Wide Constriction Research Articles

Geometry‐driven coalescence of water droplets in a highly viscous fluid

AbstractThis work is dedicated to numerical studies of the coalescence of water droplets moving in bitumen. The Navier–Stokes equations coupled with the volume of fluid model (VOF) using adaptive grids techniques, available in the commercial computational fluid dynamics (CFD) software Ansys Fluent 20R2, were solved numerically to investigate the behaviour of water droplets with diameters from 1 to 100 μm moving in rapidly converging and diverging microchannels of different sizes. The model has been validated against 3‐D experimental data published in the literature. A good agreement has been demonstrated. The results of simulations revealed that the main parameter influencing coalescence is the bulk flow velocity in the channel. Analysis of unsteady simulations showed the existence of a critical flow velocity, above which no coalescence occurs, corresponding to capillary number Ca < 0.5 for droplets Reynolds number Re < 0.1. Besides, image processing analysis has been used for a mean droplet size estimation in different geometries. A mean size significantly increased due to the late coalescence occurring in a wider constriction.

Giant Magnetoresistance in a Chemical Vapor Deposition Graphene Constriction.

Magnetic field-driven insulating states in graphene are associated with samples of very high quality. Here, this state is shown to exist in monolayer graphene grown by chemical vapor deposition (CVD) and wet transferred on Al2O3 without encapsulation with hexagonal boron nitride (h-BN) or other specialized fabrication techniques associated with superior devices. Two-terminal measurements are performed at low temperature using a GaAs-based multiplexer. During high-throughput testing, insulating properties are found in a 10 μm long graphene device which is 10 μm wide at one contact with an ≈440 nm wide constriction at the other. The low magnetic field mobility is ≈6000 cm2 V–1 s–1. An energy gap induced by the magnetic field opens at charge neutrality, leading to diverging resistance and current switching on the order of 104 with DC bias voltage at an approximate electric field strength of ≈0.04 V μm–1 at high magnetic field. DC source–drain bias measurements show behavior associated with tunneling through a potential barrier and a transition between direct tunneling at low bias to Fowler-Nordheim tunneling at high bias from which the tunneling region is estimated to be on the order of ≈100 nm. Transport becomes activated with temperature from which the gap size is estimated to be 2.4 to 2.8 meV at B = 10 T. Results suggest that a local electronically high quality region exists within the constriction, which dominates transport at high B, causing the device to become insulating and act as a tunnel junction. The use of wet transfer fabrication techniques of CVD material without encapsulation with h-BN and the combination with multiplexing illustrates the convenience of these scalable and reasonably simple methods to find high quality devices for fundamental physics research and with functional properties.

Revisiting abnormal slow translocation of ssDNA through the 2.3 nm wide gamma-hemolysin nanopore

Biological nanopores have been used extensively in biomolecule sensing, especially in sequencing DNA and protein. The wildly studied three biological nanopores are α-hemolysin, MspA, and Aerolysin, whose constriction zones are 1.4 nm, 1.2 nm, and 1.0 nm, respectively; while the translocation times of ssDNA are about 1-5 μs/nt. Interestingly, γ-hemolysin (γ-HL) protein channel with a 2.3-nm wide constriction zone can assemble into an octameric pore on a planar lipid bilayer, however, the translocation rate of ssDNA through the γ-HL pore is 16 μs/nt.

Sub-10 nm patterning of few-layer MoS2 and MoSe2 nanolectronic devices by oxidation scanning probe lithography

The properties of 2D materials devices are very sensitive to the physical, chemical and structural interactions that might happen during processing. Low-invasive patterning methods are required to fabricate devices at the nanoscale. Here we developed a process that combines oxidation scanning probe lithography (o-SPL) and oxygen plasma to fabricate nanoribbon field-effect transistors and nano-constrictions on few-layer MoS2 and MoSe2. The oxygen plasma has a double role in this process. First, it forms a thin, uniform oxide layer on top of the flake surface to enable o-SPL nanopatterning with full control of shape and size. Second, the oxide layer thins down the flake. Both plasma-based and o-SPL oxides are soluble in deionized H2O, which enabled etching and the definition of electrically isolated nano-constrictions and nanoribbons. The accuracy and robustness of the process was applied to pattern sub-10 nm wide constrictions and nanoribbon transistors.

Mechanical Low-Pass Filtering of Cells for Detection of Circulating Tumor Cells in Whole Blood.

