Capillary Geometry Research Articles

Development workflow based on in situ synchrotron investigations to optimize laser processing of copper pins

To design future laser manufacturing processes for welding of copper materials, more and more high-end analysis methods are required. A fundamental process understanding by analyzing cause-and-effect relations of process dynamics using inline in situ diagnostics allows for an improved description of laser material interaction. Strategies for a reliable and robust welding process are derived from the findings. In this study, a four-step advanced methodical approach is presented and discussed. In the first step, a fundamental process description of the geometry of the vapor capillary and the formation of weld defects is developed. Therefore, welds on electrolytic tough pitch copper (Cu-ETP) and CuSn6 are carried out to analyze the temporal and spatial vapor capillary dynamics depending on laser power, welding speed, and focal diameter. This fundamental process understanding is transferred to the welding of copper pins in the form of I-pins. For this purpose, impurities and imperfections were applied to the pin surface to investigate the effects on the process result. As a third step, strategies by means of laser intensity distributions were adapted to compensate for imperfections in the welding process. Finally, a sensor vision system is adapted for ideal welding results. Investigations are based on in situ synchrotron analysis at Petra III, DESY in Hamburg. For the experiments, a TRUMPF TruDisk laser (100/400 μm fiber diameter), a TRUMPF TruFiber 6000P (50/100 μm fiber diameter), and a single-mode fiber laser (14 μm fiber diameter) were used. The focal diameter was adjusted with the optical system depending on the investigation.

The Influence of Capillary Geometry on the Stiffness of Hydrostatic Bearings in Medium-Sized Cylindrical Grinding Machines: A Simulation Analysis

The Influence of Capillary Geometry on the Stiffness of Hydrostatic Bearings in Medium-Sized Cylindrical Grinding Machines: A Simulation Analysis

Effect of Heat Exchanger and Capillary Geometry on the performance of Joule- Thomson Refrigerators operating with different mixtures

Abstract Joule-Thomson (J-T) refrigerators or J-T cryocoolers are extensively used in many low-temperature applications. J-T refrigerators operating with nitrogen-hydrocarbon (N2-HC) refrigerant mixtures offer several advantages, such as low operating pressures (<20 bar), high exergy efficiency, no moving parts in the cold section, and low cost. The cooling power or cooling capacity of the J-T refrigerator depends on the hardware used as well as the refrigerant composition. The proposed work focuses on estimating the cooling capacity of a mixed refrigerant J-T (MRJ-T) refrigerator of the given hardware and specified refrigerant. An iterative steady-state full-cycle simulation procedure has been presented in this work to simulate the complete system and estimate the cooling capacity, taking into account the possibility of choking of the expansion capillary. Some of the results have been validated against experimental results of an MRJT refrigerator available in the open literature. The details of the simulation model and the results of our studies on the prediction of stable operating range, maximum cooling capacity, the effect of heat exchanger geometry, expansion capillary geometry, mixture composition, and choking of the refrigerant mixture on the performance of an MRJ-T refrigerator are presented in this paper

In Situ Synchrotron Investigations of Beam Diameter Influence on Vapor Capillary Formation during Laser Beam Welding of Copper Alloy with a Blue Laser Beam Source

Laser beam welding as a reliable tool for high-precision joining of batteries or microelectronics is more and more the choice for achieving reproducible results in production processes. In addition to a high automation capability, the precise control of the energy deposition into the material plays an important role, especially when highly reflective materials, such as copper or aluminum, must be welded together. Alongside the use of highly brilliant fiber lasers in the near-infrared range with a focal diameter of a few tens of micrometers, diode lasers in the wavelength range of 445 nm are increasingly being used. Here, beam diameters of a few hundred micrometers can be achieved. With a wavelength of 445 nm, the absorptivity in copper can be increased by more than a factor of 10 compared to a near-infrared laser beam sources in solid state at room temperature. This paper presents the in situ X-ray observation of laser welding processes on CuSn6 with a laser beam source with a wavelength of 445 nm using synchrotron radiation at DESY Petra III Beamline P07 EH4 in Hamburg, Germany. For the experiments, the laser radiation was focused via two separate optics to focal diameters of 362 µm and 609 µm. To characterize the dynamics of the vapor capillaries depending on the different focal diameters dF, the parameters were varied with respect to laser power PL and feed rate v. For the investigations, a synchrotron beam of 2 × 2 mm2 in size with a photon energy of 89 keV was used, and the material samples were analyzed by means of phase-contrast videography to show the boundaries between solid, liquid, and gaseous material phases. The results of this paper show the welding depths achieved and how the geometry of the vapor capillary behaves by changing the focal diameter, laser power and feed rate.

