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An atom is cooled by a standing-wave light field between two high-quality mirrors. Cavity cooling avoids the usual light scatter into the surroundings. Instead, the light leaking out of the mirrors is blue-shifted to a higher frequency (image credit: Pepijn Pinkse Max Planck Institute of Quantum Optics)
Artist's impression of an atom cooling device
NECF Meeting Abstracts

78th New England Complex Fluids Meeting
Northeastern University | Friday, March 8, 2019
Registration deadline: Wednesday, March 6, 2019

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abstract list | attendees list

Abstracts for Invited Talks and Sound Bites:

Invited Talks

1. Conrad, Jacinta

University of Houston
"Tracer transport in slowly-relaxing glassy matrices"
Dynamic coupling of small penetrants to slow, cooperative relaxations within crowded cells, supercooled liquids, and glassy polymer matrices has broad consequences for applications ranging from drug delivery to nanocomposite processing. Interactions between the constituents of these and other disordered media alter the cooperative relaxations, but their effect on penetrant dynamics remains incompletely understood. We use molecular dynamics simulations, supplemented by dynamic x-ray scattering experiments, to probe the effects of matrix structure and dynamics on the transport and localization of penetrants. Using molecular dynamics simulations, we show that the motions of hard-sphere tracer particles probe differences in local structure and cooperative relaxation processes in attractive and repulsive glassy liquid matrices with equal bulk packing fractions and long-time diffusivities. Coupling of the tracer dynamics to collective relaxations in each matrix affects the shape of tracer trajectories, which are fractal within the repulsive matrix and more compact in the attractive. These results reveal that the structure of relaxations controls penetrant transport and dispersion in cooperatively relaxing systems and provide insight into dynamical heterogeneity within glassy liquids. Matrices at higher volume fractions, by contrast, do not structurally relax on long time scales, but remain out of equilibrium and evolve via a slow aging process. In these systems, tracers can instead become localized within the disordered structure of the matrix. We investigate the transport and localization of tracer probes in a glassy matrix as a function of relative size using dynamic x-ray scattering experiments and molecular dynamics simulations. The quiescent alpha-relaxations of tracer particles evolve with increasing waiting time $t_w$. The corresponding relaxation times increase exponentially at small tw and then transition to a power-law behavior at longer $t_w$. As tracer size decreases, the aging behavior weakens and the particles become less localized within the matrix. These localized dynamics are coupled to the matrix elasticity until the tracers delocalize at a critical size ratio $delta_0 = 0.38$. Localization does not vary with sample age even as the relaxations slow by approximately an order of magnitude, suggesting that matrix structure controls tracer localization when the penetrant dynamics are well separated in time from those of the matrix.

2. Doyle, Patrick

"Nanoemulsions: formation, assembly and applications"

3. Hsiao, Lilian

North Carolina State University
"Designing and understanding friction within soft materials"
Even in 2017, friction accounts for approximately 20 percent of the global energy consumption. Energy losses arise from viscous and solid dissipation as two contacting surfaces move against each other. As technologies such as wearable electronics, functional textiles, and soft robotics emerge, the need to engineer friction at the microscale becomes increasingly important. In this talk, I discuss ways in which our group studies soft matter mechanics using high speed confocal imaging and rheometry. The first part of the talk will focus on the role of particle roughness in the rheology and dynamics of concentrated colloidal suspensions. A series of experiments and simulations demonstrates that surface roughness is key to shear thickening, jamming, and changes in rotational dynamics within colloidal suspensions. Transient creep experiments show that rough particles exhibit stochastic strain jumps that are reminiscent of large-scale mudslides and avalanches. The second part of the talk will discuss the role of microtexturing on the lubrication properties of soft elastomeric substrates, and how hydrodynamics and friction may be combined to generate new scaling theories for the tribology of biomimetic interfaces.

4. Invited, Speaker; Samanvaya Srivastava

"Complexation Driven Self-Assembly of Block Polyelectrolytes"
Polyelectrolyte complexes (PEC) form when oppositely charged polyelectrolyte chains spontaneously associate and phase separate in aqueous media. Conjugating one or both the polyelectrolytes with neutral polymers restricts bulk phase separation of the PECs, and thus leads to self-assembled structures with PEC domains surrounded by neutral polymer coronae, forming micelles and hydrogels. The PEC domains in these assemblies can encapsulate hydrophilic cargo and have tremendous potential for biomedicine, chemical-sensing applications, food systems and cosmetics. This talk will focus on physical properties of model PEC hydrogel assemblies comprising oppositely charged block copolyelectrolytes. Created using a novel two-step synthesis scheme, the copolyelectrolytes self-assemble to form hydrogel structures with polyelectrolyte complex domains serving as physical crosslinks. Precise PEC domain size, morphology, spacing and ordering is achieved via tuning of the polymer architecture and loading, ideal for fundamental studies. Structural investigations employing complementary X-ray and neutron scattering elucidate the contributions of the charged and the neutral blocks in defining and directing the structure of these self-assemblies, respectively. Extending these ideas to assembly of model oppositely charged triblock copolyelectrolytes, we find that at low concentrations the materials spontaneously assemble into phase separated inter-connected networks of PEC cores, underscoring the disparity between complexation-driven assembly of triblock copolyelectrolytes and hydrophobicity driven assembly of their uncharged amphiphilic counterparts. Molecular dynamics simulations are employed to provide insights on the driving forces behind these unique assemblies and their relationships to corresponding assemblies of amphiphilic molecules.