The detection of circulating tumor cells (CTCs) from liquid biopsies using microfluidic devices is attracting a considerable amount of attention as a new, less-invasive cancer diagnostic and prognostic method. One of the drawbacks of the existing antibody-based detection systems is the false negatives for epithelial cell adhesion molecule detection of CTCs. Here we report a mechanical low-pass filtering technique based on a microfluidic constriction and electrical current sensing system for the novel CTC detection in whole blood without any specific antigen-antibody interaction or biochemical modification of the cell surface. The mechanical response of model cells of CTCs, such as HeLa, A549, and MDA-MB-231 cells, clearly demonstrated different behaviors from that of Jurkat cells, a human T-lymphocyte cell line, when they passed through the 6-μm wide constriction channel. A 6-μm wide constriction channel was determined as the optimum size to identify CTCs in whole blood with an accuracy greater than 95% in tens of milliseconds. The mechanical filtering of cells at a single cell level was achieved from whole blood without any pretreatment (e.g., dilution of lysing) and prelabeling (e.g., fluorophores or antibodies).

Abstract 1897: The role of nuclear deformation and rupture in breast cancer cell migration

Abstract Invasion of tumor cells into the surrounding tissue is a critical step in the metastatic cascade, which is responsible for the vast majority of cancer-related deaths. Recent findings have begun to demonstrate that the biophysical properties of the cell nucleus play a critical role in this process. During invasion, cancer cells must often pass through interstitial spaces much smaller than the cell nucleus. While the cell cytoplasm easily maneuvers through confined spaces, the deformability of the nucleus, which is the largest and stiffest organelle, ultimately determines how efficiently a cell can pass through a tight space. Furthermore, as the nucleus deforms to move through a confined space, it can exhibit transient loss of nuclear envelope (NE) integrity, where the nuclear membranes rupture and nuclear and cytoplasmic components interchange. We previously showed that chromatin is exposed to the cell cytoplasm during NE rupture, and cells can incur DNA damage. While this work revealed that cancer cell migration requires nuclear deformation and can involve NE rupture, it is currently unknown whether such events correlate with metastatic potential. We used a panel of human breast cancer cells lines, including claudin-low, basal-like, and lumen-like cells. Cells were stably labeled with a green fluorescent protein fused to a nuclear localization sequence (NLS-copGFP) to investigate whether some cell lines were particularly well suited to migrate through tight spaces, and if any were particularly prone to NE rupture. The cells were loaded into microfluidic devices that were designed to mimic interstitial in vivo spacing and monitored by time-lapse microscopy. Devices contained constrictions 5 µm tall and 1, 2, or 15 µm wide. We found that claudin-low cell lines such as BT-549 migrated through confined spaces (1+2 µm wide constrictions) significantly faster and more efficiently than basal-like HCC70 cancer cells. In contrast, the various cell lines displayed similar migration rates in the 15 µm wide constrictions, which do not require nuclear deformation. Thus, the differences observed between the cell lines reflect specific defects in migration through confined spaces, and not general migration defects. We are currently expanding our studies to a larger panel of cancer cell lines and to mouse breast cancer models. If confirmed, these results could indicate that highly invasive cancer cells may benefit from more deformable nuclei that facilitate movement through tight spaces encountered in the tissue microenvironment. Citation Format: Alexandra L. McGregor, Joshua J. Elacqua, Emily S. Bell, Jan Lammerding. The role of nuclear deformation and rupture in breast cancer cell migration [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1897. doi:10.1158/1538-7445.AM2017-1897

A Size-Selective Intracellular Delivery Platform.

Identifying and separating a subpopulation of cells from a heterogeneous mixture are essential elements of biological research. Current approaches require detailed knowledge of unique cell surface properties of the target cell population. A method is described that exploits size differences of cells to facilitate selective intracellular delivery using a high throughput microfluidic device. Cells traversing a constriction within this device undergo a transient disruption of the cell membrane that allows for cytoplasmic delivery of cargo. Unique constriction widths allow for optimization of delivery to cells of different sizes. For example, a 4 μm wide constriction is effective for delivery of cargo to primary human T-cells that have an average diameter of 6.7 μm. In contrast, a 6 or 7 μm wide constriction is best for large pancreatic cancer cell lines BxPc3 (10.8 μm) and PANC-1 (12.3 μm). These small differences in cell diameter are sufficient to allow for selective delivery of cargo to pancreatic cancer cells within a heterogeneous mixture containing T-cells. The application of this approach is demonstrated by selectively delivering dextran-conjugated fluorophores to circulating tumor cells in patient blood allowing for their subsequent isolation and genomic characterization.

