Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session Q24: Drops: Complex Fluids |
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Chair: Jimmy Feng, UBC Vancouver Room: 606 |
Tuesday, November 26, 2019 7:45AM - 7:58AM |
Q24.00001: Numerical Simulation of Interfacial Flows of a Hydrogel Lei Li, Pengtao Yue, Yuan-nan Young, James Feng Hydrogels are crosslinked polymer networks swollen with an aqueous solvent, and play central roles in biomicrofluidic devices. In such applications, the gel is often in contact with a flowing fluid, thus setting up a fluid-hydrogel two-phase system. Using a recently proposed model (Y.-N. Young et al, Phys. Rev. Fluids 4, 063601, 2019), we treat the hydrogel as a poroelastic material consisting of a neo-Hookean polymer network and a Newtonian viscous solvent, and numerically study the motion and deformation of gel drops suspended in viscous flows. The gel-fluid interface is tracked by using the Arbitrary Lagrangian-Eulerian method that maps the interface to a reference configuration. The interfacial deformation is coupled with the fluid and elasticity governing equations into a monolithic solution algorithm using the finite-element library deal.II. Our numerical simulation of a hydrogel drop in sedimentation and shear flow shows that it deforms in ways that differ from that of a viscous drop or elastic particle, and the solvent perfusion can have a significant effect on the hydrogel dynamics. [Preview Abstract] |
Tuesday, November 26, 2019 7:58AM - 8:11AM |
Q24.00002: Stability of chiral colloidal droplets Leroy Jia, Ephraim Bililign, Michael Shelley, William Irvine We report experiments on a two-dimensional cohesive chiral fluid consisting of millions of spinning colloidal magnets suspended in water. Droplets made of this fluid are observed to swirl around and collapse into a single central superdroplet in a process reminiscent of the accretion of matter during the formation of a black hole. We put forth a minimal but complete hydrodynamic description of this active chiral fluid and use it to analyze the stability of the droplet states. [Preview Abstract] |
Tuesday, November 26, 2019 8:11AM - 8:24AM |
Q24.00003: Central spot shaped deposition from colloidal droplet evaporation under enhanced Marangoni effect Fei Duan, Junheng Ren Evaporation of a colloidal droplet on a substrate can result in a residual deposit near the three-phase line, driven the capillary flow, and form coffee ring. Under a higher evaporation rate, the more nonuniform temperature can be generated at liquid-gas interface of the sessile droplet. The evaporation induced Marangoni flow can be observed, the particles can be carried to move toward the centerline and aggregate at the center area of the droplet. We have developed a three-dimensional (3D) diffusion limited aggregation Monte Carlo method to simulate the dried patterns under the enhanced Marangoni effect. The particle motion is controlled by calculating the probabilities of six moving directions. The interactions among particle to particle, particle to liquid, particle to substrate have been taken into account. The results show that the final dried residuals can be formed to with a central spot inside a thin coffee ring by enhancing the Marangoni flow during drying a colloidal droplet. The results are comparable to the deposited patterns under the reduced pressure evaporation conditions. The drying patterns have been analyzed as well. [Preview Abstract] |
Tuesday, November 26, 2019 8:24AM - 8:37AM |
Q24.00004: The role of surfactant in evaporation and deposition of bi-solvent biopolymer droplets Dong-Ook Kim, Arif Rokoni, Chunxiao Cui, Li-Hsin Han, Ying Sun The quality of bioprinting is determined by the solvent evaporation and deposition processes of biopolymer droplets, during which instantaneous viscosity and surface tension changes occur. Such dynamics is complex and not well understood. Using high-speed interferometry and particle image velocimetry, we directly observe in real time the instantaneous drop shape and micro flows inside inkjet-printed evaporating gelatin drops containing glycerol and water. It is observed that, for bi-solvent gelatin drops with surfactants, highly viscous gelatin and glycerol accumulated near the pinned contact line at an early stage suppress the evaporation-driven outward flow and create a stagnation zone near the contact line. Lower surface tension at the contact line as compared to the drop apex induces a strong Marangoni recirculation, which in conjunction with a stagnation zone in the contact line region, causing the drop shape to transition from a spherical cap to a volcano shape during evaporation. In contrast, the suppressed evaporation outward flow together with a weak Marangoni flow leads to a dome-like deposition for the case without surfactant. The role of surfactant in polymer drop deposition with water-only solvent is also investigated and compared against that of bi-solvent drops. [Preview Abstract] |
Tuesday, November 26, 2019 8:37AM - 8:50AM |
Q24.00005: Evaporation-driven Fracture of Colloidal Drops Undergoing a Sol-gel Transition Arandeep Uppal, Matthew Hennessy, Richard Craster, Omar Matar Evaporation of liquid from a colloidal suspension can lead to a sol-gel transition whereby the mixture is transformed into a soft, gel-like material consisting of closely-packed particles with liquid-filled voids. If a drop of colloidal fluid is placed on a solid substrate, then non-uniformities in the evaporation rate lead to a gelation front that propagates from the contact line towards the center of the drop. Afterwards, uniformly spaced, radially aligned cracks begin to form near the contact line which themselves propagate into the bulk. By varying the composition of the drop, a myriad of striking fracture patterns can be observed. To describe this evaporation-driven fracture process, we have developed a fully-coupled poroelastic-damage model. The model is systematically reduced by exploiting the small contact angle of the drop and the resulting equations are solved using the finite element method. The model confirms that fracture is driven by the generation of tensile stresses at the contact line caused by the adhesion of the solid matrix to the substrate and volumetric contraction due to fluid loss. Our simulations are able reproduce the experimentally observed fracture patterns and reveal that slip plays a key role in selecting the resulting morphology. [Preview Abstract] |
