solvents are characterized by using Dripping-onto-Substrate (DoS) rheometry. Unlike shear relaxation time that decreases with increase in concentration in the unentangled, semidilute solutions, the extensional relaxation time of PAA solutions increases with an exponent of 1/2, and the entangled semidilute solutions also exhibit a stronger concentration dependence of 3/2. In contrast, the extensional relaxation time is not measurable for the unentangled, semidilute aqueous NaPSS solutions, though entangled NaPSS solutions show concentration-dependent values. The experiments and analysis described herein elucidate how the interplay of stretching due to electrostatics and hydrodynamics influences extensional rheology response and printability of polyelectrolyte dispersions.
We show that visualization and analysis of capillary-driven thinning and pinch-off dynamics of the columnar neck in an asymmetric liquid bridge created by dripping-onto-substrate can be used for ...characterizing the extensional rheology of complex fluids. Using a particular example of dilute, aqueous PEO solutions, we show the measurement of both the extensional relaxation time and extensional viscosity of weakly elastic, polymeric complex fluids with low shear viscosity η < 20 mPa·s and relatively short relaxation time, λ < 1 ms. Characterization of elastic effects and extensional relaxation times in these dilute solutions is beyond the range measurable in the standard geometries used in commercially available shear and extensional rheometers (including CaBER, capillary breakup extensional rheometer). As the radius of the neck that connects a sessile drop to a nozzle is detected optically, and the extensional response for viscoelastic fluids is characterized by analyzing their elastocapillary self-thinning, we refer to this technique as optically-detected elastocapillary self-thinning dripping-onto-substrate (ODES-DOS) extensional rheometry.
The injection of polymer solutions into an oil basin can lead to enhanced oil recovery (EOR) by increasing the microscopic sweep of the reservoir, improving the water-oil motility ratio, and thus ...leading to greater yield from oil fields. In this contribution, we characterize both shear and extensional rheological response of aqueous solutions of partially hydrolyzed polyacrylamide (HPAM), the most commonly used polymer for EOR, for velocity gradients in both the flow direction (extensional) and perpendicular to flow (shear) arise in EOR applications. As HPAM is a charged polymer, to better emulate the environment in oil basins, the rheological response was investigated in presence of salt, sodium chloride, and calcium chloride, with concentrations 3.7 × 10
−4
− 1.5 M, as a function of polymer molecular weight (2–10 million g/mol) and concentration (0.005–0.3 wt%). The extensional relaxation times and extensional viscosity are measured using dripping-onto-substrate (DoS) rheometry protocols, and a commercial shear rheometer was utilized for characterizing the shear rheology response. The polyelectrolyte solutions formed by HPAM exhibit shear thinning in steady shear, but show strain hardening in response to extensional flow. Even though an increase in monovalent salt concentration leads to a decrease in both shear viscosity and extensional relaxation times, an increase in divalent salt concentration leads to an increase in extensional viscosity and relaxation time, implying that ion coordination can play a role in the presence of multivalent ions.
The relationship between ordered plasma membrane nanodomains, known as lipid rafts, and actin filaments is the focus of this study. Plasma membrane order was followed in live cells at 37°C using ...laurdan and di-4-ANEPPDHQ to report on lipid packing. Disrupting actin polymerisation decreased the fraction of ordered domains, which strongly argue that unstimulated cells have a basal level of ordered domains. Stabilising actin filaments had the opposite effect and increased the proportion of ordered domains. Decreasing the plasma membrane level of 4-phosphate-inositides lowers the number of attachment points for actin filaments and reduced the proportion of ordered domains. Aggregation of plasma membrane molecules, both lipid raft and non-lipid raft markers, lead to the formation of ordered domains. The increase in ordered domains was correlated with an increase in actin filaments just beneath the plasma membrane. In live cell plasma membrane blebs, which are detached from the underlying actin filaments, the fraction of ordered domains was low and GM1 could not be patched to form ordered domains. We conclude that ordered domains form when actin filaments attach to the plasma membrane. This downplays lipid-lipid interactions as the main driving force behind the formation of ordered membrane domains in vivo, giving greater prominence to membrane-intracellular filament interactions.
Display omitted
► Ordered plasma membrane domains exist in resting cells. ► Ordered membrane domains form when actin filaments attach to the plasma membrane. ► Phosphoinositides link actin dynamics to plasma membrane order. ► Intracellular actin dynamics are reflected in the outer leaflet of the plasma membrane. ► Molecular patching induces actin polymerisation and ordered domain formation.
PDF
