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1.
Phys Rev Lett ; 132(11): 118203, 2024 Mar 15.
Artigo em Inglês | MEDLINE | ID: mdl-38563929

RESUMO

Predicting the rheology of dense suspensions under inhomogeneous flow is crucial in many industrial and geophysical applications, yet the conventional "µ(J)" framework is limited to homogeneous conditions in which the shear rate and solids fraction are spatially invariant. To address this shortcoming, we use particle-based simulations of frictionless dense suspensions to derive new constitutive laws that unify the rheological response under both homogeneous and inhomogeneous conditions. By defining a new dimensionless number associated with particle velocity fluctuations and combining it with the viscous number, the macroscopic friction, and the solids fraction, we obtain scaling relations that collapse data from homogeneous and inhomogeneous simulations. The relations allow prediction of the steady state velocity, stress, and volume fraction fields using only knowledge of the applied driving force.

2.
Soft Matter ; 19(7): 1342-1347, 2023 Feb 15.
Artigo em Inglês | MEDLINE | ID: mdl-36723039

RESUMO

Colloidal gels formed from small attractive particles are commonly used in formulations to keep larger components in suspension. Despite extensive work characterising unfilled gels, little is known about how the larger inclusions alter the phase behavior and microstructure of the colloidal system. Here we use numerical simulations to examine how larger 'granular' particles can alter the gel transition phase boundaries. We find two distinct regimes depending on both the filler size and native gel structure: a 'passive' regime where the filler fits into already-present voids, giving little change in the transition, and an 'active' regime where the filler no longer fits in these voids and instead perturbs the native structure. In this second regime the phase boundary is controlled by an effective colloidal volume fraction given by the available free volume.

3.
Eur J Dent Educ ; 25(4): 664-669, 2021 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-33314572

RESUMO

INTRODUCTION: Reflection and self-assessment are critical skills for healthcare providers. Identification of gaps in knowledge, skills and attitudes, along with the ability to critically think and problem solve to fill gaps, is the ultimate outcome for lifelong learning. The aims of this study were to (a) refine an instrument used for measuring reflective ability, and conduct comprehensive reliability testing, and (b) describe a process for rater calibration. MATERIALS AND METHODS: Students develop e-portfolios over a four-year span with assignments that require reflection and self-assessment. The final piece of the portfolio includes a global reflection written the last semester of the programme. Three faculty raters independently evaluated 106 dental students' global reflections using the revised grading rubric. An intraclass correlation coefficient measured the level of agreement between the three raters. RESULTS: Analysis of the 318 faculty ratings (106/rater) resulted in an intraclass correlation of .708. Based on a 5-point grading scale (0 = does not respond to the assignment to 5 = reconstructing), the ratings of the 106 global reflections ranged from 1.3 to 5.0 (M = 3.1, SD =0.66). DISCUSSION: This study provides confidence in the reliability of a grading rubric designed to assess reflective ability, along with suggestions for calibration. An overall mean of 3.1 (Level 3 = relating-includes evidence of lessons learned) illustrates the complexity of teaching reflection and self-assessment. CONCLUSION: Use of a reliable grading rubric for assessing reflective writing could assist schools interested in incorporating reflection and self-assessment into the curriculum, ultimately supporting lifelong and enhanced health care.


Assuntos
Avaliação Educacional , Estudantes de Odontologia , Educação em Odontologia , Humanos , Reprodutibilidade dos Testes , Autoavaliação (Psicologia)
4.
Phys Rev Lett ; 124(8): 088004, 2020 Feb 28.
Artigo em Inglês | MEDLINE | ID: mdl-32167320

RESUMO

We consider a model for driven particulate matter in which absorbing states can be reached both by particle isolation and by particle caging. The model predicts a nonequilibrium phase diagram in which analogs of hydrodynamic and elastic reversibility emerge at low and high volume fractions respectively, partially separated by a diffusive, nonabsorbing region. We thus find a single phase boundary that spans the onset of chaos in sheared suspensions to the onset of yielding in jammed packings. This boundary has the properties of a nonequilibrium second order phase transition, leading us to write a Manna-like mean field description that captures the model predictions. Dependent on contact details, jamming marks either a direct transition between the two absorbing states, or occurs within the diffusive region.

