Passive solute separation in AC electroosmosis including surface charge-coupled hydrodynamic slip effects.

Apparent wall-slip of colloidal hard-sphere suspensions in microchannel flow

We study the origins of the dynamic contact angle in a two-dimensional lattice-Boltzmann model of immiscible fluids. We show that the dynamic contact angle changes as a function of capillary number as observed in laboratory experiments and explain how this dependence arises in the lattice-Boltzmann model. We also explain how the fluid-fluid interface can move while retaining its shape. The interface has an apparent slip length. The apparent slip follows the classical Navier slipping rule where the velocity of the fluid at the wall is proportional to the viscous stress at the wall. This apparent slip length is proportional to the viscous length scale associated with the spurious flow induced by uncompensated stress at the three-phase contact point.

Velocity slip in electroosmotic flows (EOF) is investigated using an analytical solution of Poisson–Boltzmann and Stokes (PB-S) equations with slip correction, and the results are compared with the predictions from molecular dynamics (MD) simulations. Particularly, EOFs of NaCl solution in silicon nanochannels are simulated, and channel dimensions are picked to exhibit plug flow behavior in the bulk region. Electrochemical conditions are specified within the validity region of PB theory to avoid charge inversion and flow reversal effects. The analytical model shows normalization of the slip length (an interfacial property) with the Debye length determined by the ionic concentration, implying that the slip enhancement in EOFs is independent of the channel height. MD results are compared with the analytical solution using the apparent viscosity and slip lengths determined by force-driven flow simulations in charged silicon nanochannels. Onset of slip velocity within the electrical double layer and its effects on EOF are investigated as a function of the liquid–wall interaction strength. Good agreements between the analytical solutions and MD results validate PB-S equations in nanochannels as small as 3.5 nm. Results validate that slip enhancement in EOFs is independent of the channel height, which constitutes a theoretical basis for the slip-length measurements on hydrophobic dielectric surfaces using EOFs.

Three mathematical models are developed for meniscus roll coating in which there is steady flow of a Newtonian fluid in the narrow gap, or nip, between two contrarotating rolls in the absence of body forces.The zero flux model predicts a constant pressure gradient within the central core and two eddies, each with an inner structure, in qualitative agreement with observation. The small flux model takes account of a small inlet flux and employs the lubrication approximation to represent fluid velocity as a combination of Couette and Poiseuille flows. Results show that the meniscus coating regime is characterized by small flow rates (λ [Lt ] 1) and a sub-ambient pressure field generated by capillary action at the upstream meniscus. Such flows are found to exist for small modified capillary number, Ca(R/H0)1/2 [lsim ] 0.15, where Ca and R/H0 represent capillary number and the radius to semi-gap ratio, respectively.A third model incorporates the full effects of curved menisci and nonlinear free surface boundary conditions. The presence of a dynamic contact line, adjacent to the web on the upper roll, requires the imposition of an apparent contact angle and slip length. Numerical solutions for the velocity and pressure fields over the entire domain are obtained using the finite element method. Results are in accord with experimental observations that the flow domain consists of two large eddies and fluid transfer jets or ‘snakes’. Furthermore, the numerical results show that the sub-structure of each large eddy consists of a separatrix with one saddle point, two sub-eddies with centres, and an outer recirculation.Finally finite element solutions in tandem with lubrication analysis establish the existence of three critical flow rates corresponding to a transformation of the pressure field, the emergence of a ‘secondary snake’ (another fluid transfer jet) and the disappearance of a primary snake.

