Molecular Processes Leading to "Necking" in Extensional Flow of Polymer Solutions: Using Microfluidics and Single DNA Imaging

Pure elongation flow of an electrorheological fluid: insights on wall slip from electrorheology

In this work we study the pure elongation flow behavior of an electrorheological (ER) fluid as a model soft-jammed system, wherein the extent of jamming is controlled by an externally applied electric-field. More specifically, a pure elongation flow has been achieved by facilitating significant slip at the contact between the material and rheometer-plate while pulling it with constant pulling velocity under a constant external electric-field. The normal force exerted by the top plate on the material was measured as a function of gap during the flow for various combinations of electric-field strength and pulling velocity. For any force-gap curve, at first force increases to the maximum (region-I), then it decreases with gap (region-II). In region-II, the normal force-gap curve shifts to higher gaps with increasing electric-field strength for any given pulling velocity. Interestingly, these curves (region-II) demonstrate gap-electric field-velocity superposition, manifesting the self-similar nature of the flow. Finally, we have modeled the flow curves using a slip-layer model, which rendered a remarkable prediction of flow curves and also led to estimation of slip-layer thickness. We observed that slip-layer thickness decreases with increasing magnitude of electric field for a given pulling velocity, which suggests that the extent of jamming plays a crucial role in slip dynamics.

Suppressing the rebound of impacting droplets from solvophobic surfaces by polymer additives: polymer adsorption and molecular mechanisms

A liquid droplet impacting on a solvophobic surface normally rebounds. The rebound is suppressed by a small amount of dissolved polymer. In this work, using multi-body dissipative particle dynamics simulations, two anti-rebound mechanisms, the slow-retraction and the slow-hopping mechanisms, are identified. Which of them dominates depends on the polymer-surface attraction strength. However, these two mechanisms do not exclude each other but may coexist. During the droplet rebound, the surface-adsorbed polymer acts in two ways: the adsorbed beads mediate the solvent-surface interactions, and the highly stretching unadsorbed polymer segment exerts a retraction force on the liquid. Both actions increase the friction against retraction and the resistance against hopping. We also investigate the effects of the molecular weight and the concentration of the polymer additive, the droplet size, and the impact velocity on the rebound tendency. As the first work to provide a microscopic explanation of the anti-rebound mechanism by polymer additives, this study allows better understanding of wetting behavior by polymer-solution droplets.

Rebound Suppression of a Droplet Impacting on a Supersolvophobic Surface by a Small Amount of Polymer Additives

A small amount of polymer dissolved in a droplet suppresses droplet rebound when it impinges on a supersolvophobic surface. This work investigates impacting dynamics of a droplet of dilute polymer solution depending on the molecular weight and the concentration of the polymer by using multibody dissipative particle dynamics simulations. Either the long polymer or the high polymer concentration suppresses rebound of a droplet although its shear viscosity and the liquid-vapor surface tension are not different from those of a pure solvent droplet. We found a new mechanism of the anti-rebound in which the resistance is applied against the hopping motion, while behavior of the non-rebounding droplet at the earlier spreading and retraction stages is the same as for the rebounding droplets. Two polymer contributions to reducing the rebound tendency are quantitatively analyzed: the alteration of the substrate wettability by the polymer adsorption and the polymer elongation force.

Observation of Ethanol-Induced Condensation and Decondensation Processes at a Single-DNA Molecular Level in Microfluidic Devices Equipped with a Rapid Solution Exchange System

Conformational transitions from secondary (e.g., B- to A-form DNA) to higher-order (e.g., coil to globule) transitions play important roles in genome expression and maintenance. Several single-molecule approaches using microfluidic devices have been used to determine the kinetics of DNA chromatin assembly because microfluidic devices can afford stretched DNA molecules through laminar flow and rapid solution exchange. However, some issues, particularly the uncertainty of time 0 in the solution exchange process, are encountered. In such kinetic experiments, it is critical to determine when the target solution front approaches the target DNA molecules. Therefore, a new design for a microfluidic device is developed that enables the instantaneous exchange of solutions in the observation channel, allowing accurate measurements of DNA conformational transitions; stepwise, ethanol-induced conformational transitions are revealed. Although full DNA contraction from coil to globule is observed with >50% ethanol, no outstanding change is observed at concentrations <40% in 10 min. With 50% ethanol solution, the DNA conformational transition passes through two steps: (i) fast and constant-velocity contraction and (ii) relatively slow contraction from the free end. The first process is attributed to the B to A conformational transition by gradual dehydration. The second process is due to the coil-globule transition as the free end of DNA starts the contraction. This globular structure formation counteracts the shear force from the microfluids and decelerates the contraction velocity. This real-time observation system can be applied to the kinetic analysis of DNA conformational transitions such as kinetics of chromatin assembly and gene expression.