5.
Phys Rev Lett ; 124(24): 248005, 2020 Jun 19.
Artigo em Inglês | MEDLINE | ID: mdl-32639825

RESUMO

Particle-based simulations of discontinuous shear thickening (DST) and shear jamming (SJ) suspensions are used to study the role of stress-activated constraints, with an emphasis on resistance to gearlike rolling. Rolling friction decreases the volume fraction required for DST and SJ, in quantitative agreement with real-life suspensions with adhesive surface chemistries and "rough" particle shapes. It sets a distinct structure of the frictional force network compared to only sliding friction, and from a dynamical perspective leads to an increase in the velocity correlation length, in part responsible for the increased viscosity. The physics of rolling friction is thus a key element in achieving a comprehensive understanding of strongly shear-thickening materials.

6.
Soft Matter ; 16(1): 229-237, 2020 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-31777908

RESUMO

There is a growing consensus that shear thickening of concentrated dispersions is driven by the formation of stress-induced frictional contacts. The Wyart-Cates (WC) model of this phenomenon, in which the microphysics of the contacts enters solely via the fraction f of contacts that are frictional, can successfully fit flow curves for suspensions of weakly polydisperse spheres. However, its validity for "real-life", polydisperse suspensions has yet to be seriously tested. By performing systematic simulations on bidisperse mixtures of spheres, we show that the WC model applies only in the monodisperse limit and fails when substantial bidispersity is introduced. We trace the failure of the model to its inability to distinguish large-large, large-small and small-small frictional contacts. By fitting our data using a polydisperse analogue of f that depends separately on the fraction of each of these contact types, we show that the WC picture of shear thickening is incomplete. Systematic experiments on model shear-thickening suspensions corroborate our findings, but highlight important challenges in rigorously testing the WC model with real systems. Our results prompt new questions about the microphysics of thickening for both monodisperse and polydisperse systems.

7.
Soft Matter ; 16(4): 990-1001, 2020 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-31853526

RESUMO

Three-dimensional DNA networks, composed of tri- or higher valent nanostars with sticky, single-stranded DNA overhangs, have been previously studied in the context of designing thermally responsive, viscoelastic hydrogels. In this work, we use linker-mediated gels, where the sticky ends of two trivalent nanostars are connected through the complementary sticky ends of a linear DNA duplex. We can design this connection to be either rigid or flexible by introducing flexible, non-binding bases. The additional flexibility provided by these non-binding bases influences the effective elasticity of the percolating gel formed at low temperatures. Here we show that by choosing the right length of the linear duplex and non-binding flexible joints, we obtain a completely different phase behaviour to that observed for rigid linkers. In particular, we use dynamic light scattering as a microrheological tool to monitor the self-assembly of DNA nanostars with linear linkers as a function of temperature. While we observe classical gelation when using rigid linkers, the presence of flexible joints leads to a cluster fluid with a much-reduced viscosity. Using both the oxDNA model and a coarse-grained simulation to investigate the nanostar-linker topology, we hypothesise on the possible structure formed by the DNA clusters. Moreover, we present a systematic study of the strong viscosity increase of aqueous solutions in the presence of these DNA building blocks.


Assuntos
DNA de Cadeia Simples/química , DNA/química , Hidrogéis/química , DNA/ultraestrutura , DNA de Cadeia Simples/ultraestrutura , Difusão Dinâmica da Luz , Elasticidade , Temperatura , Viscosidade , Água/química
8.
Philos Trans A Math Phys Eng Sci ; 378(2179): 20200160, 2020 Sep 04.
Artigo em Inglês | MEDLINE | ID: mdl-32762430

RESUMO

We present the second half of the papers from the Stokes200 symposium celebrating the bicentenary of George Gabriel Stokes. This article is part of the theme issue 'Stokes at 200 (part 2)'.

9.
Philos Trans A Math Phys Eng Sci ; 378(2174): 20190505, 2020 Jun 26.
Artigo em Inglês | MEDLINE | ID: mdl-32507087

RESUMO

Sir George Gabriel Stokes PRS was for 30 years an inimitable Secretary of the Royal Society and its President from 1885 to 1890. Two hundred years after his birth, Stokes is a towering figure in physics and applied mathematics; fluids, asymptotics, optics, acoustics among many other fields. At the Stokes200 meeting, held at Pembroke College, Cambridge from 15-18th September 2019, an invited audience of about 100 discussed the state of the art in all the modern research fields that have sprung from his work in physics and mathematics, along with the history of how we have got from Stokes' contributions to where we are now. This theme issue is based on work presented at the Stokes200 meeting. In bringing together people whose work today is based upon Stokes' own, we aim to emphasize his influence and legacy at 200 to the community as a whole. This article is part of the theme issue 'Stokes at 200 (Part 1)'.