5. Karniadakis, George E.

Brown Unviersity
"DPD Alphabet and Multiscale Modeling and Simulations of Physical and Biological Systems"
A new approach to multiscale modeling of complex systems will be presented using the dissipative particle dynamics (DPD) method, which is a coarse-grained version of molecular dynamics (MD) allowing simulations on larger domains and longer time scales than MD. Domain decomposition algorithms interfacing DPD with MD and the continuum Navier-Stokes equations will also be presented. Examples will be shown from polymer dynamics, e.g. thermos-responsive polymers for drug delivery, as well as bio-cellular mechanics, including modeling of blood diseases such as malaria and sickle cell anemia. The correct governing equations of DPD are derived based on the Mori-Zwanzig formulation including formulations with correlated structures (memory effects).

Sound Bites

1. Abdelshafy, Kareem K; Botond Tyukodi(2) ,Damien Vandembroucq (3), Craig Maloney(1)

1- Northeastern University, 2-brandeis University, 3-ESPCI Paris
" Cyclic shear in a mesoscopic model of amorphous plasticity"
We present results of a mesoscopic model of amorphous plasticity in which the material is subject to oscillatory shear. We show the existence of a threshold amplitude of the shear. Below this threshold amplitude, eventually all plasticity is exhausted and the material shows a fully elastic response under the periodic shear. The time necessary to reach this fully elastic state diverges as one approaches the critical amplitude. Above the critical amplitude we observe a hysteresis in the stress response. After several cycles the hysteresis reaches a steady state, showing that plasticity never vanishes above the threshold amplitude. Right above the critical amplitude, we observe a strong localization of plastic activity and a diffusive increase of strain and displacement fluctuations. Our results are in good agreement with several recent particle simulation observations.
Keywords: Amorphous , plasticity, cyclic, mesoscopic

2. Arroyo, Julian; Carlos H. Hidrovo

Northeastern University
"In-air microfluidics under confined 3D flow focusing microchannels for droplet generation under the jetting regime"
The purpose of this study is to showcase an alternative method for droplet generation using in-air confined flow focusing microfluidic chips. Conventional methods for droplet generation use oil as the continuous phase in a microfluidic chip to generate highly uniform droplets. Previous studies proved the possibility of using highly inertial gases as the continuous phase to generate liquid droplets. Other studies have generated liquid droplets by dispersing the liquid in an open gaseous environment using non-microfluidic formats. Several geometries are fabricated to study the influence of air, liquid and output channels width and height as well as the size of the neck before the output. These parameters are studied against each other to understand their physical meaning in the jet formation and its resulting droplets. The result of this experiment is represented in a flow regime map focused on the jetting region which allows to explain the physical requirements for the jetting to occur and how different parameters affect it. The results indicate that is possible to control the droplet size and generation frequency by adjusting the flow rates of the continuous (gas) and the dispersed (liquid) phases. We obtain droplets between 50 µm and below 10 µm at frequencies higher than 100 kHz. The system shows stability for long periods of time. The outcomes of this work are useful in many areas, namely pharmaceutical and food industries where uniform droplets can be generated purely in air and without any extra liquid contaminants. Also, the ability to create these droplets within a confined microchannel allows for additional modules for developing integrated Lab-on-a-chip platforms suitable for material synthesis and nanotechnology.
Keywords: Droplet generation, Microfluidics,

3. Balciunaite, Aiste; Maria Santore

University of Massachusetts Amherst
"The Effect of Particle Shape on Adhesion to Surfaces in Shear Flow"
The behavior of microparticles in flow and their adhesion to surfaces is a topic of key interest within the medical field, specifically due to the applications for drug delivery. Recent research suggests advantages to using non-spherical particles for targeted drug delivery rather than spherical particles, but little is known about the behavior of non-spherical particles in flow, as a result of the paucity of well-characterized uniform particle systems. This work address the near-surface flow behavior and adhesion of rod-shaped microparticles. Silica rod microparticles with varying aspect ratios but similar diameters were synthesized, and their near-surface flow behavior and adhesion was compared to silica microspheres of comparable hydrodynamic diameters. The microparticles were flowed through a laterally mounted microfluidic device, and electrostatic attractions were exploited to adhere particles to a poly-l-lysine covered glass surface. The surface capture rates of spheres and rods were studied and compared. The Leveque equation was used to model the capture rate of spherical particles as a function of shear rate and size, and extended to rod-shaped microparticles. The effect of flow on the orientation of adhered rods is also considered.