Production and measurement of MeV proton microbeams in atmospheric environment based on glass capillary

<sec>Traditionally, ion microbeam is produced by focusing or/and collimating to reduce the beam size to submicron level. The traditional setup for producing the microbeam consists of an expensive focusing and collimating system with a large space, based on electromagnetic fields. Meanwhile, the microbeam obtained through pure collimation of metal micro-tubes is limited by the fabrication processing, i.e. the size of beam spot is largely limited to a few microns and its manufacture is not as simple as that of a glass capillary. Inspired by early studies of the guiding effect, the use of inexpensive and easy-to-make glass capillaries as the tool for ion external microbeam production has become a new direction.</sec><sec>In this work, we use a glass capillary with an open outlet (108 μm in diameter), which serves as a vacuum differential and collimating component, to produce a 2.5 MeV-proton microbeam directly from the linear accelerator into the atmosphere for measurements. We measure the beam spot diameter and energy distribution of the microbeam as a function of the tilt angle of the capillary. We also conduct calculations and ion trajectory analysis on the scattering process of 2.5 MeV protons on the inner walls.</sec><sec>The measurement results show that when the tilt angle is around 0°, there are a direct transmission part that maintains the initial incident energy, and a scattering part with the energy loss in the microbeam. It is found that the proportion of directly transmitted protons and the beam spot size are highest near zero tilt angle. As the tilt angle increases, the beam spot diameter decreases; when the tilt angle is greater than the geometric angle, all the microbeams come from the scattering with the energy loss. The simulation combined with the ion trajectory analysis based on the scattering process can explain the experimental results. It is found that the large angle scattering determines the entire external microbeam spot, and the central region of the beam spot is composed of directly penetrating ions, whose size is determined by the geometric shape of the glass capillary, i.e. the outlet diameter and aspect ratio.</sec><sec>The natural advantage of producing external micobeames easily and inexpensively through glass capillaries is their relative safety and stable operation, and the last but not least point is to simply locate the microbeams on the sample without complex diagnostic tools. The microbeams are expected to be widely used in fields such as radiation biology, medicine, and materials.</sec>

Investigation on spontaneous liquid–liquid imbibition in capillaries with varying axial geometries using lattice Boltzmann method

Spontaneous liquid–liquid imbibition in capillaries with irregular axial geometries is common in the petroleum industry. Monitoring the real-time dynamic contact angle (DCA) of the meniscus is crucial during such processes. In this work, we extend the Bell–Cameron–Lucas–Washburn (BCLW) equation by considering the axial shape of the capillaries, inertial force, and non-wetting fluid viscosity. We also develop a cascaded multi-component Shan–Chen lattice Boltzmann method (CLBM) with a modified mass-conservative curved boundary scheme to accurately simulate imbibition processes in sinusoidal capillaries. The results indicate that the DCA is highly sensitive to variations in the axial geometry of the capillary during imbibition, displaying a periodic time evolution pattern. When the axial geometry diverges, the DCA increases, and when it converges, the DCA decreases. The viscosity ratio affects the imbibition velocity, controlling the evolution period and extreme values of the DCA. A critical contact angle exists for a fixed capillary axial geometry and viscosity ratio. Continuous spontaneous imbibition occurs if the static contact angle is smaller than this critical value. However, if it exceeds this threshold, imbibition ceases within regions where axial geometry divergence. Moreover, we noticed a discrepancy in imbibition lengths predicted by the extended BCLW equation that ignores the DCA compared to those computed through the CLBM. To address this issue, we employed CLBM to monitor the DCA in real time and used the gathered data to refine the extended BCLW equation. As a result, the prediction of imbibition lengths by the extended BCLW equation for coupling the DCA became more accurate.