10.
Proc Natl Acad Sci U S A ; 113(39): 10774-8, 2016 09 27.
Artigo em Inglês | MEDLINE | ID: mdl-27621472

RESUMO

Shear thickening, an increase of viscosity with shear rate, is a ubiquitous phenomenon in suspended materials that has implications for broad technological applications. Controlling this thickening behavior remains a major challenge and has led to empirical strategies ranging from altering the particle surfaces and shape to modifying the solvent properties. However, none of these methods allows for tuning of flow properties during shear itself. Here, we demonstrate that by strategic imposition of a high-frequency and low-amplitude shear perturbation orthogonal to the primary shearing flow, we can largely eradicate shear thickening. The orthogonal shear effectively becomes a regulator for controlling thickening in the suspension, allowing the viscosity to be reduced by up to 2 decades on demand. In a separate setup, we show that such effects can be induced by simply agitating the sample transversely to the primary shear direction. Overall, the ability of in situ manipulation of shear thickening paves a route toward creating materials whose mechanical properties can be controlled.

11.
Soft Matter ; 14(42): 8475-8482, 2018 Oct 31.
Artigo em Inglês | MEDLINE | ID: mdl-30152833

RESUMO

We study the viscoelastic response of amorphous polymers using theory and simulations. By accounting for internal stresses and considering instantaneous normal modes (INMs) within athermal non-affine theory, we make parameter-free predictions of the dynamic viscoelastic moduli obtained in coarse-grained simulations of polymer glasses at non-zero temperatures. The theoretical results show very good correspondence with rheology data collected from molecular dynamics simulations over five orders of magnitude in frequency, with some instabilities that accumulate in the low-frequency part on approach to the glass transition. These results provide evidence that the mechanical glass transition itself is continuous and thus represents a crossover rather than a true phase transition. The relatively sharp drop of the low-frequency storage modulus across the glass transition temperature can be explained mechanistically within the proposed theory: the proliferation of low-eigenfrequency vibrational excitations (boson peak and nearly-zero energy excitations) is directly responsible for the rapid growth of a negative non-affine contribution to the storage modulus.

12.
Soft Matter ; 13(19): 3664-3674, 2017 May 21.
Artigo em Inglês | MEDLINE | ID: mdl-28451674

RESUMO

The viscosity of a dense suspension has contributions from hydrodynamics and particle interactions, both of which depend upon the flow-induced arrangement of particles into fragile structures. Here, we study the response of nearly hard sphere suspensions to oscillatory shear using simulations and experiments in the athermal, non-inertial limit. Three distinct regimes are observed as a function of the strain amplitude γ0. For γ0 < 10-1, initially non-contacting particles remain separated and the suspension behaves similarly to a Newtonian fluid, with the shear stress proportional to the strain rate, and the normal stresses close to zero. For γ0 > 101, the microstructure becomes well-established at the beginning of each shear cycle and the rheology is quasi-Newtonian: the shear stress varies with the rate, but flow-induced structures lead to non-zero normal stresses. At intermediate γ0, particle-particle contacts break and reform across entire oscillatory cycles, and we probe a non-linear regime that reveals the fragility of the material. Guided by these features, we further show that oscillatory shear may serve as a diagnostic tool to isolate specific stress contributions in dense suspensions, and more generally in those materials whose rheology has contributions with both hydrodynamic and non-hydrodynamic origin.

13.
Soft Matter ; 12(3): 914-24, 2016 Jan 21.
Artigo em Inglês | MEDLINE | ID: mdl-26555249

RESUMO

We propose a unifying rheological framework for dense suspensions of non-Brownian spheres, predicting the onsets of particle friction and particle inertia as distinct shear thickening mechanisms, while capturing quasistatic and soft particle rheology at high volume fractions and shear rates respectively. Discrete element method simulations that take suitable account of hydrodynamic and particle-contact interactions corroborate the model predictions, demonstrating both mechanisms of shear thickening, and showing that they can occur concurrently with carefully selected particle surface properties under certain flow conditions. Microstructural transitions associated with frictional shear thickening are presented. We find very distinctive divergences of both microstructural and dynamic variables with respect to volume fraction in the thickened and non-thickened states.