Imaging coherent transport in chemical vapor deposition graphene wide constriction by scanning gate microscopy

We use a scanning gate microscopy to perturb coherent transport in chemical vapor deposition (CVD) graphene wide constriction. Particularly, we observe conductance oscillations in the wide constriction region (W ∼ 800 nm) characterized by spatial conductance variations, which imply formation of the nanometer-scale ring structure due to the merged domains and intrinsic grain boundaries. Moreover, additional hot charges from high current can suppress the coherent transport, suggesting that the hot carriers with a wide spreading kinetic energy could easily tunnel merged domains and intrinsic grain boundaries in CVD-grown graphene due to the heating effect, a great advantage for applications in graphene-based interference-type nano-electronics.

Probing weak localization in chemical vapor deposition graphene wide constriction using scanning gate microscopy

Low-temperature scanning gate microscopy (LT-SGM) studies of graphene allow one to obtain important spatial information regarding coherent transport such as weak localization (WL) and universal conductance fluctuations. Although fascinating LT-SGM results on pristine graphene prepared by mechanical exfoliation have been reported in the literature, there appears to be a dearth of LT-SGM results on chemical vapor deposition (CVD)-grown graphene whose large scale and flexible substrate transferability make it an ideal candidate for coherent electronic applications. To this end, we have performed LT-SGM studies on CVD-grown graphene wide constriction (0.8 μm), which can be readily prepared by cost-effective optical lithography fully compatible with those in wafer foundry, in the WL regime. We find that the movable local gate can sensitively modulate the total conductance of the CVD graphene constriction possibly due to the intrinsic grain boundaries and merged domains, a great advantage for applications in coherent electronics. Moreover, such a conductance modulation by LT-SGM provides an additional, approximately magnetic-field-independent probe for studying coherent transport such as WL in graphene and spatial conductance variation.

High Current-Induced Electron Redistribution in a CVD-Grown Graphene Wide Constriction

Investigating the charge transport behavior in one-dimensional quantum confined system such as the localized states and interference effects due to the nanoscale grain boundaries and merged domains in wide chemical vapor deposition graphene constriction is highly desirable since it would help to realize industrial graphene-based electronic device applications. Our data suggests a crossover from interference coherent transport to carriers flushing into grain boundaries and merged domains when increasing the current. Moreover, many-body fermionic carriers with disordered system in our case can be statistically described by mean-field Gross-Pitaevskii equation via a single wave function by means of the quantum hydrodynamic approximation. The novel numerical simulation method supports the experimental results and suggests that the extreme high barrier potential regions on graphene from the grain boundaries and merged domains can be strongly affected by additional hot charges. Such interesting results could pave the way for quantum transport device by supplying additional hot current to flood into the grain boundaries and merged domains in one-dimensional quantum confined CVD graphene, a great advantage for developing graphene-based coherent electronic devices.

Scanning gate microscopy of localized states in wide graphene constrictions

In graphene nanoconstrictions, charge puddles due to substrate-induced potential inhomogeneities can form quantum dots. We use a scanning gate microscope to probe a short, wide graphene constriction and observe a set of ring-shaped resonances in maps of the graphene conductance as a function of the local potential perturbation, a signature of quantum dot-mediated transport. By varying the scanning tip height and applied tip voltage, we observe the screening effect of the nearby graphene. Introducing a simple capacitive model to account for screening, we model the conduction of the graphene constriction and how the tip gates the quantum dots. We identify the locations of some of the quantum dots and extract their parameters.

Low-Noise Nano Superconducting Quantum Interference Device Operating in Tesla Magnetic Fields

Superconductivity in the cuprate YBa(2)Cu(3)O(7) (YBCO) persists up to huge magnetic fields (B) up to several tens of Teslas, and sensitive direct current (dc) superconducting quantum interference devices (SQUIDs) can be realized in epitaxially grown YBCO films by using grain boundary Josephson junctions (GBJs). Here we present the realization of high-quality YBCO nanoSQUIDs, patterned by focused ion beam milling. We demonstrate low-noise performance of such a SQUID up to B = 1 T applied parallel to the plane of the SQUID loop at the temperature T = 4.2 K. The GBJs are shunted by a thin Au layer to provide nonhysteretic current voltage characteristics, and the SQUID incorporates a 90 nm wide constriction which is used for on-chip modulation of the magnetic flux through the SQUID loop. The white flux noise of the device increases only slightly from 1.3 μΦ(0)/(Hz)(1/2) at B = 0 to 2.3 μΦ(0)/(Hz))(1/2) at 1 T. Assuming that a point-like magnetic particle with magnetization in the plane of the SQUID loop is placed directly on top of the constriction and taking into account the geometry of the SQUID, we calculate a spin sensitivity S(μ)(1/2) = 62 μ(B)/(Hz))(1/2) at B = 0 and 110 μ(B)/(Hz))(1/2) at 1 T. The demonstration of low noise of such a SQUID in Tesla fields is a decisive step toward utilizing the full potential of ultrasensitive nanoSQUIDs for direct measurements of magnetic hysteresis curves of magnetic nanoparticles and molecular magnets.

