实验流体力学

Research progress on mechanical and flow properties of blood cells in microcirculation using microfluidic devices

QI Xiaojing, LI Xuejin

2020, 34(2): 1-10. doi: 10.11729/syltlx20190158

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Abstract:
Microfluidics (or lab-on-a-chip) is an important technology suitable for a wide range of biomedical applications from single-cell analysis to point-of-care diagnosis. In this paper, we review recent advances in the applications of the microfluidic technology in the field of cell biology and biomechanics. We highlight examples of some successful applications of microfluidic devices in probing the mechanical and rheological characteristics of blood cells in healthy and diseased states at single-cell and multi-cell levels, and in investigating the cell migration and separation at the whole-cell population.

State of the art of the methods and techniques in modeling analysis and in vitro simulation of arterial endothelial hemodynamic microenvironment

QIN Kairong, LIANG Fuyou, NA Jingtong

2020, 34(2): 11-24. doi: 10.11729/syltlx20200029

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Abstract:
Hemodynamic variables in arterial endothelial microenvironment, such as blood pressure, wall shear stress and tensile stress, play critical roles in maintaining the normal physiological function of endothelium. Modeling analysis and in vitro simulation of in vivo arterial endothelial hemodynamic microenvironment may not only offer important physiological parameters for early diagnosis and prevention, treatment and rehabilitation of cardiovascular diseases but also establish a fundamental basis for further understanding the underlying mechanisms of disease initiation and progression, and therefore have important scientific significance and value of clinic application. This paper provides an overall review of research progresses in this area from three perspectives, i.e. modeling analysis of in vivo arterial endothelial hemodynamic microenvironment, in vitro mock circulatory system (MCS) for studying the characteristics of arterial endothelial hemodynamic microenvironment, and endothelial cell culture model (ECCM) for investigating cell mechanobiology under controlled in vitro conditions. We raise several methodological and technical problems of urgent need for better solution based on a summary and systematic analysis of major literatures in this field aiming to offer some references for relevant future studies.

Study on behaviors of droplets and particles within microchannels

WANG Xiang, PANG Yan, SHEN Feng, LIU Zhaomiao

2020, 34(2): 25-38. doi: 10.11729/syltlx20190137

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Abstract:
The requirements for miniaturization and integration of new detection devices as well as the demands for interdisciplinary manipulation platform by current scientific researches and engineering applications prompt the rapid development of microfluidics. Droplet and particle are two important targets that are manipulated by microfluidics, which usually works at the laminar flow domain. Nonlinear factors are introduced into microscale flow by scale effect and interface effect, which are influenced by multiple parameters including the channel geometry and flow condition. In order to understand the complex flow phenomena, physical mechanisms should be studied from the fundamental perspective of hydrodynamics. Related work of our group on behaviors of droplets and particles over recent years is summarized. By analyzing the variation of characteristic parameters of droplet or particle, distinct flow regimes and corresponding critical conditions can be specified. Key control parameters dominating the flow can be confirmed and theoretical models can be constructed to pursue the manipulation methods of different behaviors. This study can provide references for the improvement of the theoretical system of complex flow at microscale and the related engineering applications.

Experimental study on circulating filtration of micro particles based on metal rubber and dielectrophoretic effect

SONG Chunlei, REN Yukun, HE Wenjun, JIANG Tianyi, JIANG Hongyuan

2020, 34(2): 39-45. doi: 10.11729/syltlx20190152

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Abstract:
In order to improve the filtration capacity of the metal rubber sheet, the porous metal rubber sheet is combined with the dielectrophoresis, and the high-efficiency pumping function of the liquid metal droplet in the confined space is utilized to research the filtration performance of the metal rubber sheet on the 5 μm polystyrene microspheres. A pair of metal rubber sheets is parallelly embedded in the close-looped circulating microfluidics channel to achieve an AC field gradient for effective dielectrophoresis. The results show that it is difficult to realize the filtering function without applying an AC electric field as the aperture of the pores inside the metal rubber sheet is much larger than the diameter of latex beads. As long as the two pairs of electrodes are energized synergistically, the suspended colloids can be trapped by short-range dielectrophoresis force near the metal rubber sheets, which can significantly improve the filtering quality.