14.
Phys Rev E ; 109(4): L042601, 2024 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-38755845

RESUMO

We use the discrete element method, taking particle contact and hydrodynamic lubrication into account, to unveil the shear rheology of suspensions of frictionless non-Brownian rods in the dense packing fraction regime. We find that, analogously to the random close packing volume fraction, the shear-driven jamming point of this system varies in a nonmonotonic fashion as a function of the rod aspect ratio. The latter strongly influences how the addition of rodlike particles affects the rheological response of a suspension of frictionless non-Brownian spheres to an external shear flow. At fixed values of the total (rods plus spheres) packing fraction, the viscosity of the suspension is reduced by the addition of "short"(≤2) rods but is instead increased by the addition of "long"(≥2) rods. A mechanistic interpretation is provided in terms of packing and excluded-volume arguments.

15.
J Dent Educ ; 88 Suppl 1: 690-698, 2024 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-38758034

RESUMO

The aim of this study was to provide a follow-up to the 2015 national survey of women in dental education to re-examine factors and perceived barriers to pursuing administrative and leadership roles. At the beginning of 2023, a survey was administered to full-time women faculty in predoctoral dental programs in the United States. The survey instrument employed a structured format along with open-ended questions to capture qualitative data. The overall response rate was 26% (527/1994) and included respondents from fifty-five of the sixty-seven dental schools contacted. Half of respondents indicated holding an administrative position and 20% reported administration as their primary appointment. There was a significant increase in the number of women holding a leadership role in 2023 (76%) compared to 2015 (53%). Those seeking a leadership position in 2023 were less likely to have participated in leadership training (p<0.01) and less likely to have previously sought an administrative position (p>0.01). Women in leadership and administrative roles were more likely to indicate receiving extra compensation for administrative duties (p<0.01) and a desire for higher rank (p = 0.04) compared to 2015, but perceptions persist among the majority of respondents (71%) that women are not being compensated equally to their male counterparts. Respondents highlighted the negative impact of the COVID-19 pandemic on career advancement and the positive culture changes that have contributed to greater workplace flexibility. Overall, respondents sent a resounding message that women in dental education need structural change to achieve success: pay equity, mentorship, and inclusive and supportive work environments.


Assuntos
Odontólogas , Docentes de Odontologia , Liderança , Humanos , Feminino , Estados Unidos , Odontólogas/estatística & dados numéricos , Educação em Odontologia , Inquéritos e Questionários , Seguimentos , Adulto , COVID-19/epidemiologia , Faculdades de Odontologia
16.
Phys Rev E ; 104(5-1): 054602, 2021 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-34942818

RESUMO

We measure by experiment and particle-based simulation the rheology of concentrated, non-Brownian droplet emulsions functionalized with surface-bound single-stranded (ss), "sticky," DNA. In the absence of ssDNA, the emulsion viscosity increases with the dispersed phase volume fraction ϕ, before passing through a liquid-solid transition at a critical ϕ_{c} related to random close packing. Introducing ssDNA leads to a liquid-solid transition at ϕ<ϕ_{c}, the onset being set by the droplet valency N and the ssDNA concentration (or simulated binding strength ε). Using insight from simulation, we identify three key behaviors: (i) jammed suspensions (ϕ>ϕ_{c}≈0.64) show weak effects of functionalization, with elastic rheology instead governed by droplet stiffness; (ii) suspensions with ϕ<ϕ_{c} and N=1, 2 always exhibit viscous rheology, regardless of functionalization; and (iii) for ϕ<ϕ_{c} and N>3, functionalization leads to a controllable viscous-elastic transition. We present state diagrams showing the range of rheological tuning attainable by these means.