To characterize fast and slow diffusion components in diffusion-weighted magnetic resonance imaging (DW-MRI) of pediatric Crohn's disease (CD). Overall diffusivity reduction as measured by the apparent diffusion coefficient (ADC) in patients with CD has been previously demonstrated. However, the ADC reduction may be due to changes in either fast or slow diffusion components. In this study we distinguished between the fast and slow diffusion components in the DW-MRI signal decay of pediatric CD. We acquired MRI from 24 patients, including MR enterography (MRE) and DW-MRI with 8 b-values (0-800 s/mm(2)). We characterized fast and slow diffusivity by intravoxel incoherent motion (IVIM) model parameters (f, D*, D), and overall diffusivity by ADC values. We determined which model best described the DW-MRI signal decay. We assessed the influence of the IVIM model parameters on the ADC. We evaluated differences in model parameter values between the enhancing and nonenhancing groups. The IVIM model described the observed data significantly better than the ADC model (P = 0.0088). The ADC was correlated with f (r = 0.67, P = 0.0003), but not with D (r = 0.39, P = 0.062) and D* (r = -0.39, P = 0.057). f values were significantly lower (P < 0.003) and D* values were significantly higher (P = 0.03) in the enhancing segments, while D values were not significantly different between the groups (P = 0.14). For this study population the IVIM model provides a better description of the DW-MRI signal decay than the ADC model. The reduced ADC is related to changes in the fast diffusion rather than to changes in the slow diffusion.

Experiments are reported for Te and Ag implantation in silicon, as examples of slow and fast diffusers, after furnace or laser annealing. Slow diffusers are substitutionally located at concentrations in great excess of the maximum solid solubility after both processes. Fast diffusers inhibit the solid-phase epitaxial regrowth or are rejected at the sample surface after laser irradiation. Although the epitaxial growth occurs with velocities which differ up to ten orders of magnitude after furnace or laser annealing, the supersaturation is interpreted as due to the same basic mechanism: solute trapping at the moving interface when the residence time is larger than the one monolayer regrowth time. This process is controlled by the diffusion coefficient in the two adjacent phases.

In this paper, a series of experiments are presented which demonstrate drag reduction for the laminar flow of water through microchannels using superhydrophobic surfaces with random surface microstructure. These superhydrophobic surfaces were fabricated with a simple, inexpensive technique of sanding polytetrafluoroethylene (PTFE) with sandpaper having grit sizes between 120- and 600-grit. A microfluidic device was used to measure the pressure drop as a function of the flow rate to determine the drag reduction and slip length of each surface. A maximum pressure drop reduction of 27 % and a maximum apparent slip length of b = 20 μm were obtained for the superhydrophobic surfaces created by sanding PTFE with a 240-grit sandpaper. The pressure drop reduction and slip length were found to increase with increasing mean particle size of the sandpaper up to 240-grit. Beyond that grit size, increasing the pitch of the surface roughness was found to cause the interface to transition from the Cassie–Baxter state to the Wenzel state. This transition was observed both as an increase in the contact angle hysteresis and simultaneously as a reduction in the pressure drop reduction. For these randomly rough surfaces, a correlation between the slip length and the contact angle hysteresis was found. The surfaces with the smallest contact angle hysteresis were found to also have the largest slip length. Finally, a number of sanding protocols were tested by sanding preferentially along the flow direction, across the flow direction and with a random circular pattern. In all cases, sanding in the flow direction was found to produce the largest pressure drop reduction.

The goal of medical image translation tasks is to design models that transform source domain images to target domain images, with the potential to significantly enhance diagnostic accuracy and reduce patient radiation exposure. Current advanced methods employ generative models to characterize the conditional distribution of target domain images, typically incorporating high frequency edge information from source domain images as generation conditions to maintain anatomical consistency between the two domains. However, directly introducing edge information may introduce noise and misleading information, adversely affecting the generative model. To address this issue, we propose a Fast and Slow Diffusion framework with Fuzzy Contextual Representation. Specifically, High-frequency edge images undergo fast diffusion while original images follow a slow diffusion process, coupled by a contextual pool that dynamically stores edge patterns from the fast path. Furthermore, we apply fuzzification to the contextual pool to enhance its expressiveness and use the resulting fuzzy representations to guide the reverse process of the slow diffusion, ensuring the accurate preservation of edge structures. Comprehensive experiments demonstrate that our proposed framework achieves performance comparable to existing state-of-the-art methods in medical image translation tasks. In addition, quantitative experiments demonstrate that fuzzified edge representations enhance generative models' capability in precisely perceiving and retaining high-frequency components.

