Session H53: Fluid Mechanics for Soft Matter III: Cells, Particles, and Drops

Revealing the mechanical nature of active embryonic tissues with magnetic droplets

The sculpting of tissues into their functional morphologies requires a tight spatiotemporal control of their mechanics. While cell-generated mechanical forces power morphogenesis, the resulting tissue movements depend on the local tissue mechanical properties, which govern the system's response to the internally generated forces. Despite their relevance, the role of mechanical forces and mechanical properties, as well as their spatiotemporal variations, in developmental processes remains largely unknown, mainly because of a lack in methodologies enabling direct in vivo and in situ measurements of cell-generated forces and mechanical properties within developing 3D tissues and organs. I will present a novel technique that employs magnetic microdroplets to quantify the local mechanical properties and stresses in soft materials, including living tissues. Using zebrafish as model system, I will show that the spatiotemporal control of tissue mechanical properties, rather than cellular forces, plays a key role in the shaping of embryonic tissues.   

Competition between Red Blood Cells aggregation and break-up: Depletion force due to filamentous viruses vs shear flow

Human blood is a shear thinning fluid with complex response that strongly depends on the Red Blood Cells (RBC) ability to form aggregates, called rouleaux.
Both, depletion and bridging have long been believed to play a role in rouleaux formation, mediated by the presence of macromolecules such as fibrinogen in the plasma. However, despite numerous investigations, microscopic understanding of the formation and break up of RBC aggregates has not been fully elucidated.
In order to distinguish the mechanisms behind RBCs aggregation we employ a depletant agent with very long ranged interaction force, the filamentous fd bacteriophage. These colloidal rod-like particles have high length-to-diameter ratio and carry the same charge as the RBCs. Fine-tuning of the depletion force is achieved through mixtures of RBCs in PBS at different concentrations of rods. We study the breakup of aggregates during shear flow to quantify the interaction between the cells, combining a home-build counter-rotating cone-plate shear cell with an ultra-fast confocal microscope enabling visualization of the structures while shearing.
We present a non-equilibrium phase diagram of shear rates versus depletant concentrations, showing regions for different flow response and stages of separation of the RBCs doublets.   

Combining Rheology and Simulations to Model Bulk Blood Flow

Understanding blood flow is an active area of research due to its potential to improve drug delivery, importance in design of medical devices, and role in health complications. However, modeling blood flow is difficult because of both the complex flow geometries and the unique rheology of blood. The rheology of blood is characterized by a shear thinning behavior with a nonzero yield stress, viscoelasticity, and thixotropy – a time dependent change in the viscosity. These features arise as a result of the coin stack microstructures called rouleaux that red blood cells (RBCs) form under stasis and low shear. In this work, we present new data on human blood bulk rheology and offer a thixotropic rheological model for transient blood flow. The model includes a structural term and a viscoelastic term relating to the RBC deformability. Additionally, flow inhomogeneities that occur within blood are discussed and a representation of these effects is proposed. Finally, the rheological model is incorporated into flow simulations of blood through circulatory system portions, and from a comparison with a Newtonian model, the significance of the rheology in blood flow simulations is identified.   

The reference map technique for simulating dense suspensions of flexible particles

Conventional computational methods often create a dilemma for fluid-structure interaction problems. Typically, solids are simulated using a Lagrangian approach with a grid that moves with the material, whereas fluids are simulated using an Eulerian approach with a fixed spatial grid, requiring some type of interfacial coupling between the two different perspectives. Here, a fully Eulerian method for simulating structures immersed in a fluid will be presented. By introducing a reference map variable to model finite-deformation constitutive relations in the structures on the same grid as the fluid, the interfacial coupling problem is highly simplified. The method is particularly well suited for simulating soft, highly-deformable materials and many-body contact problems. We demonstrate the method by using a large-scale, three-dimensional, parallel implementation of it to simulate dense suspensions of flexible particles.   

Unusual Rheology of Aqueous Dispersions of Soft Phytoglycogen Nanoparticles

Phytoglycogen is a natural polysaccharide produced in the form of compact 35 nm diameter nanoparticles by some varieties of plants such as sweet corn. The highly-branched, dendrimeric structure of phytoglycogen leads to interesting and useful properties that make the particles ideal as unique additives in personal care, nutrition and biomedical formulations. One such property is the unusual rheology of aqueous dispersions of phytoglycogen nanoparticles. When added to water, the zero-shear viscosity of the dispersions remains small up to large concentrations (~20% w/w). For higher concentrations, the zero-shear viscosity increases dramatically, reaching values that exceed that of water by more than a factor of 10⁶ at the highest concentrations of 30% w/w. These very large values of the zero-shear viscosity are coupled with very small values of the yield stress (below the detection limit of the rheometer) and significant compression of the soft nanoparticles. This unusual combination of rheological properties offers new opportunities for applications of the phytoglycogen nanoparticles.   

Orientation dynamics of stiff polymeric nanoparticles

Hydrodynamic assembly of elongated nanoparticles is of paramount importance for producing large and ordered structures with excellent functionalities. However, performing such bottom-up assembly, where diffusion processes play a fundamental role, necessitates quantification of the nanoparticle dynamics in flowing systems. In this presentation, we expose for the first time an experimental method for characterizing the orientation dynamics of nanorod suspensions in a flow also used for assembly. The parameters controlling the dynamics are investigated. The concept, based on birefringence relaxation, is demonstrated by flowing nanocelluloses (cellulose nanocrystals and nanofibrils), used as model systems, through a flow-focusing channel but it can be easily extended to different flows and to various birefringent nanoparticle suspensions.   