4. Clark, Andrew

Tufts University
"Dynamic fragility measurements of polyzwitterions using fast scanning calorimetry"
Glass forming systems can be characterized as strong or weak glass-formers based on their deviations from Arrhenius temperature dependence. The dynamic fragility of a system is a measure of the deviation from Arrhenius behavior, and can thus be used to classify strong vs. weak glass-formers. In this work the dynamic fragility of 3 polyzwitterionic copolymers is measured using fast scanning calorimetry (FSC). The copoylmers were comprised of sulfobetaine vinylimadzole (SBVI) and 2-methacryloyloxyethyl phosphorylcholine (MPC) in the ratios of 2:1, 1:1 and 1:2 SBVI to MPC. The copolymers showed pronounced bowing in the heat flow signature after ejection of bound water, due to bubbling of the samples. A full symmetry correction was employed to reduce the bowing, and to clearly resolve the glass transition. Using the Moynihan method the fictive temperature as a function of previous cooling rate was assessed, and fit to the Williams-Landel-Ferry model. The data fit the model well, howing super-Arrhenius temperature dependence in the Angell plot. The dynamic fragility for the samples was measured, with the 1:1 copolymer showing the largest fragility.

5. Cochard, Thomas; T. Cochard1, Y. Song, L. Xiao, D.A. Weiz

Harvard University, Weitz lab
"Working towards an energetic approach of hydraulic fracture dynamics"
Most of the research on hydraulic fracture is focusing on the pressure related evolution during the fracture propagation. In the present research, we are developing an energy balance approach to the phenomena. Indeed, by only considering the pressure evolution within the system one important information is being neglected, the total volume of the fracking fluid (water in our system). In most of the application, the compressed fluid is incompressible as the compressibility being very high, but our current research is showing that the amount of fluid which is being compressed cannot be neglected because the pressure that must be applied to the material to observe its failure is at least 350 time the atmospheric pressure.
Keywords: Hydraulic fractures, high speed imaging, water compressibility

6. Cui, Naiwen; Yamei Cai; Huidan Zhang; Haichuan Hu; David Weitz

"Ultra High-Throughput Targeted sequencing in single cells using droplet barcoding microfluidics"
Single cell targeted sequencing is a powerful tool for genetic mutation detection, which play an important role in studying genetic heterogeneity and clonal evolution in many complex illness and diseases. Studies show that clonal evolution can contribute to treatment failure, drug resistance and metastasis in cancer. For example, Intratumoural EGFR heterogeneity in Non-Small Cell Lung Cancer (NSCLC) revealed a close relationship with tumor shrinkage under chemotherapy treatment. To provide a good representation of the whole tumor, large number of cells need to be processed due to large number of tumor cells present. Current technology could only study 96 cells at a time using plate based assays or commercially available platform, which is very difficult and expansive to scale up. We have developed a droplet-based microfluidics technology that is capable of first encapsulating single cells in droplet, amplifying multiple targeted genes in each drop and then molecularly barcode them using hydrogel barcoded beads with very high efficiency. The ability to barcode tens of thousands of cells in each experiment with such low cost makes this a great tool for many clinical applications. 
Keywords: single cell; sequencing; drop-based microfluidics

7. Das, Amit; Amit Das 1 , Jennifer A Mitchel 2 , Dapeng Bi 1 , Jin-Ah Park 2

Northeastern University, Boston MA 1 ; Harvard T.H. Chan School of Public Health, Boston MA 2
"Different modes of fluidization in a dense biological tissue"
Epithelial tissues remain non-migratory and behave as a jammed system under homeostatic conditions. Using a jammed layer of human lung epithelial cells we compare a new mode of tissue fluidization, induced by mechanical compression, namely the unjamming transition (UJT) with the canonical epithelial-mesenchymal transition (EMT). Our analyses of experimental data reveal the following distinct structural and dynamical signatures: Strong cellular elongation and large fast moving nematic swirls during the UJT while retaining the epithelial characteristics. All these feature lost during the EMT when the cells become mesenchymal. To further our understanding we developed a dynamic vertex model (DVM) which differs from previous vertex models in that the cell edges can now become curved and can thus reflect the competition of the forces acting on the edge locally. These forces include cortical tension, intracellular- pressure differences, and polarized motility forces. We explore different paths of solid-to-fluid transitions based on different parameters in the model, such as individual cell motility and preferred cell shapes, and compare our predictions with the experimental observations on UJT and EMT. Based on our comparisons, we propose that the UJT could be an alternative route to fluidization of jammed epithelial tissues, independent of the EMT.
Keywords: Unjamming transition; EMT; Dynamic Vertex Model; Human lung cells

8. Deveney, Brendan; Julie Brouchon, Perry Ellis, Raoul Rosenthal, John Heyman, David Weitz

"Gelling of microfluidic droplets for high-throughput capture, sorting and analysis of cells"
Gelling of microfluidic droplets can facilitate the capture of cells and their transfer from an oil to an aqueous continuous phase, with applications in cell isolation, incubation, selection and sorting.
Keywords: high-throughput microfluidics

