Thermal delay of drop coalescence

We present the results of a combined experimental and theoretical study of drop coalescence in the presence of an initial temperature difference T[subscript 0] between a drop and a bath of the same liquid. We characterize experimentally the dependence of the residence time before coalescence on T[subscript 0] for silicone oils with different viscosities. Delayed coalescence arises above a critical temperature difference T[subscript c] that depends on the fluid viscosity: for T[subscript 0] > T[subscript c], the delay time increases T[subscript 0] [superscript 2/3] as for all liquids examined. This observed dependence is rationalized theoretically through consideration of the thermocapillary flows generated within the drop, the bath and the intervening air layer.National Science Foundation (U.S.) (Grant CMMI-1727565)National Science Foundation (U.S.) (Grant DMS-1614043

Ligament Mediated Fragmentation of Viscoelastic Liquids

The breakup and atomization of complex fluids can be markedly different than the analogous processes in a simple Newtonian fluid. Atomization of paint, combustion of fuels containing antimisting agents, as well as physiological processes such as sneezing are common examples in which the atomized liquid contains synthetic or biological macromolecules that result in viscoelastic fluid characteristics. Here, we investigate the ligament-mediated fragmentation dynamics of viscoelastic fluids in three different canonical flows. The size distributions measured in each viscoelastic fragmentation process show a systematic broadening from the Newtonian solvent. In each case, the droplet sizes are well described by Gamma distributions which correspond to a fragmentation-coalescence scenario. We use a prototypical axial step strain experiment together with high-speed video imaging to show that this broadening results from the pronounced change in the corrugated shape of viscoelastic ligaments as they separate from the liquid core. These corrugations saturate in amplitude and the measured distributions for viscoelastic liquids in each process are given by a universal probability density function, corresponding to a Gamma distribution with n_{min}=4. The breadth of this size distribution for viscoelastic filaments is shown to be constrained by a geometrical limit which can not be exceeded in ligament-mediated fragmentation phenomena.DuPont MIT Allianc

Computing the linear viscoelastic properties of soft gels using an Optimally Windowed Chirp protocol

We use molecular dynamics simulations to investigate the linear viscoelastic response of a model three-dimensional particulate gel. The numerical simulations are combined with a novel test protocol (the optimally windowed chirp or OWCh), in which a continuous exponentially varying frequency sweep windowed by a tapered cosine function is applied. The mechanical response of the gel is then analyzed in the Fourier domain. We show that (i) OWCh leads to an accurate computation of the full frequency spectrum at a rate significantly faster than with the traditional discrete frequency sweeps, and with a reasonably high signal-to-noise ratio, and (ii) the bulk viscoelastic response of the microscopic model can be described in terms of a simple mesoscopic constitutive model. The simulated gel response is in fact well described by a mechanical model corresponding to a fractional Kelvin-Voigt model with a single Scott-Blair (or springpot) element and a spring in parallel. By varying the viscous damping and the particle mass used in the microscopic simulations over a wide range of values, we demonstrate the existence of a single master curve for the frequency dependence of the viscoelastic response of the gel that is fully predicted by the constitutive model. By developing a fast and robust protocol for evaluating the linear viscoelastic spectrum of these soft solids, we open the path toward novel multiscale insight into the rheological response for such complex materials

Time-Resolved Mechanical Spectroscopy of Soft Materials via Optimally Windowed Chirps

