Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session Z03: Drops: Complex Fluids (12:15pm - 1:00pm CST)Interactive On Demand
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Z03.00001: Wetting, dewetting and multiplicity of solutions for nematic liquid crystal ridges Joseph R. L. Cousins, Lindsey T. Corson, Brian R. Duffy, Stephen K. Wilson, Nigel J. Mottram Situations involving droplets of nematic liquid crystal (nematic) on a solid substrate are of great interest, both scientifically and technologically. In such situations there is a solid--nematic interface between the solid substrate and the nematic, a nematic--isotropic interface (i.e.\ a free surface) between the nematic and the surrounding atmosphere, and a three-phase contact line. The static continuum description of these interfaces was first obtained by Jenkins and Barratt (1974), who derived equations for the balance of couple and balance of stress on the free surface and the force at the contact line in terms of the average nematic molecular orientation and the free surface height. Using a complementary approach we minimise the energy functional of a static two-dimensional ridge of nematic to derive the full governing equations. We then specialise these equations to a thin ridge of nematic and construct parameter planes in terms of the nematic material parameters, which describe the wetting and dewetting behaviour as well as the possibility of multiple solutions for the free surface height. Our findings allow us to speculate on similar important transitions for droplets of nematic and provide a framework for future investigations. [Preview Abstract] |
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Z03.00002: Thinning of Active and Passive Cylindrical Interfaces Dominated by Surface Forces Alejandro Mart\'inez-Calvo, Alejandro Sevilla Cylindrical interfaces occur in countless natural phenomena and engineering contexts: elongated vesicles are ubiquitous in biological environments, and liquid jets are routinely used for additive manufacturing applications. The most noteworthy aspect of liquid cylinders is the Plateau-Rayleigh instability induced by the interfacial tension. At sufficiently small scales the surface energy associated with the interfacial structure dominate over the volumetric one, leading to phenomena like pearling on vesicles or droplet formation of surfactant-laden threads. Here, by means of theory and numerical simulations we report the existence of an asymptotic regime where interfacial tension balances surface viscous stresses, leading to an exponential thinning of the interface. The potential use of this phenomenon to measure the surface viscosity coefficients will be discussed. Moreover, we will also consider biological and active systems, where other surface and volumetric forces can enter in the dominant balance. We will discuss the effect of surface elastic forces and the role of active stresses that occur beneath the interface in some living systems, namely in the cytokinesis of cells. The outcome of this study may have potential impact in morphogenic processes and developmental studies. [Preview Abstract] |
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Z03.00003: Suppression of Satellite Droplet Formation by Very Dilute Viscoelastic Solutions Uddalok Sen, Tim Segers, Herman Wijshoff, Michel Versluis, Detlef Lohse The presence of satellite droplets during inkjet printing is extremely undesirable since it degrades the quality and reproducibility of the print. Existing strategies for the suppression of satellite droplet formation involve increasing the viscosity of the ink, thus increasing the stability of the jetted filament and suppressing satellite droplet formation. However, such a mitigation strategy is usually at the cost of the jetting velocity. In the present work, we demonstrate that very dilute viscoelastic solutions can suppress satellite droplet formation, without any appreciable loss of droplet velocity as compared to the case with pure water (where satellite droplet formation is observed). Furthermore, we show that, for a given driving condition, there exist upper and lower bounds of polymer concentrations within which satellite droplets are suppressed. Satellite droplets are formed at concentrations below the lower bound, while droplet formation ceases for concentrations above the upper bound. Finally, we attempt to present scaling arguments that shed light on the underlying interplay between inertia, capillarity, and viscoelasticity, which leads to the suppression of satellite droplet formation. [Preview Abstract] |
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Z03.00004: Spreading of hygroscopic ionic solution droplets during vapor absorption Zhenying Wang, George Karapetsas, Prashant Valluri, Khellil Sefiane, Yasuyuki Takata Studies on the evaporation of multi-component droplets have revealed complex and important physical mechanisms. With the addition of hygroscopic salts, the adhesive property of the droplet can be tuned, and the direction of water vapor mass flux inverses. This study focuses on the dynamics of hygroscopic aqueous solution droplets, and analyzes the interacting physical processes, including the capillary effect by spatiotemporally varying droplet geometry, the thermal Marangoni effect by non-uniform absorptive heating, and the solutal Marangoni effect by preferential vapor uptake. Specifically, a lubrication-type model is established, and an expression for the absorptive mass flux is derived combining the balance of chemical potential across the solution-air interface and the Hertz-Knudsen equation. Depending on the droplet state and the ambient condition, evaporation or vapor absorption issues. The evaporative/absorptive mass flux varies both spatially and temporally as the droplet approaches equilibrium with the ambient. It is demonstrated that the dominating mechanisms, $i.e. $capillary, thermal Marangoni and solutal Marangoni, compete with each other, and may lead to diverse droplet dynamics at different stages of evaporation or vapor absorption. The findings shed light on the physical processes within droplets with both positive and negative interfacial phase change, and provides rational explanations to our experimental observations. [Preview Abstract] |