9. Duwe, Lukas; Michael Molinski, Arijit Bose

University of Rhode Island
"Designing Safe and Stable All-Solid-State Lithium-Ion Batteries"
Safety and cycle stability are important issues for rechargeable batteries. For this purpose, we propose an all-solid-state lithium-ion battery (ASSLIB) consisting of a polyethylene oxide (PEO)-based polymer electrolyte with a lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt, lithium titanate (LTO) anode and lithium iron phosphate (LFP) cathode. In general, two major drawbacks have to be surmounted in the ASSLIB field: first, the polymer electrolyte impedance is orders of magnitude higher when compared to liquid electrolytes due to its lower bulk ionic conductivity; second, a high interfacial resistance is present due to insufficient contact of the solid electrolyte with the electrode’s active material particles. In spite of these hindrances, cycling data is obtained for the proposed ASSLIB with a Coulombic efficiency is close to 100%. However, the observed discharge capacity is only about 20% of the theoretical, which is most likely due to limited charge transfer at the electrode-electrolyte interface. In order to further optimize the capacity, a melting procedure as well as adding additional polymer and lithium salt into the electrodes is proposed to decrease interfacial resistance.
Keywords: all-solid-state lithium-ion battery, polymer electrolyte

10. Elgailani, Ahmed; Craig E Maloney

Northeastern University
"Multi-particle finite element simulation of highly compressed microgel-packings"
Packings of hydrogel particles, like any granular material, become rigid when volumetrically confined. At low confinement near the onset of rigidity, linear elastic contact mechanics should provide a good description of the interparticle forces. At higher confinement, the particles become strongly deformed and linear elastic contact mechanics no longer provides a reasonable description of the interparticle forces. Here, we report on simulations using a multiparticle finite element technique, employing the Flory-Rehner constitutive law, to model the full non-linear elastic deformation of all particles in the packing. We show that the shear modulus (μ) of the packing depends strongly on confining pressure (p) with a crossover from a low-pressure regime where μ ~ p1/2 to a high-pressure strongly-faceted regime where μ ~ p1/4 where interstitial space has essentially vanished and the facet geometry no longer evolves with pressure.
Keywords: MPFEM, Hydrogels, Jamming

11. Ellis, Perry; Giridhar Anand, David A. Weitz, Sharad Ramanathan

Harvard University
"Identifying pathogenic bacteria by phenotyping individual cells"
While estimates give the number of bacterial species on this earth to be greater than 10^10, we have currently cataloged fewer than 10^7. Of all the species we have cataloged, only 10^3 are pathogenic. As even a single pathogenic bacterium can colonize humans and cause disease, identifying pathogenic bacteria in natural sample has important consequences for human health. We work to address this problem using high-throughput methods. Our goal is to identify even a single pathogenic bacterium in a collection of 10^6 bacteria obtained from a natural sample. Our current approach relies on droplet microfluidics: we co-encapsulate human tissue with the bacteria of interest, forming a functional assay capable of screening 10^6 bacteria per day.

12. Faust, Jessica; Gavin Winter, Marilyn Minus, Randall Erb

Northeastern University
"Creating a hierarchical interphase in polymer-ceramic composites"
Discontinuous polymer-ceramic composites are of specific interest due to their high use in bulk manufacturing techniques and promising material properties. However, polymer-ceramic composites often show poor interfacial interaction and delamination at the interface. In this work, we suggest introducing nanomaterials at the ceramic interface to improve interfacial interactions and locally crystallize the polymer matrix. The proposed structure was created by building a hierarchical nano-micro-macro structure using colloidal self-assembly and solvent-based polymer crystallization techniques. Alumina micro-platelets were used as the reinforcing particles within an isotactic polypropylene (iPP) matrix. To improve interactions with the polymer matrix, single wall carbon nanotubes were uniformly decorated on the surface of the alumina via Van der Waals forces. Polymer crystals were then grown on the surface of SWNT-alumina platelets using a solvent/non-solvent based process. The immiscibility of the two fluids allows polymer crystallization to take place at the liquid-liquid interface, avoids aggregation of the SWNT-alumina platelets, and improves homogeneous crystallization around the particles. This technique creates a hierarchically structured surface on the ceramic micro-particles and shows a promising avenue to improve the material properties of polymer-ceramic composites.
Keywords: Polypropylene, crystallization, single walled carbon nanotubes, alumina, composite, interphase

13. Filippov, Sergey; Bart Verbraeken, Peter Konarev, Dmitri Svergun,Christine M. Papadakis,Sarah Rogers, Aurel Radulescu,Timothee Courtin,José C. Martins, Larisa Starovoytova, Potemkin Potemkin, Richard Hoogenboom

Harvard University, SEAS
"Block and gradient copoly(2-oxazoline) micelles: striking different on the inside"
Herein, we provide a direct proof for differences in the micellar structure of amphiphilic diblock and gradient copolymers, thereby unambiguously demonstrating the influence of monomer distribution along the polymer chains on the micellization behavior[1]. The internal structure of amphiphilic block and gradient co poly(2-oxazolines) based on the hydrophilic poly(2-methyl-2-oxazoline) (PMeOx) and the hydrophobic poly(2-phenyl-2-oxazoline) (PPhOx) was studied in water and water-ethanol mixtures by Small-Angle X-ray Scattering (SAXS), Small-Angle Neutron Scattering (SANS), Static and Dynamic Light Scattering (SLS/DLS), and 1H NMR spectroscopy. Contrast matching small angle neutron scattering (SANS) experiments revealed that block copolymers form micelles with a uniform density profile of the core. In contrast to popular assumption, the outer part of the core of the gradient copolymer micelles has a distinctly higher density than the middle of the core. We attribute the latter finding to back-folding of chains resulting from hydrophilic-hydrophobic interactions, leading to a new type of micelles that we refer to as micelles with a “bitterball-core” structure. References [1] S.K. Filippov, J. Phys. Chem. Let. 2017, 8, 3800-3804
Keywords: gradient copolymers, micelles, SANS, SAXS