Tuesday, November 26, 2019 8:50AM - 9:03AM |
Q24.00006: To jam or not to jam? Srishti Arora, Michelle Driscoll The extraordinary hydrodynamic phenomenon of drop impact has inspired numerous studies exploring a variety of Newtonian and complex fluids. Although this is a century-old problem, there are only a few studies on the impact dynamics of colloidal suspension drops. The impact of a suspension drop provides a unique model system to probe the complicated interplay between hydrodynamic instability and the non-trivial rheology of complex fluids. Here, we present a comprehensive study of colloidal suspension drop impact to explore the rich flow behaviors in conditions that are inaccessible via conventional rheometry. We study the dynamics of suspension drops by impacting a millimetric size droplet on a solid glass surface while varying the volume fraction and impact conditions. We find that the extent of spreading decreases with increasing volume fraction or decreasing impact velocity. Moreover, we present a state diagram that delineates the inertial spreading regime from thickening regime and the jammed solid regime. Furthermore, we observe a variety of elastic behaviors which appears in the jammed regime and are controlled by adjusting volume fraction and impact velocity. [Preview Abstract] |
(Author Not Attending)
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Q24.00007: Onset of rebound suppression in non--Newtonian droplets post--impact on superhydrophobic surfaces Devranjan Samanta, Soumya Ranjan Mishra, Purbarun Dhar Droplet deposition after impact on superhydrophobic surfaces has been an important area of study in recent years for its potential application in reduction of pesticides usage. Minute amounts of long chain polymers added to water has been known to arrest the droplet rebound effect on superhydrophobic surfaces. Previous studies have attributed different reasons like extensional viscosity, dominance of elastic stresses or slowing down of contact line in retraction phase due to stretching of polymer chains. The present study attempts to unravel the existence of critical criteria of polymer concentration and impact velocity on the inhibition of droplet rebound. The impact velocity will indirectly influence the shear rate during the retraction phase, and the polymer concentration dictates the relaxation timescale of the elastic fluids. Finally, we show that the governing Weissenberg number (at onset of retraction), which quantifies both the elastic effects of polymer chains and the hydrodynamics, is the critical parameter in determining the regime of onset of rebound suppression, and that there is a critical value which determines the onset of bounce arrest. The previous three causes, which are manifestations of elastic effects in non-Newtonian fluids, can be related to the proposed Weissenberg number criterion. [Preview Abstract] |
Tuesday, November 26, 2019 9:16AM - 9:29AM |
Q24.00008: Drop impact of extensible yield-stress fluids Samya Sen, Randy H Ewoldt We study the role of extensional rheology on the impact behavior of drops of elastoviscoplastic fluids on thin films, showing that a recently proposed and successful dimensionless group for viscoplastic fluids fails to predict impact regimes when elastic extensional effects are significant. We do this by creating a new formulation for an extensible yield-stress fluid in which extensional properties vary dramatically. The non-Newtonian fluid is an aqueous suspension of Carbopol microgel particles (a well-studied system) with the addition of high molecular weight poly(ethylene oxide) (PEO) at varying concentration. The fluids fit into the new paradigm of extensible yield-stress fluids [1]. Drop impacts onto substrates coated with the same material are recorded using high-speed cameras, and different impact regimes are identified as a function of droplet size, velocity, coating thickness, and droplet rheology. Whereas the previous dimensionless group only involves steady shear rheological properties (yield stress and Bingham plastic viscosity), we consider additional modification to account for the elastic extensional effects. [1] Nelson, A.Z., R.E. Bras, J. Liu, and R.H. Ewoldt, ``Extending yield-stress fluid paradigms'', \textit{J. Rheol.} \textbf{62}, 357 (2018) [Preview Abstract] |
Tuesday, November 26, 2019 9:29AM - 9:42AM |
Q24.00009: Drop impact of thixotropic yield-stress fluids Randy H Ewoldt, Samya Sen, Anthony G Morales We use high-speed imaging to study the effect of thixotropy on drop impact of thixotropic-viscoplastic fluids onto thin films. Using a dimensionless group proposed earlier for predicting impact behavior of Carbopol, a glassy aqueous suspension of soft microgel particles, we predict the behavior of an aqueous suspension of Laponite, a colloidal clay with an attractive gel microstructure. For rejuvenated samples of both materials, the dimensionless group separates various impact regimes, gives a constant critical value for stick-splash transition as a function of dimensionless coating thickness, and gives a similar critical value across the varying microstructures studied. This is remarkable considering the different chemistries and microstructures involved. The group is less effective at predicting impact behavior of thixotropically aged Laponite suspensions, demonstrating the role of thixotropy in drop impact with viscoplastic fluids. We tune this dimensionless group to include thixotropic effects by proposing rheological techniques to estimate shear properties of thixotropically aged samples. The modified group is tested with a wide range of experimental conditions and is shown to be effective in predicting the drop impact regimes of aged Laponite samples. [Preview Abstract] |
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