Towards an Understanding of the Challenges in Laser Beam Welding of Copper – Observation of the Laser-Matter Interaction Zone in Laser Beam Welding of Copper and Steel Using in Situ Synchrotron X-Ray Imaging

The increasing demand for contacting applications in electric components such as batteries, power electronics and electric drives is boosting the use of laser-based copper processing. Laser beam welding is a key for an efficient and high-quality electric vehicle production due to its local, non-contact energy input and high automation capability enabling reproducible weld quality. Nevertheless, a major challenge in process design is the combination of energy-efficiency and precise process guidance with regard to weld seam depth and defect prevention (i.e. spatter, melt ejections), partly caused by the high thermal conductivity of copper. High power lasers in the near infrared range and emerging visible laser beam sources with excellent beam quality can provide a suitable joining solution for this purpose. However, the underlying physical phenomena are currently only partly understood and a reflection on the challenges of laser beam welding of copper compared to well researched steel processing has not yet been carried out. In order to improve the understanding of the effect of the different material properties and the influence of process parameters on the vapor capillary and melt pool geometry in laser beam welding, in situ synchrotron investigations on Cu-ETP and S235 using 515 and 1030 nm laser sources were conducted. The material phase contrast analysis was successfully used to distinguish vapor capillary and melt pool phase boundaries during the welding process with high spatial and temporal resolution up to 5 kHz. A significantly different vapor capillary geometry and sensitivity to parameter variation were found between the steel and copper material. In addition, the visualization of characteristic melt flows revealed different melt pool dynamics and a pronounced eddy close to the melt pool surface for copper, which is assumed to be causal for the observation of pronounced spatter formation during copper welding in a certain process window.

Nature-Inspired Stalactite Nanopores for Biosensing and Energy Harvesting.

Nature provides a wide range of self-assembled structures from the nanoscale to the macroscale. Under the right thermodynamic conditions and with the appropriate material supply, structures like stalactites, icicles, and corals can grow. However, the natural growth process is time-consuming. This work demonstrates a fast, nature-inspired method for growing stalactite nanopores using heterogeneous atomic deposition of hafnium dioxide at the orifice of templated silicon nitride apertures. The stalactite nanostructures combine the benefits of reduced sensing region typically for 2-dimensional material nanopores with the asymmetric geometry of capillaries, resulting in ionic selectivity, stability, and scalability. The proposed growing method provides an adaptable nanopore platform for basic and applied nanofluidic research, including biosensing, energy science, and filtration technologies.

Lego-Inspired Glass Capillary Microfluidic Device: A Technique for Bespoke Microencapsulation of Phase Change Materials.

We report a Lego-inspired glass capillary microfluidic device capable of encapsulating both organic and aqueous phase change materials (PCMs) with high reproducibility and 100% PCM yield. Oil-in-oil-in-water (O/O/W) and water-in-oil-in-water (W/O/W) core-shell double emulsion droplets were formed to encapsulate hexadecane (HD, an organic PCM) and salt hydrate SP21EK (an aqueous PCM) in a UV-curable polymeric shell, Norland Optical Adhesive (NOA). The double emulsions were consolidated through on-the-fly polymerization, which followed thiol-ene click chemistry for photoinitiation. The particle diameters and shell thicknesses of the microcapsules were controlled by manipulating the geometry of glass capillaries and fluid flow rates. The microcapsules were monodispersed and exhibited the highest encapsulation efficiencies of 65.4 and 44.3% for HD and SP21EK-based materials, respectively, as determined using differential scanning calorimetry (DSC). The thermogravimetric (TGA) analysis confirmed much higher thermal stability of both encapsulated PCMs compared to pure PCMs. Polarization microscopy revealed that microcapsules could sustain over 100 melting-crystallization cycles without any structural changes. Bifunctional microcapsules with remarkable photocatalytic activity along with thermal energy storage performance were produced after the addition of 1 wt % titanium dioxide (TiO2) nanoparticles (NPs) into the polymeric shell. The presence of TiO2 NPs in the shell was confirmed by higher opacity and whiteness of these microcapsules and was quantified by energy dispersive X-ray (EDX) spectroscopy. Young's modulus of HD-based microcapsules estimated using micromanipulation analysis increased from 58.5 to 224 MPa after TiO2 incorporation in the shell.