14. Garry, Ryan; Julie Brouchon, Kirk Mutafopulous, Dave Weitz

Harvard University
"High-throughput hydro-gel encapsulated cell sorting using Traveling Surface Acoustic Waves (TSAW)"
Microfluidic devices enable the encapsulation of cells or molecules into individual droplets that can be sorted based on their fluorescent emission. Our method involves, encapsulating immune cells into droplets that are later converted into alginate gels. Our aim is to isolate a cancer-specific immune cell for immunotherapy. These isolated gels contain a specific fluorescent marker that is interrogated by our optical system and is sorted using a pulsed traveling surface acoustic wave (TSAW). These specific immune cells are rare and therefore require high sample throughput. Current fluorescent activated cell sorting (FACS) jet-in-air machines can sort at throughput rates of up to fifty kilohertz, but aerosols produced by these machines are considered a biohazard and gels bursting as a result of high vibrational pressure exerted at the nozzle of the FACS machine restrict its use in our application. Alternative to jet-in-air is microfluidic FACS (uFACS) which overcomes these issues. However, not all uFACS can achieve our high throughput requirements, for example, dielectrophoresis are unable to surpass throughputs of a few kilohertz. Our approach offers high throughput in the region of ten kilohertz whilst maintaining gel integrity and achieving a high level of accuracy and precision.
Keywords: Fluorescent activated cell sorting (FACS), Traveling surface acoustic wave (TSAW)

15. Gerber, Gaetan; M. Bensouda, D.A. Weitz, P. Coussot

Université Paris-Est, Navier, Champs-sur-Marne, Frange ; Exp. Soft Condensed Matter Groups, Harvard SEAS, USA
"Colloid accumulation in 3D porous media"
Colloid deposition is of great importance in a multitude of industrial or hydrologic processes, like filtration or soil remediation. Still, the description of this process, happening inside complex 3D porous structures, is vastly empirical. Here we propose a direct internal observation of the transport and accumulation of charged colloids in a model column of borosilicate packed grains of opposite charge. We vary the inter-colloidal interactions to modulate the dynamics of deposition, from langmuirian single-layers to major clogging of the pore space. We show that the deposition dynamic is driven and limited by a strong coupling between the cohesive forces and drag forces applied on the particles. Through this simple model, we provide a better prediction of the dynamics of colloid deposition.
Keywords: colloid, porous media, adsorption, transport, accumulation

16. Hu, Xiaoyi; Thomas Cubaud

Stony Brook University
"Microfluidic viscosity-stratified flows: from droplets to waves"
We study experimentally viscosity-stratified flows in microchannels for a wide range of flow patterns including dispersed flows with droplets and separated flows with interfacial waves. We carefully analyze the dynamic characteristics of droplets and waves, such as emission frequency and propagation velocity. Based on experiment observations, we find that the transition from dispersed to separated flows occurs when the propagation velocity equals the wave celerity and develop an analytical formula which agrees with experiments well.
Keywords: microfluidics, multiphase flow, droplet, waves

17. Julien, Elisa; E. Julien, G. Gerber, T. Cochard, Y. Hue, W. Steinhardt, D.A. Weitz

Harvard University
"Particles motion during hydraulic fracture propagation and collapse"
Proppants are particles (sand like materials) injected within fracking fluid during the hydraulic fracturing process. They keep the newly fractures open to hold high permeability paths to recover the oil trapped in common shales reservoirs. We developed an experimental approach where we use a transparent model (polymeric gels) to observe the particles motion at different stages of the process: transport during fracture propagation and deposition during fracture collapse. By measuring the pressure evolution and imaging both fractures and particles we provide a general understanding of proppants transport and deposition during the hydraulic fracking process.
Keywords: Hydraulic fracture, proppants transport

18. Li, Xinzhi; Amit Das, Dapeng(Max) Bi

Northeastern University
"Cell-level mechanical heterogeneity promotes rigidity in confluent tissues"
Intra-tumor heterogeneity is one of the hallmarks of cancer, which describes the phenotypic differences among cells in a tumor or cellular collective. While genetic heterogeneity has been an intense focus of study, how mechanical variations among cells influence tissue mechanics is not well understood. Here, we investigate the effect of cell-to-cell mechanical heterogeneity on the overall bulk mechanics of a confluent 2d tissue using a vertex model-based approach. We find that the rigidity of a confluent tissue depends on overall statistical properties of single-cell properties such as mean and variance, rather than the specific functional form of its distribution. A single universal parameter - the fraction of mechanically rigid cells, $f_r$, can be used to characterize the tissue mechanical state. As $f_r$ is tuned, the tissue undergoes a rigidity percolation at a critical threshold of $f_r$. Remarkably, this rigidity percolation occurs at a much lower value than what is required for rigid-cell to form a spanning cluster. A mean field model is proposed to explain the discrepancy between rigidity and contact percolations.
Keywords: heterogeneity, confluent tissue

