Capillary Flow Dynamics Research Articles

The dynamics of capillary flow in an open-channel system featuring trigger valves.

Trigger valves are fundamental features in capillary-driven microfluidic systems that stop fluid at an abrupt geometric expansion and release fluid when there is flow in an orthogonal channel connected to the valve. The concept was originally demonstrated in closed-channel capillary circuits. We show here that trigger valves can be successfully implemented in open channels. We also show that a series of open-channel trigger valves can be placed alongside or opposite a main channel resulting in a layered capillary flow. We developed a closed form model for the dynamics of the flow at trigger valves based on the concept of average friction length and successfully validated the model against experiments. For the main channel, we discuss layered flow behavior in the light of the Taylor-Aris dispersion theory and in the channel turns by considering Dean theory of mixing. This work has potential applications in autonomous microfluidics systems for biosensing, at-home or point-of-care sample preparation devices, hydrogel patterning for 3D cell culture and organ-on-a-chip models.

Carbonic anhydrase inhibitors prevent presymptomatic capillary flow disturbances in a model of cerebral amyloidosis.

To enhance early diagnosis and treatment of Alzheimer·s disease (AD), understanding the pathological changes before symptoms arise is crucial. The continuum model of AD suggest that Aβ beta (Aβ) accumulation precedes symptoms by at least 15 years, with vascular changes detectable around this time. Disturbances in capillary flow dynamics have been linked to reduced oxygen delivery to brain tissue, but evidence in presymptomatic AD remains elusive. We examined capillary flow dynamics in presymptomatic Tg-SwDI mice and the capacity of carbonic anhydrase inhibitors (CAIs) to prevent capillary flow disturbances. Our study revealed capillary flow disturbances associated with alterations in capillary morphology, adhesion molecule expression, and Aβ load in cognitively normal 9-10-month-old Tg-SwDI mice. Treated mice showed ameliorated capillary flow disturbances, enhanced oxygen availability, and reduced Aβ load. These findings underscore the importance of capillary flow disturbances in presymptomatic AD and highlight CAIs· potential for preserving vascular integrity in early AD.

Measuring capillary flow dynamics using interlaced two-photon volumetric scanning

Two photon microscopy and optical coherence tomography (OCT) are two standard methods for measuring flow speeds of red blood cells in microvessels, particularly in animal models. However, traditional two photon microscopy lacks the depth of field to adequately capture the full volumetric complexity of the cerebral microvasculature and OCT lacks the specificity offered by fluorescent labeling. In addition, the traditional raster scanning technique utilized in both modalities requires a balance of image frame rate and field of view, which severely limits the study of RBC velocities in the microvascular network. Here, we overcome this by using a custom two photon system with an axicon based Bessel beam to obtain volumetric images of the microvascular network with fluorescent specificity. We combine this with a novel scan pattern that generates pairs of frames with short time delay sufficient for tracking red blood cell flow in capillaries. We track RBC flow speeds in 10 or more capillaries simultaneously at 1 Hz in a 237 µm × 237 µm × 120 µm volume and quantified both their spatial and temporal variability in speed. We also demonstrate the ability to track flow speed changes around stalls in capillary flow and measure to 300 µm in depth.

Capillary Flow Dynamics in Composite Rectangular Microchannels with Rough Walls.

In this article, we consider rectangular microchannels composed of glass and thin polymeric walls with different roughness in which opposed walls are of the same material but adjacent walls are not. We propose a model for fluid capillary transport into these rectangular microchannels when horizontally positioned and focus our research on how the microchannel aspect ratio modifies the motion during the initial viscous regimes. The model relies on an effective static contact angle and an effective friction coefficient that averages local magnitudes in the cross section. An extensive experimental investigation with different microchannels enabled us to obtain these coefficients for different aspect ratios. While for low aspect ratios, the effective contact angle presents the smallest values, the effective friction coefficient shows the larger ones. With rough surfaces, the spontaneous occurrence of pinning and depinning events associated with sharp wall defects notably reduces the effective static contact angle even when high aspect ratios are used. The obtained values of the effective friction coefficient show good agreement with previous literature investigations for rough and smooth lateral wall surfaces. Finally, we propose a nondimensional time to establish when contact angle effects dominate the dynamics. We found that for the materials and fluid properties used in this work, these effects become negligible for times larger than t ∼ 1 s.