Brain capillary geometry development for multiscale modelling study

Due to low spatial resolution of functional magnetic resonance imaging (fMRI), mathematical modelling of human brain is usually developed in ischaemic stroke study in order to enhance the understanding of ischaemic stroke mechanism so that proper treatment plan can be decided to a particular stroke patient. A 3D brain geometry is developed in this project using AutoLISP, which is a programming language within AutoCAD. The objective of this project is to re-create the geometry developed by El-Bouri et al. by replacing the 1-dimensional line with 3-dimensional capillary which can be meshed for the simulation. Simulation of blood flow using Stokes' cell equation is done in the 3D geometry in order to obtain effective hydraulic conductivity, K. Finally, the hydraulic conductivity obtained from the simulation is analysed and compared with previous work done by El-Bouri et al. Based on result in section 3, the initial geometry developed is further modified by adding few small cylinders and also bigger volume of the geometry with 15.21% percentage difference. Non-diagonal elements obtained in matrix K show non-zero values due to small voxel size used for the simulation (180µm), which should be increased to more than 250µm. The percentage difference of diagonal elements K 11, K 22, and K 33 are 8.05%, 2.65% and 21.43% respectively. The large percentage differences are probably due to additional volume added to the 3D geometry to maintain its periodicity with neighbouring voxels. Besides, meshing process and mesh size chosen are also contributing factor to the large percentage difference between both hydraulic conductivity values.

Characterization of Vapor Capillary Geometry in Laser Beam Welding of Copper with 515 nm and 1030 nm Laser Beam Sources by Means of In Situ Synchrotron X-ray Imaging

Laser welding of copper is being used with increasing demand for contacting applications in electric components such as batteries, power electronics, and electric drives. With its local, non-contact energy input and high automation capability enabling reproducible weld quality, this joining technology represents a key enabler of future mobility systems. However, a major challenge in process design is the combination of energy efficiency and precise process guidance in terms of weld seam depth and defect prevention (i.e., spatter and melt ejections) due to the high electrical and thermal conductivity of copper. High-power lasers in the near infrared wavelength range (𝜆 ≈ 1 μm) and excellent beam quality provide an established joining solution for this purpose; nevertheless, the low absorptivity (≤5%) advocates novel beam sources at visible wavelengths due to altered absorptivity (40% at 515 nm) characteristics as an improved tool. In order to understand the influence of laser wavelength and process parameters on the vapor capillary geometry, in situ synchrotron investigations on Cu-ETP with 515 nm and 1030 nm laser sources with the same spot diameter are compared. The material phase contrast analysis was successfully used to distinguish keyhole and melt pool phase boundaries during the welding process. A significantly different sensitivity of the keyhole depth in relation to the feed rate was found, which is increased for the infrared laser. This behavior could be attributed to the increased effect of multiple reflections at 1030 nm.

Pulmonary Oxygen Exchange in a Rhythmically Expanding–Contracting Alveolus–Capillary Model

Pulmonary gas exchanges are vital to human health, and disruptions to this process have been associated with many respiratory diseases. Previous gas exchange studies have predominately relied on whole-body testing and theoretical analysis with 1D or static models. However, pulmonary gas exchanges are inherently a dynamic process in 3D spaces with instantaneous interactions between air, blood, and tissue. This study aimed to develop a computational model for oxygen exchange that considered all factors mentioned above. Therefore, an integrated alveolus–membrane–capillary geometry was developed with prescribed rhythmic expansion/contraction. Airflow ventilation, blood perfusion, and oxygen diffusion were simulated using COMSOL. The temporal and spatial distribution of blood flow and oxygen within the capillaries were simulated under varying breathing depths and cardiac outputs. The results showed highly nonuniform blood flow distributions in the capillary network, while the rhythmic oscillation further increased this nonuniformity, leading to stagnant blood flow in the distal vessels. A static alveolus–capillary geometry underestimated perfusion by 11% for normal respirations, and the deviation grew with breathing depth. The rhythmic motion caused a phase lag in the blood flow. The blood PO2 reached equilibrium with the alveolar air after traveling 1/5–1/3 of the capillary network. The time to reach this equilibrium was significantly influenced by the air–blood barrier diffusivity, while it was only slightly affected by the perfusion rate. The computational platform in this study could be instrumental in obtaining refined knowledge of pulmonary O2 exchanges.

Emerging Insights into Glomerular Vascular Pole and Microcirculation.

