5,675 research outputs found
A multi-scale model for coupling strands with shear-dependent liquid
We propose a framework for simulating the complex dynamics of strands interacting with compressible, shear-dependent liquids, such as oil paint, mud, cream, melted chocolate, and pasta sauce. Our framework contains three main components: the strands modeled as discrete rods, the bulk liquid represented as a continuum (material point method), and a reduced-dimensional flow of liquid on the surface of the strands with detailed elastoviscoplastic behavior. These three components are tightly coupled together. To enable discrete strands interacting with continuum-based liquid, we develop models that account for the volume change of the liquid as it passes through strands and the momentum exchange between the strands and the liquid. We also develop an extended constraint-based collision handling method that supports cohesion between strands. Furthermore, we present a principled method to preserve the total momentum of a strand and its surface flow, as well as an analytic plastic flow approach for Herschel-Bulkley fluid that enables stable semi-implicit integration at larger time steps. We explore a series of challenging scenarios, involving splashing, shaking, and agitating the liquid which causes the strands to stick together and become entangled.This work was supported in part by the National Science Foundation under Grant Nos.: 1717178, 1319483, CAREER-1453101, the Natu- ral Sciences and Engineering Research Council of Canada under Grant No. RGPIN-04360-2014, SoftBank Group, Pixar, Adobe, and SideFX
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Multi-Scale Models to Simulate Interactions between Liquid and Thin Structures
In this dissertation, we introduce a framework for simulating the dynamics between liquid and thin structures, including the effects of buoyancy, drag, capillary cohesion, dripping, and diffusion. After introducing related works, Part I begins with a discussion on the interactions between Newtonian fluid and fabrics. In this discussion, we treat both the fluid and the fabrics as continuum media; thus, the physical model is built from mixture theory. In Part II, we discuss the interactions between Newtonian fluid and hairs. To have more detailed dynamics, we no longer treat the hairs as continuum media. Instead, we treat them as discrete Kirchhoff rods. To deal with the thin layer of liquid that clings to the hairs, we augment each hair strand with a height field representation, through which we introduce a new reduced-dimensional flow model to solve the motion of liquid along the longitudinal direction of each hair. In addition, we develop a faithful model for the hairs' cohesion induced by surface tension, where a penalty force is applied to simulate the collision and cohesion between hairs. To enable the discrete strands interact with continuum-based, shear-dependent liquid, in Part III, we develop models that account for the volume change of the liquid as it passes through strands and the momentum exchange between the strands and the liquid. Accordingly, we extend the reduced-dimensional flow model to simulate liquid with elastoviscoplastic behavior. Furthermore, we use a constraint-based model to replace the penalty-force model to handle contact, which enables an accurate simulation of the frictional and adhesive effects between wet strands. We also present a principled method to preserve the total momentum of a strand and its surface flow, as well as an analytic plastic flow approach for Herschel-Bulkley fluid that enables stable semi-implicit integration at larger time steps.
We demonstrate a wide range of effects, including the challenging animation scenarios involving splashing, wringing, and colliding of wet clothes, as well as flipping of hair, animals shaking, spinning roller brushes from car washes being dunked in water, and intricate hair coalescence effects. For complex liquids, we explore a series of challenging scenarios, including strands interacting with oil paint, mud, cream, melted chocolate, and pasta sauce
Structural Properties of the Sliding Columnar Phase in Layered Liquid Crystalline Systems
Under appropriate conditions, mixtures of cationic and neutral lipids and DNA
in water condense into complexes in which DNA strands form local 2D smectic
lattices intercalated between lipid bilayer membranes in a lamellar stack.
These lamellar DNA-cationic-lipid complexes can in principle exhibit a variety
of equilibrium phases, including a columnar phase in which parallel DNA strands
from a 2D lattice, a nematic lamellar phase in which DNA strands align along a
common direction but exhibit no long-range positional order, and a possible new
intermediate phase, the sliding columnar (SC) phase, characterized by a
vanishing shear modulus for relative displacement of DNA lattices but a
nonvanishing modulus for compressing these lattices. We develop a model capable
of describing all phases and transitions among them and use it to calculate
structural properties of the sliding columnar phase. We calculate displacement
and density correlation functions and x-ray scattering intensities in this
phase and show, in particular, that density correlations within a layer have an
unusual dependence on separation r. We
investigate the stability of the SC phase with respect to shear couplings
leading to the columnar phase and dislocation unbinding leading to the lamellar
nematic phase. For models with interactions only between nearest neighbor
planes, we conclude that the SC phase is not thermodynamically stable.
Correlation functions in the nematic lamellar phase, however, exhibit SC
behavior over a range of length scalesComment: 28 pages, 4 figure
Active colloids in complex fluids
We review recent work on active colloids or swimmers, such as self-propelled
microorganisms, phoretic colloidal particles, and artificial micro-robotic
systems, moving in fluid-like environments. These environments can be
water-like and Newtonian but can frequently contain macromolecules, flexible
polymers, soft cells, or hard particles, which impart complex, nonlinear
rheological features to the fluid. While significant progress has been made on
understanding how active colloids move and interact in Newtonian fluids, little
is known on how active colloids behave in complex and non-Newtonian fluids. An
emerging literature is starting to show how fluid rheology can dramatically
change the gaits and speeds of individual swimmers. Simultaneously, a moving
swimmer induces time dependent, three dimensional fluid flows, that can modify
the medium (fluid) rheological properties. This two-way, non-linear coupling at
microscopic scales has profound implications at meso- and macro-scales: steady
state suspension properties, emergent collective behavior, and transport of
passive tracer particles. Recent exciting theoretical results and current
debate on quantifying these complex active fluids highlight the need for
conceptually simple experiments to guide our understanding.Comment: 6 figure
Slow dynamics, aging, and glassy rheology in soft and living matter
We explore the origins of slow dynamics, aging and glassy rheology in soft
and living matter. Non-diffusive slow dynamics and aging in materials
characterised by crowding of the constituents can be explained in terms of
structural rearrangement or remodelling events that occur within the jammed
state. In this context, we introduce the jamming phase diagram proposed by Liu
and Nagel to understand the ergodic-nonergodic transition in these systems, and
discuss recent theoretical attempts to explain the unusual,
faster-than-exponential dynamical structure factors observed in jammed soft
materials. We next focus on the anomalous rheology (flow and deformation
behaviour) ubiquitous in soft matter characterised by metastability and
structural disorder, and refer to the Soft Glassy Rheology (SGR) model that
quantifies the mechanical response of these systems and predicts aging under
suitable conditions. As part of a survey of experimental work related to these
issues, we present x-ray photon correlation spectroscopy (XPCS) results of the
aging of laponite clay suspensions following rejuvenation. We conclude by
exploring the scientific literature for recent theoretical advances in the
understanding of these models and for experimental investigations aimed at
testing their predictions.Comment: 22 pages, 5 postscript figures; invited review aricle, to appear in
special issue on soft matter in Solid State Communication
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