Dynamics of colloids under periodic reversed sedimentation.

Motivated by the exploration of novel ways of assembling materials, we investigate a periodic reversed sedimentation protocol on micrometer-sized colloidal particles under sedimentation in a Hele-Shaw cell. The experiment consists in letting colloids sediment in a cell for a certain time before reversing the sedimentation by rotating the cell 180 degrees. Using direct stroboscope analysis imaging, the resulting strobe dynamics of the colloids is found to be strongly anisotropic. The equivalent diffusion coefficient, is extracted from the strobe dynamics analysis and is found to increase with colloids’ sedimentation time. We also show how the diffusion coefficient depends on the shape of sedimentation “cycle” that is applied. These results pave the path for novel methods of mixing grains and colloids in flows at low Reynolds numbers.
  

Printing of Ultra-thin Layers of Colloidal Inks using Nanoporous Stamps for High Resolution Flexography

There is a growing industrial need for manufacturing technologies that can print devices with high resolution (< 10 microns) and at high throughput. Conventional flexography is limited in resolution due to the instabilities in the ink loading and transfer mechanisms. A recent invention from our research group, engineered nanoporous stamps composed of polymer coated carbon nanotube (CNT) forests, are highly porous (>90%) and can retain the ink within their volume rather than on their surface only and has been used to print micron-scale features with highly uniform ink layer thickness (<100 nm). We conducted experiments using a custom-built printing apparatus which enables printing on flat and curved substrates by precisely controlling the force and printing speeds while observing the printing process in the time scales of milliseconds using a high magnification high speed imaging system. Using observations, we present the mechanics of spreading during contact and the evolution of a capillary liquid bridge which after rupture transfers liquid on a precursor film formed during contact. We study the influence of stamp design, ink properties, and printing speed on the ink transfer process, and present models to eludicate how printed layer thickness can be controlled precisely.   

Drop Impact on Hairy Surfaces

Hairy textures - comprised of regularly patterned slender structures- play a key role in water-repellency and uptake for many plants and animals. Using a combination of experiments and theory, we investigate the effect of a millimeter-scale hairy texture on impact of liquid drops. By varying the speed of the drop at impact and the spacing of the hairs, we observe a variety of behaviors. For dense hairs and low impact velocity, the liquid drop sits on top of the hair, similar to a Cassie-Baxter state. For higher impact velocity, and intermediate to high density of hairs, the drops penetrate through the surface, but the hairs resist their spreading. For low hair density and high impact velocity, the drops penetrate and eject droplets upon impact.   

Jumping and bouncing of hydrogel drops on a heated plate

The contact between a liquid drop and a solid surface is ubiquitous. Significant efforts have been invested in understanding how liquid drops move along, bounce from, or roll off different types of surfaces. On super-hot surfaces, drops of liquid hover over a layer of their over vapor, due to rapid evaporation – this is known as the Leidenfrost effect. Interestingly, when the drop possesses some elasticity by exchanging it for a hydrogel, the drop no longer hovers, but it instead bounces. Using high speed video microscopy, we demonstrate that hydrogel balls, initially at rest, jump upon rapidly heating the surface. Jumping is governed by both the surface as well as the hydrogel properties. After jumping, the hydrogel can continue to bounce for extended periods of time. Our results illustrate how the interplay between solid and liquid characteristics of hydrogels can result in intriguing dynamics.   

Dynamic Friction and Lubrication in Soft Hydrogels

Hydrogel's use ranges from synthetic cartilage and contact lenses, to gel-based soft robotics. Across its applications, hydrogel interfaces experience dynamic and static loads, hence minimizing friction increases longevity. A gel's frictional behavior is strongly dependent on factors such as contact area, sliding velocity, normal force, and gel surface chemistry. Using a custom low-force tribometer, we have probed the single-contact frictional properties of spherical hydrogel particles on flat surfaces under a variety of environmental conditions. On hard surfaces, (aluminum, glass, etc.) we have examined a dynamic frictional transition at a critical sliding velocity (≈ 0.5 cm/s) which is in agreement with previous research. We have varied both the slider and surface materials, and the solvent. Interestingly, upon inverting our system to that of solid particles (steel, glass, etc.) on hydrogel surfaces, this frictional phenomena disappears. These results imply a connection between the relaxation time of the polymers and this dynamic transition. A physical model utilizing the hydrodynamic lubrication layer between the deformed surfaces and the polymer interactions will be discussed.   

Quasi-static microdroplet production in a capillary trap

We have developed a method to produce aqueous microdroplets in an oil phase, based on the periodic extraction of a pending droplet across the oil/air interface. This interface forms a capillary trap inside which a droplet can be captured and detached. This process is found to be capillary- based and quasi-static. The droplet size and emission rate are independently governed by the injected volume per cycle and the extraction frequency. We find that the minimum droplet diameter is close to the injection glass capillary diameter and that variations in surface tension moderately perturb the droplet size. A theoretical model based on surface energy minimization in the oil/water/air phases was derived and captures the experimental results. This method enables robust, versatile and tunable production of microdroplets at low production rates.