The glomerular vascular pole is the gate for the afferent and efferent arterioles and mesangial cells and a frequent location of peripolar cells with an unclear function. It has been studied in definitive detail for >30 years, and functionally interrogated in the context of signal transduction from the macula densa to the mesangial cells and afferent arteriolar smooth muscle cells from 10 to 20 years ago. Two recent discoveries shed additional light on the vascular pole, with possibly far-reaching implications. One, which uses novel serial section electron microscopy, reveals a shorter capillary pathway between the basins of the afferent and efferent arterioles. Such a pathway, when patent, may short-circuit the multitude of capillaries in the glomerular tuft. Notably, this shorter capillary route is enclosed within the glomerular mesangium. The second study used anti-Thy1.1-induced mesangiolysis and intravital microscopy to unequivocally establish in vivo the long-suspected contractile function of mesangial cells, which have the ability to change the geometry and curvature of glomerular capillaries. These studies led me to hypothesize the existence of a glomerular perfusion rheostat, in which the shorter path periodically fluctuates between being more and less patent. This action reduces or increases blood flow through the entire glomerular capillary tuft. A corollary is that the GFR is a net product of balance between the states of capillary perfusion, and that deviations from the balanced state would increase or decrease GFR. Taken together, these studies may pave the way to a more profound understanding of glomerular microcirculation under basal conditions and in progression of glomerulopathies.

Derivation of a Qualitative Model for the Spatial Characteristic Wavelength of Extrusion Flow Instabilities: Investigation of a Polybutadiene Rubber through Capillary, Slit and Complex Geometry Extrusion Dies

AbstractThe extrusion flow instabilities of commercial polybutadiene (PBD) are investigated as a function of the different extrusion die geometries, such as round capillary, slit, and complex cross‐section profile slit dies via capillary rheology. Qualitative models are used to fit the experimental data for the spatial characteristic wavelength (λ) of the appearing extrusion flow instabilities. A new qualitative model for the slit die geometry, rectangular cross‐section, is derived based on the theoretical concept of the “two layers” extrudate and the force balance at the die exit region. The proposed qualitative model for the slit die geometry is used to predict the spatial characteristic wavelength (λ) for extrudates obtained by complex cross‐section profile slit die geometries similar to industrial manufacturing. Correlation between the ratio of the extensional (Ys) and shear (σx) stress at the die exit area and the characteristic dimension, height H for slit dies and diameter D for round capillary dies, is presented. Moreover, a geometry‐dependent model is used to predict the spatial characteristic wavelength (λ) of the extrusion flow instabilities from a round capillary die to a slit die and vice versa.

An improved recoil pressure boundary condition for the simulation of deep penetration laser beam welding using the SPH method

Deep penetration laser beam welding is simulated using the mesh-free Lagrangian Smoothed Particle Hydrodynamics (SPH) method. The physical phenomena modeled are the elastic material behavior of the solid phase, the fluid dynamics of the liquid phase including temperature-dependent surface tension, heat transfer by heat conduction, and the phase transitions melting, solidification, and evaporation. The energy input of the laser beam is determined by a ray-tracing scheme that calculates the absorbed irradiance distribution in the capillary. Based on the locally absorbed irradiance, an improved recoil pressure model is introduced and compared with a temperature-dependent model frequently used in the literature. To transfer the momentum of the recoil pressure to the surrounding melt, two boundary conditions based on either dynamically detected boundary particles or surface forces are presented and validated using simple examples. The results show that the recoil pressure is a key factor for the formation of a deep capillary and the acceleration of the melt around the capillary. Comparison of capillary geometry and melt pool size from simulation and experiment shows good agreement for the irradiance-dependent recoil pressure model.

A Volume-of-Fluid Heat Pipe Model Resolving Pressure Discontinuities at the Wick-Vapor Interface

Heat pipes are passive cooling devices which play an increasingly important role in a vast field of cooling solutions for technologies like electric machines or consumer electronics. Simulations that reliably predict the heat pipe’s performance and operational limits are the key for optimizing cooling efficiency. In this paper, a novel Volume-of-Fluid model is presented which resolves temperature and pressure discontinuities at the wick-vapor interface. The flow resistance in the wick structure is modeled using momentum sources based on the Darcy-Forchheimer formulation. Additional momentum sources perpendicular to the phase boundary account for capillary forces and ultimately lead to distinct pressure regimes in the wick and vapor flow. Evaporation and condensation are modeled using a data set which takes wick microstructure effects into account and is based on statistic rate theory. The involved thermophysical fluid properties are obtained from a model by the International Association for the Properties of Water and Steam depending on temperature and pressure. Eventually, the simulation approach is validated against experimental and numerical data from literature. The final model enables thermal engineers to predict dry-out conditions and to optimize the capillary structure and heat pipe geometry. Furthermore, this paper reveals a methodical error in the state-of-the-art simulation approach which leads to a significant underestimation of mass flow and pressure drop.

