Thermal delay of drop coalescence

We present the results of a combined experimental and theoretical study of drop coalescence in the presence of an initial temperature difference T[subscript 0] between a drop and a bath of the same liquid. We characterize experimentally the dependence of the residence time before coalescence on T[subscript 0] for silicone oils with different viscosities. Delayed coalescence arises above a critical temperature difference T[subscript c] that depends on the fluid viscosity: for T[subscript 0] > T[subscript c], the delay time increases T[subscript 0] [superscript 2/3] as for all liquids examined. This observed dependence is rationalized theoretically through consideration of the thermocapillary flows generated within the drop, the bath and the intervening air layer.National Science Foundation (U.S.) (Grant CMMI-1727565)National Science Foundation (U.S.) (Grant DMS-1614043

Computing the linear viscoelastic properties of soft gels using an Optimally Windowed Chirp protocol

We use molecular dynamics simulations to investigate the linear viscoelastic response of a model three-dimensional particulate gel. The numerical simulations are combined with a novel test protocol (the optimally windowed chirp or OWCh), in which a continuous exponentially varying frequency sweep windowed by a tapered cosine function is applied. The mechanical response of the gel is then analyzed in the Fourier domain. We show that (i) OWCh leads to an accurate computation of the full frequency spectrum at a rate significantly faster than with the traditional discrete frequency sweeps, and with a reasonably high signal-to-noise ratio, and (ii) the bulk viscoelastic response of the microscopic model can be described in terms of a simple mesoscopic constitutive model. The simulated gel response is in fact well described by a mechanical model corresponding to a fractional Kelvin-Voigt model with a single Scott-Blair (or springpot) element and a spring in parallel. By varying the viscous damping and the particle mass used in the microscopic simulations over a wide range of values, we demonstrate the existence of a single master curve for the frequency dependence of the viscoelastic response of the gel that is fully predicted by the constitutive model. By developing a fast and robust protocol for evaluating the linear viscoelastic spectrum of these soft solids, we open the path toward novel multiscale insight into the rheological response for such complex materials

Cell separation using tilted-angle standing surface acoustic waves

Separation of cells is a critical process for studying cell properties, disease diagnostics, and therapeutics. Cell sorting by acoustic waves offers a means to separate cells on the basis of their size and physical properties in a label-free, contactless, and biocompatible manner. The separation sensitivity and efficiency of currently available acoustic-based approaches, however, are limited, thereby restricting their widespread application in research and health diagnostics. In this work, we introduce a unique configuration of tilted-angle standing surface acoustic waves (taSSAW), which are oriented at an optimally designed inclination to the flow direction in the microfluidic channel. We demonstrate that this design significantly improves the efficiency and sensitivity of acoustic separation techniques. To optimize our device design, we carried out systematic simulations of cell trajectories, matching closely with experimental results. Using numerically optimized design of taSSAW, we successfully separated 2- and 10-µm-diameter polystyrene beads with a separation efficiency of ~99%, and separated 7.3- and 9.9-µm-polystyrene beads with an efficiency of ~97%. We illustrate that taSSAW is capable of effectively separating particles–cells of approximately the same size and density but different compressibility. Finally, we demonstrate the effectiveness of the present technique for biological–biomedical applications by sorting MCF-7 human breast cancer cells from nonmalignant leukocytes, while preserving the integrity of the separated cells. The method introduced here thus offers a unique route for separating circulating tumor cells, and for label-free cell separation with potential applications in biological research, disease diagnostics, and clinical practice.National Institutes of Health (U.S.) (Grant U01HL114476)National Institutes of Health (U.S.) (New Innovator Award 1DP2OD007209-01)National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Grant DMR-0820404

Time-Resolved Mechanical Spectroscopy of Soft Materials via Optimally Windowed Chirps

The ability to measure the bulk dynamic behavior of soft materials with combined time and frequency resolution is instrumental for improving our fundamental understanding of connections between the microstructural dynamics and the macroscopic mechanical response. Current state-of-the-art techniques are often limited by a compromise between resolution in the time and frequency domains, mainly due to the use of elementary input signals that have not been designed for fast time-evolving systems such as materials undergoing gelation, curing, or self-healing. In this work, we develop an optimized and robust excitation signal for time-resolved mechanical spectroscopy through the introduction of joint frequency- and amplitude-modulated exponential chirps. Inspired by the biosonar signals of bats and dolphins, we optimize the signal profile to maximize the signal-to-noise ratio while minimizing spectral leakage with a carefully designed modulation of the envelope of the chirp, obtained using a cosine-tapered window function. A combined experimental and numerical investigation reveals that there exists an optimal range of window profiles (around 10% of the total signal length) that minimizes the error with respect to standard single-frequency sweep techniques. The minimum error is set by the noise floor of the instrument, suggesting that the accuracy of an optimally windowed-chirp (OWCh) sequence is directly comparable to that achievable with a standard frequency sweep, while the acquisition time can be reduced by up to 2 orders of magnitude, for comparable spectral content. Finally, we demonstrate the ability of this optimized signal to provide time- and frequency-resolved rheometric data by studying the fast gelation process of an acid-induced protein gel using repeated OWCh pulse sequences. The use of optimally windowed chirps enables a robust time-resolved rheological characterization of a wide range of soft materials undergoing rapid mutation and has the potential to become an invaluable rheometric tool for researchers across different disciplines