19. Miranda de Lima, Nicolle ; Shima Parsa, David Weitz, Márcio Carvalho

PUC-Rio/Harvard University
"Foam formation analysis during drainage of surfactant solution"
Foam is widely used in oil recovery operations to maximize oil production because of its lower sensitivity to gravity and permeability heterogeneities. Foam, that can be pre-formed and injected in the reservoir or produced in-situ through the pore space, fills the high permeability areas and diverts the displacing fluid into the direction of trapped oil, reducing the relative permeability of gas and leading to a more stable flood front. The presence of liquid lamellae between gas bubbles also reduces the gas mobility, by the increase of the gas apparent viscosity. The flow mobility is a function of the pore geometry and foam properties. However, the dynamics of foam in porous media is not fully understood due to its complexity. The goal of this research is to study foam formation during drainage of a two-dimensional porous media glass model by visualizing the pore scale displacement flow of a surfactant solution by injected gas.
Keywords: foam formation, enhanced oil recovery, lamella, microfluidic

20. Mohammed, Danahe Zeyna; David Weitz

"Modulation of tumoral cell behavior by osmotic shocks"
The physico-chemical properties of the cellular environment impact on cell fate. Recent studies showed that osmotic pressure affects functional properties of tumoral cells. The role of osmotic pressure on cell behavior was recently investigated but only for single cells and not on tissues yet. In this research project, we propose to study the role of the osmotic pressure in the regulation of the migration and proliferation of tumoral cells. The main goal of this project is to understand the modulation of tumoral cell behavior by osmotic shocks. To address this project, we will use albumin (protein which control the osmotic pressure) and PEG (control) to modulate the osmotic pressure of breast epithelial cells and comparing with tumoral cells (MCF-10A and MDA-MB-231). Furthermore, the concentration of albumin has been identified as a robust prognostic marker of breast cancer. We will also study the intracellular organization with confocal microscopy and the nuclear deformation of breast cancer cells in response to osmotic shocks to understand how osmotic pressure can regulate breast cancer cell migration and proliferation.
Keywords: cell migration, osmotic pressure

21. Molavi, Amir; Alain Karma

Center for Inter-Disciplinary Research on Complex Systems, Department of Physics, Northeastern University, Boston
"Spatiotemporal organization of excitation and mechanical waves during life-threatening re-entrant cardiac arrhythmias"
Two-dimensional spiral waves and three-dimensional scroll waves of electrical excitation in the heart are well-established mechanisms of high-frequency re-entrant arrhythmias such as atrial/ventricular tachycardia and fibrillation. While conventional optical mapping techniques can only traditionally be used to image excitation waves at the surface of the heart, recent progress has been made to use high-resolution ultrasound imaging to study mechanical waves inside the heart muscle [1]. This new imaging modality has created an unprecedented opportunity to investigate the three-dimensional spatiotemporal organization of electromechanical waves in the heart. At the same time, it provides a framework to address basic questions: How are mechanical strain waves spatiotemporally organized? Do they exhibit phase singularities that track excitation scroll-wave filaments or more complex patterns? Do different mechanisms of high-frequency arrhythmias, reentry in the form of scroll waves versus focal excitations (sources of waves from small isolated tissue regions) give rise to different mechanical wave patterns? This talk will summarize recent progress made to answer those questions using two- and three-dimensional computational models that couple electrical excitation to mechanical contraction. The results highlight how the non-locality of elastic interactions yields a complex geometrical relationship between wave patterns of active tension, which track electrical excitation waves, and waves of tissue deformation imaged by ultra-sound. Our main conclusion is that this relationship can be understood as a basis to use ultra-sound imaging to locate electrical excitation waves, thereby opening up new avenues for improving current techniques of radiofrequency ablation and low-energy defibrillation.
Keywords: spiral wave, tissue mechanics, phase singularity, excitable media

22. Oliveira, Tania Thalyta Silva de ; Arijit Bose

University of Rhode Island
"Response of Synechococcus elongatus PCC 7942 to microplastics"
About 150 million tons of plastic are in the world’s oceans currently, and 8 million additional tons are dumped into the ocean each year. If this rate continues, the weight of plastics in the ocean will exceed the weight of all marine organisms by 2050. These are highly concerning statistics, since plastics, in the ocean do not degrade easily, and can impact marine life for over a hundred years. In our laboratory, we have been examining how a specific ocean bacterium, Synechococcus elongatus PCC 7942 responds when exposed to microplastics. We have exposed PCC 7942 to polystyrene and polyethylene particles of diameters varying from 0.2 µm to 10 µm and examined these samples using fluorescence microscopy and cryogenic scanning electron microscopy. The bacteria attached to the particles within 24 hrs. Biofilm formation was detected within 72 hrs of contact, and after 200 hrs of exposure it increased drastically, followed by increase in the hetero-aggregates size. The control sample within this timeline showed spread cells and few small agglomerations. We reveal yet another pathway by which microplastics affect marine life.
Keywords: Microplastics, Biofilm, Cyanobacteria