Imaging backscattering through impurity-induced antidots in quantum Hall constrictions

We exploit the biased tip of a scanning gate microscope (SGM) to induce a controlled backscattering between counter-propagating edge channels in a wide constriction in the quantum Hall regime. We compare our detailed conductance maps with a numerical percolation model and demonstrate that conductance fluctuations observed in these devices as a function of the gate voltage originate from backscattering events mediated by localized states pinned by potential fluctuations. Our imaging technique allows us to identify the necessary conditions for the activation of these backscattering processes and also to reconstruct the constriction confinement potential profile and the underlying disorder.

Microfluidic tools to investigate pathologies in the blood microcirculation

We show how microfluidics technology can be used to fabricate simple and innovative biomimetic tools to shed new light on physiopathological events occurring in the blood microcirculation. Examples of applications are given in the context of the acute respiratory distress syndrome (ARDS), an inflammatory disease of the lung triggered by a massive arrest of white blood cells in the lung microvasculature. The main challenge consists in building relevant micro–devices to reproduce key biological characteristics of blood capillaries. We present a series of tools that permit us to decouple the role of the multiple parameters involved in complex biological events. Straight narrow channels with non–adherent walls are used to characterise the passage of a cell in 4 m wide constrictions in the absence of adhesion, whereas channels covered by endothelial cells allow a quantitative measurement of cell adhesion in the absence of mechanical constraints. We show that incubation of white blood cells in sera of ARDS patients increases their stiffness, confirming the role of stiffness on the abnormal sequestration of white blood cells, whereas we could not bring to light a significant adhesion increase. The multiple branches and constrictions of the blood microvasculature network are mimicked here by series of interconnected crenelled constrictions with different symmetries. In symmetric crenels, cells adopt a stable deformed shape after a few constrictions and travel fast through successive constrictions with a constant orientation. In asymmetric channels, cell orientation and trajectory are perturbed between two constrictions. Unfavourable orientations upon entry can yield temporary or even definitive arrests with the stiffest cells. Finally, we present a new artificial micro–vessel with porous walls to mimic the porosity of real blood vessels. This new tool is useful to observe directly under a microscope the late stages of inflammation in the microvasculature such as immune cells transmigration, or the infection of a micro–vessel by pathogenic bacteria.

Possible early foraminiferans in post-Sturtian (716−635 Ma) cap carbonates

Foraminifera are an ecologically important group of modern heterotrophic amoeboid eukaryotes whose naked and testate ancestors are thought to have evolved ∼1 Ga ago. However, the single-chambered agglutinated tests of these protists appear in the fossil record only after ca. 580 Ma, coinciding with the appearance of macroscopic and mineralized animals. Here we report the discovery of small, slender tubular microfossils in the Sturtian (ca. 716–635 Ma) cap carbonate of the Rasthof Formation in Namibia. The tubes are 200–1300 μm long and 20–70 μm wide, and preserve apertures and variably wide lumens, folds, constrictions, and ridges. Their sometimes flexible walls are composed of carbonaceous material and detrital minerals. This combination of morphologic and compositional characters is also present in some species of modern single-chambered agglutinated tubular foraminiferans, and is not found in other agglutinated eukaryotes. The preservation of possible early Foraminifera in the carbonate rocks deposited in the immediate aftermath of Sturtian low-latitude glaciation indicates that various morphologically modern protists thrived in microbially dominated ecosystems, and contributed to the cycling of carbon in Neoproterozoic oceans much before the rise of complex animals.