Pericyte remodeling is deficient in the aged brain and contributes to impaired capillary flow and structure

Deterioration of brain capillary flow and architecture is a hallmark of aging and dementia. It remains unclear how loss of brain pericytes in these conditions contributes to capillary dysfunction. Here, we conduct cause-and-effect studies by optically ablating pericytes in adult and aged mice in vivo. Focal pericyte loss induces capillary dilation without blood-brain barrier disruption. These abnormal dilations are exacerbated in the aged brain, and result in increased flow heterogeneity in capillary networks. A subset of affected capillaries experience reduced perfusion due to flow steal. Some capillaries stall in flow and regress, leading to loss of capillary connectivity. Remodeling of neighboring pericytes restores endothelial coverage and vascular tone within days. Pericyte remodeling is slower in the aged brain, resulting in regions of persistent capillary dilation. These findings link pericyte loss to disruption of capillary flow and structure. They also identify pericyte remodeling as a therapeutic target to preserve capillary flow dynamics.

Capillary flow in Y-shaped junctions with open- and closed-end branches

The capillary flow in asymmetric junctions can delay the liquid invasion of the wide branch, which has important implications for the liquid imbibition in porous media with elongated pores. For the capillary flow in Y-shaped junctions with two open-end branches of different radii, the delay time of the liquid invasion of the wide branch depends on the pressure at the junction. In Y-shaped junctions with open- and closed-end branches, the gas compression in the closed-end branch is required for determining the pressure at the junction. A closer investigation of the capillary flow in such junctions is necessary to predict the liquid imbibition in porous media that comprise continuous and discontinuous elongated pores. This study presents a theoretical model considering the effect of gas compression in closed-end branches and analyzes this effect on the capillary-flow dynamics. The analytical results obtained suggest that the initiation of capillary flow in the open-end wide branch is governed by the trapped-gas pressure in the closed-end narrow branch. In this case, the delay time of the liquid invasion of the open-end branch decreases with a decrease in the length of the closed-end branch; this is because the decreased length causes the pressure difference at the junction to approach that at the liquid–gas interface in the open-end branch early. Additionally, for open- and closed-end branches, the velocity of capillary flow in the open-end branch increases because the volume rate of flow in the closed-end branch decreases owing to the increase in the trapped-gas pressure in this branch.

Automated capillary flow segmentation and mapping for nailfold video capillaroscopy.

This study aimed to develop an automated image analysis method for segmentation and mapping of capillary flow dynamics captured using nailfold video capillaroscopy (NVC). Methods were applied to compare capillary flow structures and dynamics between young and middle-aged healthy controls. NVC images were obtained in a resting state, and a region of the vessel in the image was extracted using a conventional U-Net neural network. The approximate length, diameter, and radius of the curvature were calculated automatically. Flow speed and its fluctuation over time were mapped using the Radon transform and frequency spectrum analysis from the kymograph image created along the vessel's centerline. The diameter of the curve segment (14.4μm and 13.0μm) and the interval of two straight segments (13.7μm and 32.1μm) of young and middle-aged subjects, respectively, were significantly different. Faster flow was observed in older subjects (0.48mm/s) than in younger subjects (0.26mm/s). The power spectral analysis revealed a significant correlation between the high-frequency power spectrum and the flow speed. The present method allows a spatiotemporal characterization of capillary morphology and flow dynamics with NVC, allowing a wide application such as large-scale health assessment.

Experimental, Theoretical, and Numerical Investigation of the Electric Field and Surface Wettability Effects on the Penetration Length in Capillary Flow

This study addressed the dynamics of capillary-driven flow for different surface wettabilities by concentrating on the influence of electric potential. The capillary flow dynamics were investigated by varying the wettability (plasma-treated, hydrophobic, hydrophilic, and superhydrophilic) of a capillary surface, and the applied electric potential to the liquid ranged from 0 to 500 V. When an electric potential was applied to the liquid, the fluid flow penetration length increased by 30–50% due to the electrohydrodynamic (EHD)-driven flow by the Maxwell (electric) pressure gradient effect. The results showed that the EHD effect enhanced the fluid penetration through narrow gaps. The maximum fluid penetration was attained for every surface at 500 V, particularly for the superhydrophilic surface, which exhibited the highest value. The combined effect of the electric field and wettability resulted in an enhanced fluid penetration speed, reducing the underfill time. In addition, theoretical and numerical models were developed for comparison with the experimental results. The proposed models reinforce the observed fluid flow phenomenon on various surfaces under the influence of an electric field. These findings can provide alternative strategies for controlling the dynamic features of capillary imbibition by introducing an electric field and wettability effects, which has practical implications in flip-chip packaging, microfluidic devices, and the manipulation of biocells.