23. Pan, Animesh; Geoffrey D. Bothun

University of Rhode Island
"Radiofrequency and Near-Infrared Responsive Core-Shell Multifunctional Nanostructures Using Lipid Templates for Cancer Theranostics"
We report multifunctional nanotherapeutic platform based on liposomes loaded with drug, and iron-oxide magnetic nanoparticles (MNPs), and coated with a gold nano-shell synthesized using a polyelectrolyte (layersomes) soft templating technique. MNPs and the anti-cancer drug doxorubicin (DOX) were co-encapsulated inside liposomes composed of zwitterionic phophatidylcholine (PC) and anionic phosphatidylglycerol (PG) lipids using reverse phase evaporation (REV) method. The liposomes were coated with positively charge poly-L-lysine to enrich the interface with gold anion, exposed to a reducing agent to form a gold nanoshell, and then the surface of shell modified with thiol-terminated polyethylene glycol (SH-PEG2000) to get the better stability. The core-shell nanostructures were characterized by dynamic light scattering, transmission electron microscopy and visible-near-infrared spectroscopy. This multifunctional system achieves a variety of functions, such as radiofrequency (RF) heating and NIR laser-triggered for photothermal therapy. We also demonstrate an efficient loading and delivery system to significant cell death of human cancer cells (A549) with therapeutic capabilities. Coupled with radio frequency (RF) and near-infrared (NIR) excitation to the doxorubicin-loaded core-shell nanostructure helped in targeted and controlled drug release to the cancer cells. The present core-shell multifunctional system would be eminent candidates for cancer theranostics.
Keywords: multifunctional nanostructures, radiofrequency heating, photothermal therapy, cancer theranostics

24. Rabiei, Nastaran; Carlos H. Hidrovo, Pooyan Tirandazi

Northeastern University
"Friction Reduction Effects of Wetted Microtexturing in Microchannel Flow"
Microchannel flows are widely used in applications where small diffusion length scales are important, such as in microscale heat exchangers for electronics cooling and lab-on-a-chip biochemical processing. However, these small length scales also translate into high pumping power requirements. One possible way to alleviate the large viscous pressure losses associated with this inherent dimensional constrain is to take inspiration from nature. Recent studies have shown that the microstructures on sharks’ skin enable them to swim faster and experience less resistance. Similarly, introducing side trenches in a micro-channel can help lower the skin drag. The flow over these transverse trenches may experience two wetting states: Cassie-Baxter and Wenzel. In the Cassie-Baxter state, the air is trapped in the trenches. Whereas, in the Wenzel state the trenches are fully wetted. In both states the trapped air or water can act like a cushion and change the no-slip to slip boundary condition resulting in less shear stress. However, it has been shown that in some instances the air-water interface in the Cassie Baxter state might act like a solid boundary due to contamination. Concurrently, penetration of the flow inside the trenches in the Wenzel state and appearance of the trenches as the flow barriers, can induce the pressure drag alongside the skin drag. Therefore, the Wenzel state in the trenches can lead to a trade-off between skin and pressure drag. The aim of this work is to understand the geometrical effect that different microtextures have on the total drag reduction by testing trenches with different aspect ratios and measuring the pressure drop through the micro-channel.
Keywords: Micro-texturing, Superhydrophobic, Drag reduction, Cassie-Baxter state, Wenzel state

25. Ran, Ran; Tingting Zhu, Kaizhen Zhang, Sinan Müftü, Kai-tak Wan

Northeastern University
"Mechanical Characterization and Fusion of Giant Unilamellar Lipid Vesicles"
The cell membrane, consisting of a lipid bilayer with embedded proteins, isolates the interior of cell from the outside environment, while also controls the exchange on both sides. The giant unilamellar lipid vesicle is a perfect model to study the mechanical properties of the cell membrane. Our overarching goal is to investigate the mechanical behavior of single vesicle and the fusion process of two adjacent vesicles under external mechanical load using an atomic force microscopy (AFM). The giant unilamellar lipid vesicles were designed and fabricated using a high throughput glass capillary micro-fluidic device. Mechanical properties of single lipid vesicle were measured by parallel plate compression operated by AFM to extract the material parameters such as stretching modulus. In the study of fusion mechanism, the AFM experiment of one lipid vesicle approaching another clearly showed the force changes and energy barriers corresponding to the hemi-fusion and fusion intermediates. We envision the results could ultimately facilitate efficacious targeted drug delivery for medical treatments for cancers and other diseases.
Keywords: Lipid Vesicle, Atomic Force Microscopy, Mechanical Properties, Fusion