The ability to measure the bulk dynamic behavior of soft materials with combined time and frequency resolution is instrumental for improving our fundamental understanding of connections between the microstructural dynamics and the macroscopic mechanical response. Current state-of-the-art techniques are often limited by a compromise between resolution in the time and frequency domains, mainly due to the use of elementary input signals that have not been designed for fast time-evolving systems such as materials undergoing gelation, curing, or self-healing. In this work, we develop an optimized and robust excitation signal for time-resolved mechanical spectroscopy through the introduction of joint frequency- and amplitude-modulated exponential chirps. Inspired by the biosonar signals of bats and dolphins, we optimize the signal profile to maximize the signal-to-noise ratio while minimizing spectral leakage with a carefully designed modulation of the envelope of the chirp, obtained using a cosine-tapered window function. A combined experimental and numerical investigation reveals that there exists an optimal range of window profiles (around 10% of the total signal length) that minimizes the error with respect to standard single-frequency sweep techniques. The minimum error is set by the noise floor of the instrument, suggesting that the accuracy of an optimally windowed-chirp (OWCh) sequence is directly comparable to that achievable with a standard frequency sweep, while the acquisition time can be reduced by up to 2 orders of magnitude, for comparable spectral content. Finally, we demonstrate the ability of this optimized signal to provide time- and frequency-resolved rheometric data by studying the fast gelation process of an acid-induced protein gel using repeated OWCh pulse sequences. The use of optimally windowed chirps enables a robust time-resolved rheological characterization of a wide range of soft materials undergoing rapid mutation and has the potential to become an invaluable rheometric tool for researchers across different disciplines

Newtonian and elastic liquid jet interaction with a moving surface

In the railroad industry a friction modifying agent may be applied to the rail or to the wheel in the form of a liquid jet. In this mode of application the interaction between the high speed liquid jet and a fast moving surface is important. Seven different Newtonian liquids with widely varying shear viscosities along with twelve different solutions of polyethylenoxide (PEO) and water with varying relaxation times were tested to isolate the effect of viscosity and elasticity from other fluid properties. Tests for the Newtonian liquids were done with five surfaces having different roughness heights to investigate the effects of surface roughness. High speed video imaging was employed to scrutinize the interaction between the impacting jet and the moving surface. For both Newtonian and Elastic liquids and all surfaces, decreasing the Reynolds number reduced the incidence of splash and consequently enhanced the transfer efficiency. At the elevated Weber numbers of the testing, the Weber number had a much smaller impact on splash than did the Reynolds number. The ratio of the surface velocity to the jet velocity has only a small effect on the splash, whereas increasing the roughness-height-to-jet-diameter ratio substantially decreased the splash threshold. Moreover, the Deborah number was also salient to the splash of elastic liquids.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat

Nonlinear dynamics of complex fluids in fragmentation and fracture

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017.Cataloged from PDF version of thesis.Includes bibliographical references (pages 309-337).The fragmentation and breakup of complex fluids are fundamental elements of many industrial and biological processes. The fracture of food gels, atomization of paints, combustion of fuels containing anti-misting agents and application of pharmaceutical and agricultural sprays, as well as involuntary physiological processes such as sneezing, are common examples in which the atomized/fractured material contains synthetic or biological macromolecules that result in viscoelastic fluid characteristics. For many of these processes the effects of varying the rheological properties on the dynamics of fragmentation or fracture are still poorly understood. In this thesis, we investigate some of the underlying complexities associated with varying the rheology of such materials in both shear and elongation. The complex nonlinear rheology of these complex fluids under representative conditions of large strain and deformation rate is difficult to quantify experimentally and is a known challenge for existing constitutive models. The contribution of this thesis is therefore to develop and exploit several new experimental tools that enable precise rheological measurements under appropriate test conditions. A better experimental understanding of the dynamics of fragmentation/fracture in complex fluids will also help guide the development of new theoretical models that can quantitatively predict the mechanical response of complex fluids in such flows. Two distinct classes of model fluids/gels are studied in this thesis. First, a series of model viscoelastic solutions composed of a flexible homopolymer, poly(ethylene oxide) or PEO, dissolved in a water/glycerol mixture. These dilute solutions are known to behave very similarly to their Newtonian solvent in shearing deformations but exhibit markedly different extensional rheological properties due to the onset of a coil-stretch transition in the solvated microstructure at high elongation rates. Secondly we also consider a family of biopolymer networks: acid-induced casein gels. These canonical protein gels display a multiscale microstructure that is responsible for their gel-like viscoelastic properties. Upon external deformation, these soft viscoelastic solids exhibit a generic power-law rheological response followed by pronounced stress- or strain-stiffening prior to irreversible damage and failure, most often through macroscopic fractures. We study the dynamics of fragmentation for the dilute PEO solutions in different canonical flows: air-assisted atomization, drop impact on a small target, jet impact atomization and rotary spraying. We also study the fracture of the casein protein gels under conditions of both constant applied stress and constant applied shear rate. Through quantitative study of these high strain and high deformation rate phenomena, we reach several conclusions about how the rheological properties of these materials can affect their mechanical behavior in fragmentation/fracture. First, for dilute viscoelastic solutions, the breakup and atomization of these fluids is markedly different than the analogous processes in a simple Newtonian fluid. The average droplet diameter shows a monotonic increase with added viscoelasticity, which is precisely monitored by accurate measurements of elongational relaxation times through a novel characterization method we have developed; Rayleigh Ohnesorge Jet Elongational Rheometry (ROJER). Based on our measurements of the material relaxation time scale a new theoretical model for the evolution in the average droplet diameter is developed for viscoelastic sprays. Second, the size distributions measured in each viscoelastic fragmentation process show a systematic broadening from the Newtonian solvent. In each case the droplet sizes are well described by Gamma distributions that correspond to an underlying fragmentation/coalescence scenario. We show that this broadening results from the pronounced change in the corrugated shape of viscoelastic ligaments as they separate from the liquid core. These corrugations saturate in amplitude and the measured distributions for viscoelastic liquids in each process are given by a universal probability density function, corresponding to a Gamma distribution with nmin = 4. The breadth of this size distribution for viscoelastic filaments is shown to be constrained by a geometrical limit, which can not be exceeded in ligament-mediated fragmentation phenomena. Third, in the fracture of the model acid-induced protein gels, we show that the fractal network of the underlying microstructure leads to a very broad power-law behavior in their linear viscoelastic response that can be precisely modeled by a simple model based on fractional calculus. We show that specific geometric properties of the microstructure set the value of the parameters that are used in the fractional model. The nonlinear viscoelastic properties of the gel can be described in terms of a 'damping function' that enables quantitative prediction of the gel mechanical response up to the onset of macroscopic failure. Using a nonlinear integral constitutive equation - built upon the experimentally-measured damping function in conjunction with power-law linear viscoelastic response - we derive the form of the stress growth in the gel following the start up of steady shear. We also couple the shear stress response with Bailey's durability criteria for brittle solids in order to predict the critical values of the stress and strain for failure of the gel, and show how they scale with the applied shear rate. This provides a generalized failure criterion for biopolymer gels across a range of different deformation histories. Results from this work are of relevance to many processes that involve breakup and rupture of complex fluids such as failure of viscoelastic gels, emulsification, spray painting and even biological processes such as pathogen transfer resulting from violent expiration. By investigating the linear and nonlinear behavior of two distinct classes of soft matter that lie on two ends of the viscoelasticity spectrum, one close to Newtonian liquids and one close to elastic solids, we provide key physical insights that can be generalized to broad classes of different complex fluids that undergo fracture and fragmentation processes.by Bavand Keshavarz.Ph. D

Micro-scale extensional rheometry using hyperbolic converging/diverging channels and jet breakup

Understanding the elongational rheology of dilute polymer solutions plays an important role in many biological and industrial applications ranging from microfluidic lab-on-a-chip diagnostics to phenomena such as fuel atomization and combustion. Making quantitative measurements of the extensional viscosity for dilute viscoelastic fluids is a long-standing challenge and it motivates developments in microfluidic fabrication techniques and high speed/strobe imaging of millifluidic capillary phenomena in order to develop new classes of instruments. In this paper, we study the elongational rheology of a family of dilute polymeric solutions in two devices: first, steady pressure-driven flow through a hyperbolic microfluidic contraction/expansion and, second, the capillary driven breakup of a thin filament formed from a small diameter jet (D[subscript j] ~ O(100 μm). The small length scale of the device allows very large deformation rates to be achieved. Our results show that in certain limits of low viscosity and elasticity, jet breakup studies offer significant advantages over the hyperbolic channel measurements despite the more complex implementation. Using our results, together with scaling estimates of the competing viscous, elastic, inertial and capillary timescales that control the dynamics, we construct a dimensionless map or nomogram summarizing the operating space for each instrument. Published by AIP Publishing.Axalta Coating System