Superwicking Functionality of Femtosecond Laser Textured Aluminum at High Temperatures.

An advanced superwicking aluminum material based on a microgroove surface structure textured with both laser-induced periodic surface structures and fine microholes was produced by direct femtosecond laser nano/microstructuring technology. The created material demonstrates excellent wicking performance in a temperature range of 23 to 120 °C. The experiments on wicking dynamics show a record-high velocity of water spreading that achieves about 450 mm/s at 23 °C and 320 mm/s at 120 °C when the spreading water undergoes intensive boiling. The lifetime of classic Washburn capillary flow dynamics shortens as the temperature increases up to 80 °C. The effects of evaporation and boiling on water spreading become significant above 80 °C, resulting in vanishing of Washburn’s dynamics. Both the inertial and visco-inertial flow regimes are insignificantly affected by evaporation at temperatures below the boiling point of water. The boiling effect on the inertial regime is small at 120 °C; however, its effect on the visco-inertial regime is essential. The created material with effective wicking performance under water boiling conditions can find applications in Maisotsenko cycle (M-cycle) high-temperature heat/mass exchangers for enhancing power generation efficiency that is an important factor in reducing CO2 emissions and mitigation of the global climate change.

Blind Spot: A Braille Patterned Novel Multiplex Lateral Flow Immunoassay Sensor Array for the Detection of Biothreat Agents.

Lateral flow immunoassays (LFIs) are simple, point-of-care diagnostic devices used for detecting biological agents or other analytes of interest in a sample. LFIs are predominantly singleplex assays, interrogating one target analyte at a time. There is a need for multiplex LFI devices, e.g., a syndromic panel to differentiate pathogens causing diseases exhibiting similar symptoms. Multiplex LFI devices would be especially valuable in instances where sample quantity is limiting and reducing assay time and costs is critical. There are limitations to the design parameters and performance characteristics of a multiplex LFI assay with many horizontal test lines due to constraints in capillary flow dynamics. To address some of the performance issues, we have developed a spot array multiplex LFI using Braille format (hence called Blind Spot) and a sensor, MACAW (Modular Automated Colorimetric Analyses Widget), that can analyze and interpret the results. As a proof of concept, we created a multiplex toxin panel, for detecting three toxins, using two letter codes for each. The results indicated that the six-plex, triple toxin assay performs as well as singleplex assays. The sensor-based calls are better compared to human interpretation in discriminating and interpreting ambiguous test results correctly especially at lower antigen concentrations and from strips with blemishes.

Dynamics of capillary flow in an undulated tube

From biology to engineering, while numerous applications are based on capillary phenomena in tubes having roughened surfaces, such as blood transport, paper-based rapid diagnostics, microfluidic fuel cells, and shale gas transport, the dynamics of such capillary flow remains poorly understood. We present a theoretical model for a circular undulated tube that has an idealized cosine-type inner wall characterized by two key morphological parameters: undulation amplitude and axial wave number. With the tube oriented at an arbitrary angle, we first characterize the apparent contact angle of the fluid as a function of local distortion angle and then establish a theoretical model involving inertia, viscosity, and gravity to describe the dynamics of capillary flow. A dimensionless number combining the three forces is introduced to quantify their influence. The model predictions reveal that, in an undulated tube with large wave numbers, the capillary height in equilibrium state is generally lower than that in a smooth tube of similar dimensions, whereas the reverse holds if the wave number becomes relatively small. When the viscosity of fluid is sufficiently small, capillary oscillation in an undulated tube is alleviated relative to that in a smooth tube, and hence stable capillary flow forms more easily in the former.