Clinical features and outcomes of elderly hospitalised patients with chronic obstructive pulmonary disease, heart failure or both

Background and objective: Chronic obstructive pulmonary disease (COPD) and heart failure (HF) mutually increase the risk of being present in the same patient, especially if older. Whether or not this coexistence may be associated with a worse prognosis is debated. Therefore, employing data derived from the REPOSI register, we evaluated the clinical features and outcomes in a population of elderly patients admitted to internal medicine wards and having COPD, HF or COPD + HF. Methods: We measured socio-demographic and anthropometric characteristics, severity and prevalence of comorbidities, clinical and laboratory features during hospitalization, mood disorders, functional independence, drug prescriptions and discharge destination. The primary study outcome was the risk of death. Results: We considered 2,343 elderly hospitalized patients (median age 81 years), of whom 1,154 (49%) had COPD, 813 (35%) HF, and 376 (16%) COPD + HF. Patients with COPD + HF had different characteristics than those with COPD or HF, such as a higher prevalence of previous hospitalizations, comorbidities (especially chronic kidney disease), higher respiratory rate at admission and number of prescribed drugs. Patients with COPD + HF (hazard ratio HR 1.74, 95% confidence intervals CI 1.16-2.61) and patients with dementia (HR 1.75, 95% CI 1.06-2.90) had a higher risk of death at one year. The Kaplan-Meier curves showed a higher mortality risk in the group of patients with COPD + HF for all causes (p = 0.010), respiratory causes (p = 0.006), cardiovascular causes (p = 0.046) and respiratory plus cardiovascular causes (p = 0.009). Conclusion: In this real-life cohort of hospitalized elderly patients, the coexistence of COPD and HF significantly worsened prognosis at one year. This finding may help to better define the care needs of this population

Infected pancreatic necrosis: outcomes and clinical predictors of mortality. A post hoc analysis of the MANCTRA-1 international study

: The identification of high-risk patients in the early stages of infected pancreatic necrosis (IPN) is critical, because it could help the clinicians to adopt more effective management strategies. We conducted a post hoc analysis of the MANCTRA-1 international study to assess the association between clinical risk factors and mortality among adult patients with IPN. Univariable and multivariable logistic regression models were used to identify prognostic factors of mortality. We identified 247 consecutive patients with IPN hospitalised between January 2019 and December 2020. History of uncontrolled arterial hypertension (p = 0.032; 95% CI 1.135-15.882; aOR 4.245), qSOFA (p = 0.005; 95% CI 1.359-5.879; aOR 2.828), renal failure (p = 0.022; 95% CI 1.138-5.442; aOR 2.489), and haemodynamic failure (p = 0.018; 95% CI 1.184-5.978; aOR 2.661), were identified as independent predictors of mortality in IPN patients. Cholangitis (p = 0.003; 95% CI 1.598-9.930; aOR 3.983), abdominal compartment syndrome (p = 0.032; 95% CI 1.090-6.967; aOR 2.735), and gastrointestinal/intra-abdominal bleeding (p = 0.009; 95% CI 1.286-5.712; aOR 2.710) were independently associated with the risk of mortality. Upfront open surgical necrosectomy was strongly associated with the risk of mortality (p < 0.001; 95% CI 1.912-7.442; aOR 3.772), whereas endoscopic drainage of pancreatic necrosis (p = 0.018; 95% CI 0.138-0.834; aOR 0.339) and enteral nutrition (p = 0.003; 95% CI 0.143-0.716; aOR 0.320) were found as protective factors. Organ failure, acute cholangitis, and upfront open surgical necrosectomy were the most significant predictors of mortality. Our study confirmed that, even in a subgroup of particularly ill patients such as those with IPN, upfront open surgery should be avoided as much as possible. Study protocol registered in ClinicalTrials.Gov (I.D. Number NCT04747990)