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Z03.00005: Numerical study of non-linear dynamics of liquid lenses spreading over a viscoplastic liquid layer Christos Dritselis, George Karapetsas The present study is focused on the investigation of the dynamics of a droplet spreading over a viscoplastic fluid substrate. The Herschel-Bulkley model is utilised in order to account for the viscoplastic behaviour of the liquid subphase. A computational model is developed based on finite volume method while employing the volume of fluid method (VOF), which uses a volumetric phase fraction for each fluid phase and a continuum surface force model, to account for the multiphase nature of the system. The time and space discretization is performed by using second order accurate schemes. The present computational model is validated through comparisons with existing analytical and numerical solutions in the case of Newtonian liquids. The results of a thorough parametric study are presented and discussed, which include the impact of the rheological characteristics of the subphase, the layer depth and the effect of density and surface tension ratios between the three phases. [Preview Abstract] |
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Z03.00006: Mass transport in a drying drop of a charged colloidal dispersion: new insights using Mach-Zehnder interferometry Benjamin Sobac, Sam Dehaeck, Anne Bouchaudy, Jean-Baptiste Salmon In the present work, we use Mach-Zehnder interferometry to thoroughly investigate the drying dynamics of a 2D confined drop of a charged colloidal dispersion. This technique makes it possible to measure the colloid concentration field during the drying of the drop at a high accuracy (about 0.5.{\%}) and with a high temporal and spatial resolution (about 1 frame/s and 5 $\mu $m/pixel). These features allow us to probe mass transport of the charged dispersion in this out-of-equilibrium situation. In particular, our experiments provide the evidence that mass transport within the drop can be described by a purely diffusive process for some range of parameters for which the buoyancy-driven convection is negligible. We are then able to extract from these experiments the collective diffusion coefficient of the dispersion $D(\varphi )$ over a wide concentration range $\varphi =$ 0.24.$-$0.5, i.e. from the liquid dispersed state to the solid glass regime, with a high accuracy. The measured values of $D(\varphi )\simeq $5.$-$12$D_{\mathrm{0}}$ are significantly larger than the simple estimate $D_{\mathrm{0}}$ given by the Stokes-Einstein relation, thus highlighting the important role played by the colloidal interactions in such dispersions. [Preview Abstract] |
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Z03.00007: Effects of Dilution on Multi-component Drying Droplets Anusuya Pal, Germano Iannacchione Bio-colloidal systems exhibit a variety of structural and functional complexity due to their ability to interact between different components. Understanding the multi-component self-assembling system, such as blood, is crucial for designing self-assembled structures with a multidisciplinary impact. This paper presents an experimental investigation that explores the whole human blood by a more in-depth understanding of the drying process involved under different dilution conditions with de-ionized water. Our results show that textural image parameters could characterize the drying stages. The parameters are acquired from the distribution of the pixel values, and their inter-spatial arrangements reveal the complexity, heterogeneity, and smoothness of the drying droplets. Consequently, a decrease in the initial blood concentration results in the disappearance of a late time feature in the contact angle dynamics. This transition is also evident in the dried morphology, where the crack width and spacing exhibit a change in their slope values. A combination of optical and scanning electron microscopy confirms how the dilution changes the initial native state by reducing the interaction between the constituent particles and minimizing their biological activities. [Preview Abstract] |
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Z03.00008: Microstructure does not matter (sort of): dimensionless groups for viscoplastic drop impact on thin films Samya Sen, Anthony G. Morales, Randy H. Ewoldt We quantitatively test dimensionless groups for predicting the onset of splash behavior in drop impact of yield-stress fluids on thin films. Results are relevant to fire suppression, painting, and spray coating with viscoplastic fluids. A single dimensionless quantity, IF(D/t), a non-trivial extension of the generalized Reynolds number, collapses high-dimensional data and separates the maps into different impact regimes. Even for fluids with different microstructure, we observe a constant critical value of IF(D/t) for a stick-splash transition, which is only a factor of two larger for ``glassy'' Carbopol microgel suspensions compared to ``gel'' Laponite suspensions. We discuss possible reasons for the mismatch in the critical value and raise a more general issue that dimensionless quantities can be sensitive to the range of available rheology data. We also test thixotropically ``aged'' Laponite, to further test the results. The results demonstrate that microstructure and chemistry do not matter (to leading order) if the right macroscopic rheological properties are selected for the dimensionless quantity. Reference: Sen et al., ``Viscoplastic drop impact on thin films,'' \textit{J. Fluid. Mech.} (2020). https://doi.org/10.1017/jfm.2020.147 [Preview Abstract] |
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Z03.00009: Axisymmetric numerical simulations of viscoelastic jet thinning and breakup Konstantinos Zinelis, Thomas Y. Abadie, Gareth H. McKinley, Omar K. Matar Spray formation in a non-Newtonian fluid is central to numerous industrial applications such as spray-drying and atomisation involving large interfacial deformations and complex dynamics. The aim of the present work is to establish a robust numerical basis for systematic examination of viscoelastic spray systems. To achieve this, we begin with two-dimensional axisymmetric numerical simulations of an impulsively-started jet entering into a stagnant gaseous phase using the volume-of-fluid technique to capture the interface and the log-conformation transformation for stable and accurate solution of the viscoelastic constitutive equation. This permits efficient exploration of material parameter space, capturing the competing effects of the elastic, viscous, and inertial forces on the ejected droplet size. First, we validate the numerical simulations against the predictions of linear stability analysis. Subsequently, the effect of shearing flow inside nozzle on the polymeric stress distribution along the jet is highlighted. Finally, we focus on the elasto-capillary regime, and systematically investigate the dependence of the rate of filament thinning on the initial flow rate, the fluid relaxation time, as well as the finite extensibility of the dissolved polymer. [Preview Abstract] |
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