Capillary-flow dynamics in open rectangular microchannels

Abstract

The effect of femtosecond laser fluence and pitches between V-shaped microgrooves on the dynamics of capillary flow

Open microgroove is one kind of capillary-driven superwicking surface structure. In this study, arrays of parallel V-shaped microgrooves were fabricated on an aluminum surface by using a femtosecond laser to obtain a superwicking surface which can quickly transport the water uphill against gravity. The relationships between the flowing time and flowing distance were investigated and compared with theoretical results. We demonstrated both laser fluence and scanning step size can affect the superwicking performance. The aluminum surfaces fabricated at a laser fluence of 18.49 J/cm2 and 52.67 J/cm2 showed the best superwicking performances with the average water flow velocities approximately 16.2 mm/s and 16.4 mm/s, respectively, in the distance of 30 mm. On the other hand, the superwicking surfaces show an anisotropic flow characteristic due to the parallel microgrooves structure. However, when the scanning step size drops to 25 µm, the surface will form irregular rough structures that result in the isotropic flow characteristics. Moreover, by using a thermal camera, we found that after a 10 µL water droplet was dropped into the heated surface, the superwicking surface temperature quickly dropped from 92.4 °C to 82.5 °C which indicated that laser processing of the superwicking surface has potential application in heat dissipation.

Temperature Effect on Capillary Flow Dynamics in 1D Array of Open Nanotextured Microchannels Produced by Femtosecond Laser on Silicon.

Capillary flow of water in an array of open nanotextured microgrooves fabricated by femtosecond laser processing of silicon is studied as a function of temperature using high-speed video recording. In a temperature range of 23–80 °C, the produced wicking material provides extremely fast liquid flow with a maximum velocity of 37 cm/s in the initial spreading stage prior to visco-inertial regime. The capillary performance of the material enhances with increasing temperature in the inertial, visco-inertial, and partially in Washburn flow regimes. The classic universal Washburn’s regime is observed at all studied temperatures, giving the evidence of its universality at high temperatures as well. The obtained results are of great significance for creating capillary materials for applications in cooling of electronics, energy harvesting, enhancing the critical heat flux of industrial boilers, and Maisotsenko cycle technologies.

3D fluid-dynamic ovarian cancer model resembling systemic drug administration for efficacy assay.

Recently, 3D in vitro cancer models have become important alternatives to animal tests for establishing the efficacy of anticancer treatments. In this work, 3D SKOV-3 cell-laden alginate hydrogels were established as ovarian tumor models and cultured within a fluid-dynamic bioreactor (MIVO®) device able to mimic the capillary flow dynamics feeding the tumor. Cisplatin efficacy tests were performed within the device over time and compared with (i) the in vitro culture under static conditions and (ii) a xenograft mouse model with SKOV-3 cells, by monitoring and measuring cell proliferation or tumor regression, respectively, over time. After one week of treatment with 10 μM cisplatin, viability of cells within the 3D hydrogels cultured under static conditions remained above 80%. In contrast, the viability of cells within the 3D hydrogels cultured within dynamic MIVO® decreased by up to 50%, and very few proliferating Ki67-positive cells were observed through immunostaining. Analysis of drug diffusion, confirmed by computational analysis, explained that these results are due to different cisplatin diffusion mechanisms in the two culture conditions. Interestingly, the outcome of the drug efficacy test in the xenograft model was about 44% of tumor regression after 5 weeks, as predicted in a shorter time in the fluid-dynamic in vitro tests carried out in the MIVO® device. These results indicate that the in vivo-like dynamic environment provided by the MIVO® device allows to better model the 3D tumor environment and predict in vivo drug efficacy than a static in vitro model.

Spatial Temporal Analysis of Fieldwise Flow in Microvasculature.

Changes in blood flow velocity and distribution are vital in maintaining tissue and organ perfusion in response to varying cellular needs. Further, appearance of defects in microcirculation can be a primary indicator in the development of multiple pathologies. Advances in optical imaging have made intravital microscopy (IVM) a practical approach, permitting imaging at the cellular and subcellular level in live animals at high-speed over time. Yet, despite the importance of maintaining adequate tissue perfusion, spatial and temporal variability in capillary flow is seldom documented. In the standard approach, a small number of capillary segments are chosen for imaging over a limited time. To comprehensively quantify capillary flow in an unbiased way we developed Spatial Temporal Analysis of Fieldwise Flow (STAFF), a macro for FIJI open-sourceimage analysis software. Using high-speed image sequences of full fields of blood flow within capillaries, STAFF producesimages that represent motion over time called kymographs for every time interval for every vascular segment. From the kymographs STAFF calculates velocities from the distance that red blood cells move over time, and outputs the velocity data as a sequence of color-coded spatial maps for visualization and tabular output for quantitative analyses. In normal mouse livers, STAFF analyses quantified profound differences in flow velocity between pericentral and periportal regions within lobules. Even more unexpected are the differences in flow velocity seen between sinusoidsthat are side by side and fluctuations seen within individual vascular segments over seconds. STAFF is a powerful new tool capable of providing novel insights by enabling measurement of the complex spatiotemporal dynamics of capillary flow.