26. Stolovicki, Elad; Elad Stolovicki, Lloyd Ung, Roy Ziblat and David A. Weitz

Harvard John A. Paulson School of Engineering and Applied Sciences
"Drop chemostats: White biotechnology on a chip"
White biotechnology, the production of chemicals using cells or enzymes, is increasingly employed as it results in higher, overall greener chemical processes. Bio-production is also ideally suited in cases where the selectivity of enzymes for a specific molecule enantiomer (chirality) is critical. By using an emulsion of small drops of growth medium in oil as a micro-reactor, we can optimize the production yield of a desired bio-product, preforming toxicity tests or response-resistance assays. Another key feature of our system is that each, individual droplet emulates a larger-scale bioreactor that can be grown using batch, fed-batch or continuous culture methods. Thus, drop micro-reactors have the advantages of reducing R&D production time and cost by having thousands of parallel experiments, greatly reduced quantities of reagents, and compact space requirements.
Keywords: White biotechnology, bio-production, drop micro-reactors, microfluidics

27. Vasudevan, Aditya; Chih-Hung Chen, Alain Karma

Department of Physics, Northeastern University
"Crack front instability in mixed mode I + III with a shear-dependent fracture energy"
Cracks under the influence of mode I (tensile mode) and mode III (tearing mode) are well known to be unstable and segment into an array of daughter cracks that produces a faceted fracture surface. Based on insights from a phase-field modelling study, which revealed that daughter cracks form as a result of a helical instability of the parent coplanar crack, a linear stability analysis carried out in the LEFM framework predicted an instability threshold much higher than observed in the experiments. However, these analyses were performed for a constant fracture energy independent of the loading conditions. Motivated by recent experiments, we propose a heuristic dependence of the fracture energy on mode-mixity and linear stability analysis and phase-field simulations with this new dependence show a significant decrease in the threshold of instability. ¬ Finally, we compare nonlinear aspects of the bifurcation to segmented fronts for cases where fracture energy is constant and depends on mode mixity.
Keywords: fracture, crack front instability, pattern formation

28. Ye, Huilin; Zhiqiang Shen, Ying Li

University of Connecticut
"Multi-scale computational method for biological system"
Study of dynamics of lipid membranes and proteins plays a crucial role in understanding the structure and conformation of complexes in the cellular membrane and peptides. Due to their small lengths and short time scales, it is difficult to access these molecular phenomena experimentally. Hence, efficient and accurate multi-scale modelling of lipids and proteins such as molecular dynamics (MD) simulations become popular in recent decades. Although multiscale modelling achieves great progress, it remains a challenge to provide a realistic description of the lipid membrane and protein behaviors, because the multiscale model should balance accuracy and efficiency. Current work aims to develop a highly efficient and adaptive fluctuating hydrodynamics simulation package to reconcile this situation.
Keywords: Multi-scale method

29. Zando, Robert

Northeastern University
"Magnetic Patterning of Fiber Suspensions for Controlled Fracture Pathways and Improved Material Toughness"
The mechanical performance of a given material may be said to broadly fall into two primary categories of interest: strength (ability to resist plastic deformation) and toughness (ability to resist fracture propagation). The mechanisms of fracture toughness are highly varied, including the formation of sheer bands and voids connected by material fibrillar just ahead of the initial fracture, as well as the bridging of the fracture wake in the case of ceramics and some polymer composites. However, among the most effective of these mechanisms is the principle of fracture deflection, in which energy that would be used to propagate the fracture is instead used to redirect its path, raising the overall energy necessary for continued crack propagation through the material matrix. Using ceramic platelet filler material, coated in ultrahigh magnetic response iron oxide nanoparticles, it is possible to align the filler material within a UV-sensitive resin by means of exposure to a rotating magnetic field, curing it with the filler particles aligned into a specific geometry. This alignment geometry thereby permits the creation of an engineered fracture path within a material via a series of controlled crack deflections, raising the overall fracture resistance (i.e. toughness) of the material.
Keywords: magnetic alignment filler suspensions fracture toughness

30. Zhang, Weixia; Weixia Zhang, Xi Xie, Alireza Abbaspourrad, Daniel Anderson, David Weitz

Harvard University, Massachusetts Institute of Technology
"Colloidal Nanomaterials Encapsulated Microcapsules for Implantable Biosensors"
Implantable sensors that detect biomarkers in vivo are critical for early disease diagnostics. While many colloidal nanomaterials have been developed into optical sensors to detect biomolecules in vitro, their application in vivo as implantable sensors is hindered by potential migration or clearance from the implantation site. One potential solution is incorporating colloidal nanosensors in hydrogel scaffold prior to implantation. However, direct contact between the nanosensors and hydrogel matrix has the potential to disrupt sensor performance. Here we develop a hollow microcapsule-based sensing platform that protects colloidal nanosensors from direct contact with hydrogel matrix. Using microfluidics, colloidal nanosensors were encapsulated in polyethylene glycol microcapsules with liquid cores. The microcapsules selectively trap the nanosensors within the core while allowing free diffusion of smaller molecules such as glucose. Glucose-responsive quantum dots were encapsulated. Microcapsules loaded with these sensors showed responsive optical signals in the presence of target biomolecules glucose. Furthermore, these microcapsules can be immobilized into biocompatible hydrogel as implantable devices for biomolecular sensing. This technique offer new opportunities to extend the utility of colloidal nanosensors from solution based detection to implantable device based detection.
Keywords: Colloidal Nanomaterials, Microcapsules, Implantable Biosensors

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