Dynamics and rheology of soft phase-change materials

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2019Cataloged from PDF version of thesis.Includes bibliographical references (pages 325-353).Many industrial processes involve multicomponent or composite materials in which one component can undergo a phase transition leading to the appearance of a solid phase dispersed in a liquid-like continuous phase. Examples of soft phase-change materials can be found in a variety of applications from food products (e.g., organogels, casein gels and gelatin), pharmaceutical products (e.g., tissue mimicking phantoms and encapsulating agents), cosmetics (e.g., foundations and lipsticks), and in the oil and gas industry, where formation of paraffin waxes and clathrate hydrates represent major issues for upstream production and flow assurance.Historically, phase-changing materials have been exploited for their unique thermal properties in energy storage applications, however soft solids and complex fluids that undergo phase transformation have broader impact in industrial and biomedical applications because of the dramatic changes in mechanical properties that result from the conditions across the phase transition. Typically, these soft phase-change materials are part of the broader class of elasto-visco-plastic materials, showing both viscoelasticity at small deformations and plasticity at large deformations. However, their material properties are greatly influenced by the specific processing conditions during formation, such as temperature and applied deformation, leading to a thermo-rheological complexity that still poses major challenges for their experimental and theoretical characterization.In this Thesis, we develop novel experimental protocols and theoretical frameworks to characterize and describe the complex rheological behavior of soft phase-changing materials, under both linear and non-linear deformations. We focus mainly on two types of materials that are of major importance in the oil and gas industry: paraffin gels, as model waxy crude oils, and clathrate hydrate suspensions. In the limit of small deformations, we are usually interested in measuring the frequency response of the material as it evolves, or mutates, over time. Current state-of-the-art techniques have major limitations in providing both time- and frequency-resolution primarily due to the type of input signals used. To overcome this, we develop a robust excitation signal that allows us to perform time-resolved mechanical spectroscopy of fast mutating systems. Inspired by the biosonar signals of bats and dolphins, we introduce a joint frequency- and amplitude- modulated chirp signal.Combining experiments and numerical simulations, we show that there exists an optimized range of amplitude modulation that minimizes the estimation error while reducing the total acquisition time by almost two orders of magnitude. With this new technique, which we call the Optimally Windowed Chirp (or OWCh), we then explore the phase transition during gelation of a series of mutating, phase-changing materials, including casein gels, gelatin and paraffin gels. To address large, non-linear deformations, we start from a thorough investigation of the steady state and transient response of paraffin gels under shear. We develop a robust protocol that enables us to systematically extract the main rheological features including the thermokinematic memory (i.e. the effect of thermal and shear history on the rheological behavior of the gel) and thixotropy (i.e. the time-dependent behavior under constant applied deformation).We show that these features can be understood in terms of microstructural rearrangements of the underlying solid particle network, which can be quantified through differential scanning calorimetry, birefringence imaging and rheometry. Based on this understanding, we present a constitutive framework that captures all of the different features while respecting thermodynamic and objectivity constraints. We also investigate mechanical instabilities that may arise during rheological measurements. Combining ultrasonic image velocimetry and rheometry, we show that both shear banding and slip can take place during steady shear below a critical value of the shear rate. However, the thixotropic nature of these materials precludes the banding instability from growing in the sheared region of the gap, ensuring that the measured stress response corresponds to the real bulk behavior. Finally, we study the visco-plastic response of clathrate hydrate suspensions.To do so, we develop a novel method to robustly control their formation, which so far has been a major issue in experimental studies due to uncontrolled nucleation and growth of hydrate crystals. Our method, based on the use of "frozen emulsions", decreases the induction time by orders of magnitude while guaranteeing that all the water droplets initially frozen into ice particles are converted into hydrate particles. Rheological measurements for different water volume fractions and shear rates reveal that the macroscopic rheological response is again governed by rearrangements of the microstructure; however, due to the very strong interparticle forces (which are the result of a continuous sintering process) the microstructure evolves towards a fully connected network that behaves as a porous solid structure.Incorporating this limit into our theoretical model, we show that the framework developed for softer interparticle interaction can also capture the macroscopic plastic response of hydrate suspensions. The results from this Thesis have the potential to impact many industrial processes that involve soft phase-change materials, such as flow assurance and oil extraction, thermal energy storage, gas transport and storage, and other processes where the dynamics of gelation are used to control the rheological properties of the ultimate product.by Michela Geri.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Mechanical Engineerin

Viscoelastic fishbones

This paper is associated with a poster winner of a 2018 APS/DFD Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original poster is available from the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2018.GFM.P0045