Dynamics of capillary flow in an open superoleophilic microchannel and its application to sensing of oil

We report the dynamics of capillary flow of oil in an open superoleophilic channel. The superoleophilic surface is fabricated by spin coating a layer of PDMS + n-hexane followed by candle sooting. The occurrence of various flow regimes, including the inertial, visco-inertial, and Lucas–Washburn regimes, are studied using analytical modelling as well as experiments. In case of a superoleophilic channel, much shorter inertial regime is observed as compared to that in an oleophilic channel due to the wicking of oil into the micro-roughness grooves ahead to moving bulk liquid meniscus. The study of the effect of channel aspect ratio $$\varepsilon$$ on the mobility parameter $$k~$$ showed that the mobility parameter $$k$$ is maximum for an aspect ratio of $$\varepsilon =1.6$$ , which is attributed to the balance between the capillary and viscous forces. Finally, we demonstrate the application of the superoleophilic channel integrated with electrodes for impedance-based sensing of oil from an oil–water emulsion.

Dynamics of liquid imbibition through paper with intra-fibre pores

We present a combined experimental and theoretical investigation of the dynamics of liquid imbibition through paper. The Washburn equation is widely used to describe the dynamics of capillary flow through paper, but this classical model has limited accuracy, which often makes it difficult to use in developing analytic systems such as paper-based microfluidic devices. We here report that the internal cavity of the cellulose fibres composing paper is significantly responsible for the limited accuracy of the Washburn equation. Our experiments demonstrated that liquid can be absorbed in the internal cavity of the cellulose fibres as well as in the inter-fibre pores formed by the fibre network. We developed a mathematical model for liquid imbibition by considering the flow through the intra-fibre pores based on experimental measurements of the intra-structure of cellulose fibres. The model markedly improves the prediction of the liquid absorption length, compared with the results of the Washburn equation, thus revealing the physics behind the limits of the Washburn equation. This study suggests that the accurate description of capillary imbibition through paper require parameters characterizing the internal pores of the cellulose fibres comprising the paper. Our results not only provide a new insight into porous media flows with different sized pores, but also provide a theoretical background for flow control in paper-based microfluidic systems.

A Capillary Flow Dynamics-Based Sensing Modality for Direct Environmental Pathogen Monitoring.

Toward ultra-simple and field-ready biosensors, we demonstrate a novel assay transducer mechanism based on interfacial property changes and capillary flow dynamics in antibody-conjugated submicron particle suspensions. Differential capillary flow is tunable, allowing pathogen quantification as a function of flow rate through a paper-based microfluidic device. Flow models based on interfacial and rheological properties indicate a significant relationship between the flow rate and the interfacial effects caused by target-particle aggregation. This mechanism is demonstrated for assays of Escherichia coli K12 in water samples and Zika virus (ZIKV) in blood serum. These assays achieved very low limits of detection compared with other demonstrated methods (1 log CFU/mL E. coli and 20 pg/mL ZIKV whole virus) with an operating time of 30 s, showing promise for environmental and health monitoring.

On numerical modeling of capillary flow dynamics in a microchannel model by using the finite element method

A simple microchannel model with submillimeter-scale geometries is proposed for studying capillary flows and investigating the dynamics in the channel. The finite element method incorporating surface tension and two-phase flow characteristic is applied. Velocity and pressure fields in the microchannel are presented. It is shown that the capillary-phase front in the microchannel is stirred, suffering small oscillations and retreating from the previous position before traveling again. Such a phenomenon is caused by nonlinear interaction of the capillary flow, surface tension, and boundary conditions.