Pore-Scale Flow Characterization of Polymer Solutions in Microfluidic Porous Media

Polymer solutions are frequently used in enhanced oil recovery and groundwater remediation to improve the recovery of trapped nonaqueous fluids. However, applications are limited by an incomplete understanding of the flow in porous media. The tortuous pore structure imposes both shear and extension, which elongates polymers; moreover, the flow is often at large Weissenberg numbers, Wi, at which polymer elasticity in turn strongly alters the flow. This dynamic elongation can even produce flow instabilities with strong spatial and temporal fluctuations despite the low Reynolds number, Re. Unfortunately, macroscopic approaches are limited in their ability to characterize the pore-scale flow. Thus, understanding how polymer conformations, flow dynamics, and pore geometry together determine these nontrivial flow patterns and impact macroscopic transport remains an outstanding challenge. This review describes how microfluidic tools can shed light on the physics underlying the flow of polymer solutions in porous media at high Wi and low Re. Specifically, microfluidic studies elucidate how steady and unsteady flow behavior depends on pore geometry and solution properties, and how polymer-induced effects impact nonaqueous fluid recovery. This work thus provides new insights for polymer dynamics, non-Newtonian fluid mechanics, and applications such as enhanced oil recovery and groundwater remediation.

Macromolecular relaxation, strain, and extensibility determine elastocapillary thinning and extensional viscosity of polymer solutions

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.

Stretching of single DNA molecules caused by accelerating flow on a microchip

DNA elongation induced by fluidic stress was investigated on a microfluidic chip composed of a large inlet pool and a narrow channel. Through single-DNA observation with fluorescence microscopy, the manner of stretching of individual T4 DNA molecules (166 kbp) was monitored near the area of accelerating flow with narrowing streamlines. The results showed that the DNA long-axis length increased in a sigmoidal manner depending on the magnitude of flow acceleration, or shear, along the DNA chain. To elucidate the physical mechanism of DNA elongation, we performed a theoretical study by adopting a model of a coarse-grained nonlinear elastic polymer chain elongated by shear stress due to acceleration flow along the chain direction.

Polymer conformation during flow in porous media

Molecular conformations of individual polymers during flow through porous media are directly observed by single-DNA imaging in microfluidics. As the Weissenberg number increases during flow (Wi > 1), we observe two types of elastic instabilities: (a) stationary dead-zone and (b) time-dependant dead-zone washing. When stretched polymer chains enter a dead-zone, they first re-coil and, once inside the dead-zone, they rotate and re-stretch again. The probability distribution of DNA chains under the stretched condition inside the dead-zone is found to be heterogeneous with a broad distribution.

Microscopic origin of wall slip during flow of an entangled DNA solution in microfluidics: Flow induced chain stretching versus chain desorption

Despite the relevance and importance of slip, a fundamental understanding of the underlying molecular mechanisms of wall slip in polymer flow is still missing. In this work, we investigate the slip behavior of an entangled DNA solution at a molecular scale using a confocal microscope coupled to a microfluidic device. From microscopic measurement, we obtain both the velocity profile and conformation of polymeric chains by visualizing DNA molecules during flow on various surfaces (ranging from weak to strong interactions with DNA molecules). In channel flow at a low Weissenberg number (Wi = 0.14), we observe a parabolic flow for an APTES-treated glass (with strong interaction with DNA) in the absence of slip, while a significant amount of slip has been observed for a regular glass (with a weak interaction with DNA). At higher flow rates (Wi > 1.0), strong slip appears during flow on APTES-treated surfaces. In this case, only immobile DNA molecules are stretched on the surface and other bulk chains remain coiled. This observation suggests that the flow induced chain stretching at the interface is the main mechanism of slip during flow on strong surfaces. Conversely, for slip flow on surfaces with weak interactions (such as unmodified or acrylate-modified glasses), polymeric chains are desorbed from the surface and a thin layer of water is present near the surface, which induces an effective slip during flow. By imaging DNA conformations during both channel and shear flows on different surfaces, we elucidate that either chain desorption or flow-induced stretching of adsorbed chains occurs depending on the surface condition. In general, we expect that these new insights into the slip phenomenon will be useful for studying the biological flow involving single DNA molecule experiments in micro/nanofluidic devices.