Experimental method and process control of capillary flow focusing

MU Kai, SI Ting

2020, 34(2): 46-56. doi: 10.11729/syltlx20190146

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Abstract:
Capillary flow focusing as one typical kind of capillary flows is able to produce droplets and capsules at micro-scales, offering promising advantages in applications. This work reviews the experimental methods of capillary flow focusing based on the 'air-injecting' and 'air-sucking' devices, and presents the complete platform for studying the fluid cone-jet structures. Moreover, the method of adding external actuations on the flow focusing device to manipulate the breakup of liquid jets is illustrated. The flow visualization methods to capture the interface evolutions of the compound fluid cones and jets are introduced. It is found that the geometric parameters and the external flow parameters significantly affect the morphologies and instabilities of the fluid cone, and would further affect the diameters, perturbation wavelengths and interface coupling of the coaxial liquid jets. The lens effect of the outer interface on the coaxial jets could cause distortion of the inner jet and droplets, and a modified method was developed to remove the distortion based on the law of refraction. For the jet breakup upon external actuation, the relationship between the jet breakup length and the excitation amplitude was studied. The effects of the actuation frequency on the diameters and monodispersity of resultant droplets were studied.

Experimental methods and recent progress in biomechanics using atomic force microscopy

GUAN Dongshi, LI Hangyu, TONG Penger

2020, 34(2): 57-66. doi: 10.11729/syltlx20200026

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Abstract:
As a micro-and nano-scale mechanical tool, Atomic Force Microscopy (AFM) is increasingly used in the experimental study of the biomechanics and promotes the further development of this interdisciplinary field. Using a variety of operation modes and modified probes, the AFM can carry out mechanical measurements on living matter at multiple scales, from subcellular structures to living cells and tissues. It can be used to study variations of the mechanical properties during different living processes, such as aging and cancerization. In this article, we will review the working principle of AFM, its experimental implementations in biomechanical measurements, and the applications of the AFM in the study of the mechanical properties of the whole cell and local variations, liquid-liquid phase-separated droplets, and epithelial cysts. We also analyze the effects of complex fluids and micro-and nano-scale flows on the AFM measurements, and make an outlook on the development of this field.

Micro-PIV study on flow field characteristics of droplets in a microcavity

SHEN Feng, YAN Chengjin, LI Mengqi, JI Deru, LIU Zhaomiao

2020, 34(2): 67-72. doi: 10.11729/syltlx20190117

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Abstract:
Droplets have become an important research content of microfluidics. In order to realize precise regulation of the droplets' microenvironment, droplets were generated and trapped in long rectangular microcavities in a microchannel, and the internal flow field characteristics were experimentally measured by using a micro-particle image velocimetry (Micro-PIV) system. The effects of the Reynolds number (Re) on the droplet morphology, internal flow velocity vector fields and the distributions of shear stress inside the trapped droplet have been investigated. The results show that at Re=11.1, a vortex structure appears inside the droplet. When Re=33.3, the flow rate at the center of the droplet reaches a maximum value of about 10 μm/s. However, when Re=44.4, the vortex structure disappears and the average flow rate decreases. Meanwhile, the droplet size decreases as the Re increases. Moreover, Re has no significant effect on the internal shear stress of the droplet, and the average value of the shear stress is extremely low (< 1.5×10^-4 Pa).

Effect of surface micro/nano-structure on gas-water interface stability and flow drag reduction