The garnet Li7La3Zr2O­12is a promising candidate as a solid electrolyte due to its stability and fast lithium diffusion. Grain boundaries have been shown to impede Li+conductivity in Li7La3Zr2O­12.1To investigate the origin of slow grain boundary diffusion a lithium deficient system Li6.75La3Zr2O­12(LLZO) was studied. An LLZO system with Frenkel defects, and an amorphous system were used as proxies for the grain boundary diffusion in classical molecular dynamics (CMD) simulations. CMD simulations were performed on a variety of cell densities in crystalline and disordered LLZO systems, especially because the grain boundary is likely to be less dense than the bulk. There are a wide range of misorientation angles so disordered systems were used as a proxy for diffusion in the grain boundaries instead of simulating highly symmetric grain boundaries. We find that the disordered system has pockets with more free volume that trap Li-ions, slowing diffusion. Results from the crystalline simulations confirm that lower density and thus more free volume decreases the rate of diffusion. Activation energies and diffusion pre-factors were extracted from Arrhenius plots of diffusion at temperatures ranging from 500 K to 2000 K for systems with volumes ranging from 92% to 108% of the experimental cell density. The activation energies were found to decrease for higher density cells even though one could expect bottleneck effects at the transition state for Li+ diffusion. The activation energies for the disordered systems were found to be higher than the ordered systems. The pre-factor and activation energy of the ordered system increase abruptly when cell density is 94% of the experimental cell density. To correlate the probability of diffusion events with higher density, Voronoi analysis using zeo++2and site analysis module Sitator3were used to quantify the size of pockets and transition state geometries.Voronoi analysis on the non-lithium sublattice reveals activation energy increases with larger diffusion channels. Site type analysis shows a correlation between long lithium residence times and larger site volumes. These results confirm that when lithium has more free volume it has slower diffusion. Furthermore, grain boundaries with densities less than crystalline LLZO can act as Li+traps slowing diffusion. Activation energies and diffusion coefficients were used as inputs to a phase-field model of polycrystalline LLZO that determines the effective diffusion with bulk and grain boundary contributions. The effects of microstructure on effective diffusion were investigated by varying grain size, percent of grain boundaries, and grain shape. The results from different microstructures provides insight into diffusion around and through grain boundaries for the LLZO system. (1) Tenhaeff, W. E.; Rangasamy, E.; Wang, Y.; Sokolov, A. P.; Wolfenstine, J.; Sakamoto, J.; Dudney, N. J. Resolving the Grain Boundary and Lattice Impedance of Hot-Pressed Li 7La 3Zr 2O 12Garnet Electrolytes. ChemElectroChem 2014,1(2), 375–378. https://doi.org/10.1002/celc.201300022. (2) Willems, T. F.; Rycroft, C. H.; Kazi, M.; Meza, J. C.; Haranczyk, M. Algorithms and Tools for High-Throughput Geometry-Based Analysis of Crystalline Porous Materials. Microporous and Mesoporous Materials 2012, 149(1), 134–141. https://doi.org/10.1016/j.micromeso.2011.08.020. (3) Kahle, L.; Musaelian, A.; Marzari, N.; Kozinsky, B. Unsupervised Landmark Analysis for Jump Detection in Molecular Dynamics Simulations. arXiv:1902.02107 [cond-mat] 2019.