Optical method to determine in vivo capillary hematocrit, hemoglobin concentration, and 3-D network geometry in skeletal muscle.

The aim of this study was to develop a tool to visualize and quantify hemodynamic information, such as hemoglobin concentration and hematocrit, within microvascular networks recorded in vivo using intravital video microscopy. Additionally, we aimed to facilitate the 3-D reconstruction of the microvascular networks. Digital images taken from an intravital video microscopy preparation of the extensor digitorum longus muscle in rats for 25 capillary segments were used. The developed algorithm was used to delineate capillaries of interest, calculate the optical density for each pixel in the image, and reconstruct the 3-D capillary geometry using the calculated light path-lengths. Subsequently, the mean corpuscular hemoglobin concentration (MCHC), hemoglobin concentration, and hematocrit for these capillaries were calculated. We evaluated the hematocrit values determined by our methodology by comparing them to those obtained using a previously published method. The hematocrit values from the proposed optical method were strongly correlated with those calculated using published methods r2 (25)=.92, p<.001, and demonstrated excellent agreement with a mean difference of 1.3% and a coefficient of variation (CV) of 11%. The average MCHC, hemoglobin concentration, and light path-lengths were 23.83g/dl, 8.06g/dl, and 3.92µm, respectively. The proposed methodology can quantify hemodynamic measurements and produce functional images for visualization of the microcirculation in vivo.

A State-of-the-Art Review on Two-Phase Flow-Induced Noise in Expansion Devices

This paper presents a state-of-the-art review of the literature concerning flow-induced noise in expansion devices. The topics covered are theoretical natural frequency models of bubbles with spherical and cylindrical shapes and flow-induced noise in capillary tubes, such as orifice, thermal and electric expansion valve. Several parametric conditions affect the flow-induced noise in capillary tubes, such as outlet capillary geometry, capillary inner diameter and length, flow orientation, outlet vapor quality, mass velocity, pressure drop, in/outlet flow pattern, evaporation temperature, and refrigeration cycle operational conditions. Despite the broad use of orifice, there is a lack of knowledge in the literature of flow-induced noise in these devices. The flow-induced noise seems primarily a function of mass velocity, vapor quality, and flow pattern. It seems that outlet intermittent flow patterns lead to significant noise in capillary tubes and expansion valves. In general, for the noise evaluated outside the expansion device, electric expansion valves exhibited higher sound pressure levels than capillary tubes. Furthermore, the present literature review indicates that efforts should be made to perform tests for a wide range of conditions evaluating the noise within and outside the expansion device, device wall acceleration and flow topology for current and new refrigerants, simultaneously.

Influence of the capillary geometry on the weld seam quality during high-speed laser welding

With high feed rates >15 m/min, the laser beam welding process is very stable and efficient. It is known that increasing the feed rate leads to an elongation of the capillary. It is also known that the capillary geometry correlates with the occurrence of undercuts, which mark the upper limit for high-speed laser welding. So far, the laser power was limited to 16 kW for these investigations. This is why the present work investigates the occurrence of undercuts at the upper limit of high-speed laser welding with a laser power of up to 24 kW and feed rates up to 1 m/s. Two processing optics were combined to combine a 16 kW and 8 kW laser source. The results show that the combination of the two laser beams influences the weld result and can be used to increase the process window to higher feed rates for high-speed laser welding.

Emerging Links between Cerebral Blood Flow Regulation and Cognitive Decline: A Role for Brain Microvascular Pericytes.

Cognitive impairment associated with vascular etiology has been of considerable interest in the development of dementia. Recent studies have started to uncover cerebral blood flow deficits in initiating cognitive deterioration. Brain microvascular pericytes, the only type of contractile cells in capillaries, are involved in the precise modulation of vascular hemodynamics due to their ability to regulate resistance in the capillaries. They exhibit potential in maintaining the capillary network geometry and basal vascular tone. In addition, pericytes can facilitate better blood flow supply in response to neurovascular coupling. Their dysfunction is thought to disturb cerebral blood flow causing metabolic imbalances or structural injuries, leading to consequent cognitive decline. In this review, we summarize the characteristics of microvascular pericytes in brain blood flow regulation and outline the framework of a two-hit hypothesis in cognitive decline, where we emphasize how pericytes serve as targets of cerebral blood flow dysregulation that occurs with neurological challenges, ranging from genetic factors, aging, and pathological proteins to ischemic stress.