YAO Zhaohui, ZHANG Jingxian, HAO Pengfei

2020, 34(2): 73-79. doi: 10.11729/syltlx20190161

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Abstract:
The pressure-flow measurement method and flow visualization are used to study the drag reduction effect of 6 types of super-hydrophobic surfaces with different micro/nano-structure dimensions and the influence of the surface microstructure shape on the gas-water interface stability. The experimental results show that the various superhydrophobic surfaces have a certain drag reduction under laminar flow and turbulent flow conditions. At the same solid area fraction, the smaller the microstructure spacing is, the better the drag reduction effect is. The maximum drag reduction rate, which is (38.6±4.5)%, is achieved on the micro/nano hierarchical structure surface with the smallest structural spacing. The visualization experiment also found that the drag reduction rate is related to the microstructure level, microstructure size, channel flow pattern and microstructure morphology, which all have certain influence on the gas-water interface stability. The hierarchical micro/nano-structure can significantly improve the drag reduction, because the addition of the nano-secondary structure reduces the solid area fraction of the surface and improves the stability of the gas-water interface. In addition, a microchannel surface with doubly reentrant structures (umbrella structure), even though made of wettable material, can capture and sustain the air-water interface, thereby achieving superhydrophobic performance.

Research progress of slip on the liquid-solid interface

ZHENG Xu, Zhanhua SILBER-LI

2020, 34(2): 80-88. doi: 10.11729/syltlx20190164

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Abstract:
Boundary slip is a long-standing scientific problem in the research of fluid mechanics, and is a highly attractive issue in micro/nanofluidics. Recently, the research community has gradually reached a consensus on the slip length of simple liquid (such as water) on smooth liquid-solid interface. However, the slip of complex fluids on liquid-solid interface has become an emerging and attractive issue. Therefore, this paper reviews the new experimental results and theoretical descriptions obtained in the research progress from the simple liquid to complex fluids. In particular, we mainly introduce how to understand the slip of complex fluids on the liquid-solid interface and its impacts, based on the recent experimental results in polyelectrolyte solutions from Prof. Charlaix's group using surface force apparatus.

Experimental study on microrheological properties of polyethylene oxide solution based on single particle tracking method

ZHOU Sijia, WANG Haoli, BAO Fubing

2020, 34(2): 89-98. doi: 10.11729/syltlx20190143

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Abstract:
The microrheological properties of polyethylene oxide (PEO) solution under different temperatures and different concentrations were studied based on the single particle tracking (SPT) method in this paper. Based on the generalized Stokes-Einstein relationship and the viscoelastic theory of complex fluid, the microrheological properties of PEO dilute solution with concentration of 0.4 wt%~1.0 wt% at 25℃, 35℃ and 45℃ were measured and analyzed by the particle tracking technique. The study results show that the restriction of Brownian motion of the probe particles increases with the increase of the solution concentration. The Brownian motion is most restricted under the concentration of 1.0 wt% and the temperature of 25℃. The solved results of the viscoelastic modulus show that PEO solution is a complex fluid with dominant viscous modulus and weak elastic modulus under the experimental conditions. The viscoelastic modulus of the solution increases with the increase of the solution concentration under the same temperature. Both the elastic modulus (G'(ω)) and the viscous modulus (G"(ω)) show the decreasing trend with the increase of the temperature, and the decreasing rate of the elastic modulus is larger than that of the viscous modulus. The analysis of MSD standard deviation indicates that the measurement errors show the increasing trend with the increase of the tracking time in the microrheological experiment based on SPT method.

Experimental study on the self-diffusiophoresis of the Janus micromotor in complex fluids

LI Nana, ZHENG Xu, Zhanhua SILBER-LI

2020, 34(2): 99-106. doi: 10.11729/syltlx20200023

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Abstract:
Janus micro/nanomotors have shown broad application prospects as drug delivery tools in biomedicine or energy generators for micro/nano-robots working under complex conditions. Existing studies have focused on the self-diffusiophoresis of Janus micro/nanomotors in simple liquids such as water, however, the research on the mechanism and characteristics of the self-diffusiophoresis in complex fluids is still lacking. This work experimentally investigates the self-diffusiophoresis of 2.06 μm Janus micromotors in polymer PEO solutions. The results systematically describe the influence of the PEO concentration on the propulsion speed of the Janus micromotor, the mean square displacements of the motion, and the rotation feature. Our findings show that the PEO polymers not only influence the viscosity of the solution, but also cause sub-diffusive and super-diffusive behaviors in the short-time and intermediate-time propulsion regime respectively, and result in an anomalous enhancement of the rotation of the Janus micromotors.

2020 Vol. 34, No. 2