Allometric Comparison of Skulls from Two Closely Related Weasels,Mustela itatsiandM. sibirica

We conducted an interspecific comparison of skulls from two closely related but differently sized mustelid species, Mustela itatsi and M. sibirica (Mammalia, Carnivora, Mustelidae); a sexual comparison within the latter species showed remarkable size dimorphism. We clarified several differences in skull proportion related to size using allometric analyses and qualitative comparisons. Allometric analysis revealed that the skulls of male M. itatsi (the smaller species) have a relatively long palate; a slender viscerocranium and postorbital constriction; a broad, short, and low neurocranium; small carnassials; and a short mandible with a thin body and small ramus compared to the skulls of male M. sibirica (the larger species). Similar results were obtained when male M. itatsi were compared to female M. sibirica, although the male M. itatsi had a broader viscerocranium than female M. sibirica. A sexual comparison in M. sibirica revealed a larger skull size among the males with a relatively wide viscerocranium; wide postorbital constriction; a slender, long, and high neurocranium; short and wide auditory bullae; short carnassials; and a long and high mandible compared to females. Qualitative comparisons revealed changes in a few characters depending on skull size or with respect to some cranial components in each species. The interspecific differences observed were clearly larger than the intraspecific differences for three qualitative characters. The allometric and qualitative differences detected between these species suggest that each species is not simply the dwarf and/or giant morph of the other, and complicated differences were clarified.

Electron interferometry in the quantum Hall regime: Aharonov-Bohm effect of interacting electrons

An apparent $h/fe$ Aharonov-Bohm flux period, where $f$ is an integer, has been reported in coherent quantum Hall devices. Such subperiod is not expected for noninteracting electrons and thus is thought to result from interelectron Coulomb interaction. Here we report experiments in a Fabry-Perot interferometer comprised of two wide constrictions enclosing an electron island. By carefully tuning the constriction front gates, we find a regime where interference oscillations with period $h/2e$ persist throughout the transition between the integer quantum Hall plateaus 2 and 3, including half-filling. In a large quantum Hall sample, a transition between integer plateaus occurs near half-filling, where the bulk of the sample becomes delocalized and thus dissipative bulk current flows between the counterpropagating edges (``backscattering''). In a quantum Hall constriction, where conductance is due to electron tunneling, a transition between forward and backscattering is expected near the half-filling. In our experiment, neither period nor amplitude of the oscillations show a discontinuity at half-filling, indicating that only one interference path exists throughout the transition. We also present experiments and an analysis of the front-gate dependence of the phase of the oscillations. The results point to a single physical mechanism of the observed conductance oscillations: Aharonov-Bohm interference of interacting electrons in quantum Hall regime.

Different Sonographic Pictures of Traumatic Hepatic Herniation and Reasons for Their Differences

Sonographic features used in the diagnosis of diaphragmatic rupture with hepatic herni-ation have been reported in several previous studies. Most of those studies demonstrated the sonographic pictures, but did not discuss the reasons behind the formation of those particular pictures. We report three cases of traumatic hepatic herniation with different sonographic pictures and discuss the reasons for their differences. Case 1 represents a negative sonographic study even though the chest film showed the impression of the hepatic herniation. In case 2, sonography revealed a relatively small volume of herniated liver compared to the larger volume of hepatic herniation seen on chest film. In case 3, a complete illustration of the hepatic herniation in one fused sonographic image included disrupted diaphragm, herniated outline with waist-like configuration of the liver, separation of the liver and right kidney, and interposed bowel gas within the separated area. From these three cases, we can conclude that different locations, narrow or wide constriction of the herniated neck, and different degrees of severity of liver herniation would affect the specific sonographic picture.

Electrokinetic flows in a microdomain.

The discrete kinetic approach and diffuse-reflection type boundary conditions are adopted to solve the transport problem for many charged particles flowing along a microslab (or channel of a wide constriction within a confined slender microdomain). The preliminary results show that there are selected orientations related to the nontrivial velocity-slip fields for a range of Knudsen numbers if there is a nonboundary-driven forcing along the streamwise direction. As the Knudsen number increases, the value of this selected orientation decreases and the cross-stream velocity profile becomes relatively flat. Our results qualitatively resemble those reported by Burgreen and Nakache or Paul et al.

Novel functionalities by guiding electrons in highly uniform quantum wire ensembles on multiatomic step arrays of GaAs(331) substrates

Coherently aligned multi-atomic step arrays on GaAs(331) substrates generate periodic ensembles of conductive quantum wires which are exploited for novel device functionalities. Sub-micrometer wide constrictions parallel to the wires with independent side-gate control allow switching between spatially separated electron channels. The strongly modulated periodic potential in constrictions perpendicular to the array opens new opportunities for realisation of coupled quantum dot structures.