The liver and spleen have a similar amount of blood perfusion, and the spleen is waterier than the liver. The spleen tissue has a higher contrast-enhanced computed tomography (CT) extracellular volume fraction than the liver. The spleen has been reported to have a much lower apparent diffusion coefficient (ADC), intravoxel incoherent motion (IVIM)-perfusion fraction (PF), and IVIM-Dslow than those of the liver, which appears to be unreasonable. As hepatocellular carcinomas (HCCs) are mostly associated with increased blood supply and increased proportion of arterial blood supply and with edema, HCC has been reported to have a lower ADC, lower IVIM-PF, and lower IVIM-Dslow than liver parenchyma, which appears to be unreasonable. Cysts are known to have a longer T2 and a higher ADC than hemangiomas. Due to the 'flushing' of blood flow inside the hemangioma, we hypothesize that the actual diffusion of hemangioma liquid is faster than the more 'static' liquid of the cysts. As ADC measure is heavily affected by T2 and in order to minimize the T2 effect, we propose a new metric reflecting tissue slow diffusion which is termed 'slow diffusion coefficient (SDC)': SDC = [S(b1 ) - S(b2 )]/(b2 - b1 ), where b1 and b2 refers to a high b-value (e.g., 400 s/mm2 for liver) and a higher b-value (e.g., 600 s/mm2) respectively, where S(b1 ) and S(b2 ) denote the corresponding diffusion weighted image signal intensity. This study utilized a random selection of authors' historical liver IVIM magnetic resonance imaging (MRI) data. For 1.5T data, SDC was calculated with b=600 and 800 s/mm2 images. For 3.0T data, SDC was calculated with b=400 and 600 s/mm2 images. With 1.5T data, SDC was calculated for 10 healthy volunteer cases' liver and spleen parenchyma, as well as two simple liver cysts and their corresponding liver parenchyma. With 3.0T data, SDC was calculated for 14 cases' liver and spleen parenchyma, as well as 13 HCC masses, 9 simple liver cysts, 13 hemangiomas and their corresponding liver parenchyma. As in vivo diffusion metrics can be only measured with MRI thus external validation is not possible, the measures of liver parenchyma were used to normalize the measures of spleen, HCC, cyst, and hemangioma, and the ratios were expressed in median value. The median ratio of SDCspleen/SDCliver was 2.47 for 1.5T data and 1.97 for 3.0T data. Two cysts at 1.5T had a median SDCcyst/SDCliver ratio of 2.92. For 3.0T data, the median ratios of SDCHCC/SDCliver, SDCcyst/SDCliver, SDChemangioma/SDCliver were 2.83, 4.23, and 5.37, respectively. However, the ADCspleen/ADCliver ratios were always <0.81 even when calculated with various combinations of high b-values. The spleen has a faster diffusion than the liver, HCCs have a faster diffusion than the adjacent liver parenchyma, and hemangiomas have a faster diffusion than simple cysts. Although it is known that cysts have a substantially longer T2 than hemangiomas, SDC of hemangioma was higher than that of cysts, suggesting 'T2 effect' is minimized for SDC.

Analyses of microchannel and microgap cooling show that galinstan, a recently developed nontoxic liquid metal that melts at −19 °C, may be more effective than water for direct liquid cooling of electronics. The thermal conductivity of galinstan is nearly 28 times that of water. However, since the volumetric specific heat of galinstan is about half that of water and its viscosity is 2.5 times that of water, caloric, rather than convective, resistance is dominant. We analytically investigate the effect of using structured surfaces (SSs) to reduce the overall thermal resistance of galinstan-based microgap cooling in the laminar flow regime. Significantly, the high surface tension of galinstan, i.e., 7 times that of water, implies that it can be stable in the nonwetting Cassie state at the requisite pressure differences for driving flow through microgaps. The flow over the SS encounters a limited liquid–solid contact area and a low viscosity gas layer interposed between the channel walls and galinstan. Consequent reductions in friction factor result in decreased caloric resistance, but accompanying reductions in Nusselt number increase convective resistance. These are accounted for by expressions in the literature for apparent hydrodynamic and thermal slip. We develop a dimensionless expression to evaluate the tradeoff between the pressure stability of the liquid–solid–gas system and hydrodynamic slip. We also consider secondary effects including entrance effects and temperature dependence of thermophysical properties. Results show that the addition of SSs enhances heat transfer.