17.
J Dent Educ ; 85(6): 747-755, 2021 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-33598917

RESUMO

PURPOSE/OBJECTIVE: Competence is expected of each beginning dentist and orthodontist. However, the broad definition of competence presents a challenge to academic programs in identifying the level of cognition for students to achieve competence. This study aimed to determine the Didactic Clinical Skills Development curriculum content and competency in predoctoral and advanced education orthodontic programs. METHOD: A modified Delphi method with a consensus threshold of 70% was employed using an expert panel of academic orthodontists. RESULTS: Round One (n = 26) identified that all topics proposed by a focus group were necessary, except for predoctoral Appliances, which was at 65%. Round Two (n = 23) included subtopics of Appliances to confirm the lack of consensus, plus subtopics of all the other topics and the level of cognition required for each subtopic. The expert panel reached a consensus that all 24 subtopics, hence all topics, were necessary. In Round Three, subtopic responses in Round Two were assigned a value between 1 (remember) and 6 (create) to generate a hierarchical level-of-learning scale. Mean values were calculated for each subtopic response. For all subtopics, the mean level of cognition for predoctoral education was at understand; for advanced education, it was at evaluate. CONCLUSION: This consensus suggests that, to be deemed competent, beginning dentists must learn these topics and subtopics in the cognitive domain of understand, and beginning orthodontists in the cognitive domain of evaluate. This study showed an expert consensus on Didactic Clinical Skills Development orthodontic curriculum content and a panorama of educational objectives that could be used as a template for curriculum design.


Assuntos
Competência Clínica , Currículo , Consenso , Técnica Delphi , Humanos , Aprendizagem
18.
Phys Rev E ; 102(2-1): 022602, 2020 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-32942369

RESUMO

We present a detailed study of the kinetic cluster growth process during gelation of weakly attractive colloidal particles by means of experiments on critical Casimir attractive colloidal systems, simulations, and analytical theory. In the experiments and simulations, we follow the mean coordination number of the particles during the growth of clusters to identify an attractive-strength independent cluster evolution as a function of mean coordination number. We relate this cluster evolution to the kinetic attachment and detachment rates of particles and particle clusters. We find that single-particle detachment dominates in the relevant weak attractive-strength regime, while association rates are almost independent of the cluster size. Using the limit of single-particle dissociation and size-independent association rates, we solve the master kinetic equation of cluster growth analytically to predict power-law cluster mass distributions with exponents -3/2 and -5/2 before and after gelation, respectively, which are consistent with the experimental and simulation data. These results suggest that the observed critical Casimir-induced gelation is a second-order nonequilibrium phase transition (with broken detailed balance). Consistent with this scenario, the size of the largest cluster is observed to diverge with power-law exponent according to three-dimensional percolation on approaching the critical mean coordination number.

19.
Nat Commun ; 11(1): 3558, 2020 07 16.
Artigo em Inglês | MEDLINE | ID: mdl-32678089

RESUMO

The dynamical arrest of attractive colloidal particles into out-of-equilibrium structures, known as gelation, is central to biophysics, materials science, nanotechnology, and food and cosmetic applications, but a complete understanding is lacking. In particular, for intermediate particle density and attraction, the structure formation process remains unclear. Here, we show that the gelation of short-range attractive particles is governed by a nonequilibrium percolation process. We combine experiments on critical Casimir colloidal suspensions, numerical simulations, and analytical modeling with a master kinetic equation to show that cluster sizes and correlation lengths diverge with exponents  ~1.6 and 0.8, respectively, consistent with percolation theory, while detailed balance in the particle attachment and detachment processes is broken. Cluster masses exhibit power-law distributions with exponents  -3/2 and  -5/2 before and after percolation, as predicted by solutions to the master kinetic equation. These results revealing a nonequilibrium continuous phase transition unify the structural arrest and yielding into related frameworks.

20.
Sci Adv ; 6(19): eaay6661, 2020 May.
Artigo em Inglês | MEDLINE | ID: mdl-32494699

RESUMO

Hundreds of YouTube videos show people running on cornstarch suspensions demonstrating that dense shear thickening suspensions solidify under impact. Such processes are mimicked by impacting and pulling out a plate from the surface of a thickening cornstarch suspension. Here, using both experiments and simulations, we show that applying fast oscillatory shear transverse to the primary impact or extension directions tunes the degree of solidification. The forces acting on the impacting surface are modified by varying the dimensionless ratio of the orthogonal shear to the compression and extension flow rate. Simulations show varying this parameter changes the number of particle contacts governing solidification. To demonstrate this strategy in an untethered context, we show the sinking speed of a cylinder dropped onto the suspension varies markedly by changing this dimensionless ratio. These results suggest applying orthogonal shear while people are running on cornstarch would de-solidify the suspension and cause them to sink.

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