Larmor frequency dependence on structural anisotropy of magnetically heterogeneous media

Hydrophobic surfaces with large slip lengths have the potential to enhance electro-osmotic flows. Existing theories of electroosmosis in hydrophobic channels postulate immobile surface charges and/or make a number of simplifying assumptions by considering mostly weakly charged surfaces and thin diffuse layers compared to channel dimension. In this paper, we extend prior models by focusing on planar and cylindrical nanochannels. Our theory accounts for a hydrodynamic slip and a mobility of surface charges, and is valid not only on the scale of the nanochannel with thin diffuse layers, but also on the scale of the overlapping diffuse layer. The model is simple enough to allow us to derive analytical approximations for the electro-osmotic velocities even when the surface potential and charge density are quite large. We also present numerical solutions to validate the analysis and illustrate the variation of electro-osmotic velocities in response to changes in the channel size, potential, surface charge and its mobility, hydrodynamic slip length, and salt concentration. Our results are directly relevant for carbon nanotubes, graphene nanochannels, and conventional nanoporous membranes.

Nanoporous anodic aluminum oxide (AAO) membranes are now being explored for use in advanced chemical separations, including in the dehydration of biofuels such as ethanol. Optimization of membrane performance requires an in-depth understanding of how solvent mixtures and solutes behave under nanoconfinement. In this work, the diffusion of rhodamine B (RhB) dye through 10 and 20 nm AAO nanopores filled with a series of ethanol/water mixtures (0–33% water) is explored by fluorescence correlation spectroscopy (FCS). RhB was found to diffuse through the pores by two distinct mechanisms with mean diffusion coefficients, Df and Ds, reflecting fast and slow diffusive motions, respectively, with values that differ by nearly 2 orders of magnitude. Both Df and Ds increased with pore size and were significantly smaller than Db, the RhB diffusion coefficient in bulk liquid. Mean Df values follow a composition-dependent trend that closely mimics the viscosity dependence of Db. Additional slowing of fast RhB diffusion is attributed to both hydrodynamic drag and electrostatic interactions with the nanopore surface. The mean Ds values exhibit a different trend with increasing water content, revealing an increase in Ds and a decrease in the contributions of slow diffusion to the observed dynamics. The fluorescence time transient data used in the analysis show that the slow diffusion process is strongly hindered and likely involves frequent adsorption of RhB to the pore surfaces. These results provide new insights into the detailed molecular-level mechanisms of mass transport in nanoporous AAO membranes.

Understanding fluid behavior at the liquid-solid interface is essential for advancing micro/nanoscale technologies, yet current models struggle to capture the complexity introduced by surface heterogeneity. In this study, we investigate the interplay between nanoscale surface roughness, wettability, and hydrodynamic slip by using molecular dynamics simulations on nanopatterned silica surfaces. By systematically varying the droplet size and surface geometry, we characterize the influence of nanoscale pinning on contact angle hysteresis and determine the macroscopic equilibrium contact angles for six distinct rough surfaces. Parallel simulations of force-driven water flow through nanochannels reveal the local and average slip lengths associated with each surface. For smooth surfaces, slip length correlates well with contact angle, aligning with predictions from well-known theoretical models by Huang et al. and Sendner et al. However, on nanopatterned surfaces, significant deviations arise due to contact line pinning, which disrupts the expected correlation between wettability and slip. We observe a clear decoupling between hydrophobicity and interfacial mobility, which can be exemplified by rose petal-like behavior where high contact angles are accompanied by strong pinning and minimal slip. These findings underscore the limitations of existing theoretical models when applied to heterogeneous interfaces and emphasize the need for refined frameworks that account for pinning effects. This study offers such a framework, providing new insight into interfacial transport and guiding the design of functionally structured surfaces.