Advancing in situ single-cell microbiological analysis through a microwell droplet array with a gradual open sidewall.

The utilization of microfluidic analysis technology has resulted in the advancement of fast pathogenic bacteria detection, which can accurately provide information on biochemical reactions in a single cell and enhance detection efficiency. Nevertheless, the achievement of rapid and effective in situ detection of single-bacteria arrays remains a challenge due to the complexity of bacterial populations and low Reynolds coefficient fluid, resulting in insufficient diffusion. We develop microwell droplet array chips from the lateral hydrodynamic wetting approach to address this issue. The sidewall of the microwell gradually opens which aids in advancing the liquid-air interface and facilitates the impregnation of the solid microwells, preserving the Wenzel state and assisting in resisting the liquid force to separation from the drop. The feasibility of preparing cell arrays and identifying them inside the microwells was demonstrated through the simulated streamlined distribution of gradual and traditional microwells with different sizes. The water-based ink diffusion experiment examined the relationship between diffusion efficiency and flow velocity, as well as the position of the microwell relative to the channel. It showed that the smaller gradual microwell still has a good diffusion efficiency rate at a flow velocity of 2.1 µL min-1 and that the infiltration state is easier to adjust. With this platform, we successfully isolated a mixed population containing E. coli and S. aureus, obtained single-bacteria arrays, and performed Gram assays after in situ propagation. After 20 hours of culture, single bacteria reproduced demonstrating the capability of this platform to isolate, cultivate, and detect pathogenic bacteria.

A method for real-time mechanical characterisation of microcapsules.

Characterising the mechanical properties of flowing microcapsules is important from both fundamental and applied points of view. In the present study, we develop a novel multilayer perceptron (MLP)-based machine learning (ML) approach, for real-time simultaneous predictions of the membrane mechanical law type, shear and area-dilatation moduli of microcapsules, from their camera-recorded steady profiles in tube flow. By MLP, we mean a neural network where many perceptrons are organised into layers. A perceptron is a basic element that conducts input-output mapping operation. We test the performance of the present approach using both simulation and experimental data. We find that with a reasonably high prediction accuracy, our method can reach an unprecedented low prediction latency of less than 1 millisecond on a personal computer. That is the overall computational time, without using parallel computing, from a single experimental image to multiple capsule mechanical parameters. It is faster than a recently proposed convolutional neural network-based approach by two orders of magnitude, for it only deals with the one-dimensional capsule boundary instead of the entire two-dimensional capsule image. Our new approach may serve as the foundation of a promising tool for real-time mechanical characterisation and online active sorting of deformable microcapsules and biological cells in microfluidic devices.

One-Step Fabrication of Droplet Arrays Using a Biomimetic Structural Chip.

In the field of one-step efficient preparation of dewetting droplet arrays, the process is hampered by the requirement for low chemical wettability of solid surfaces, which restricts the complete transition of wetting state and its broad prospects in biological applications. Inspired by the physical structure of the lotus leaf, enabling it to promote the change of the infiltration state of an aqueous solution on the surface, we developed a method of one-step fabrication of droplet arrays on the biomimetic structural chip designed in the present work. This greatly reduces the need for chemical modification techniques to achieve low wettability and reduces the reliance on complex and sophisticated surface preparation techniques, thus improving the fabrication efficiency of droplet arrays fully generated on a chip by one-step operation without the need for extra liquid phase or the control of harsh barometric pressure. We also studied the influence of dimensions of the biomimetic structure and the preparation process parameters such as number of smears and speed of smearing on the preparation rate and uniformity of the droplet arrays. The amplification of templating DNA molecules in the droplet arrays prepared in a one-step fabrication way is also performed to verify its application potential for DNA molecular diagnosis.

Cost-Effective Droplet Generator for Portable Bio-Applications.

The convenient division of aqueous samples into droplets is necessary for many biochemical and medical analysis applications. In this article, we propose the design of a cost-effective droplet generator for potential bio-chemical application, featuring two symmetric tubes. The new droplet generator revisits the relationship between capillary components and liquid flow rates. The size of generated droplets by prototype depends only on generator dimensions, without precisely needing to control external flow conditions or driving pressure, even when the relative extreme difference in flow rate for generating nL level droplets is over 57.79%, and the relative standard deviation (RSD) of the volume of droplets is barely about 9.80%. A dropper working as a pressure resource is used to verify the rapidity and robustness of this principle of droplet generation, which shows great potential for a wide range of droplet-based applications.

Cross-Sectional Dimension Dependence of Electroosmotic Flow in Fractal Treelike Rectangular Microchannel Network.

The present work theoretically and numerically studies the electroosmotic flow (EOF) within a fractal treelike rectangular microchannel network with uniform channel height. To obtain minimum EOF fluidic resistance, the microchannel cross-sectional dimensions of the fractal network are optimized. It is found that the cross-sectional dimension dependence of EOF fluidic resistance within a symmetric fractal network is only dependent on the channel width when the total channel volume is constant, and the optimal microchannel widths to reach the minimum EOF fluidic resistance satisfy the scaling law of κ = N-1 (where κ is the width ratio of the rectangular channels at two successive branching levels, N is the branching number); however, for the symmetric fractal network with constant total surface area , the optimal cross-sectional dimensions should simultaneously satisfy κ = N-1 and (where H is the channel height, S is the total channel surface area, l0 is the channel length at the original branching level, γ is the channel length ratio at two successive branching levels and m is the total branching level) to obtain the minimum EOF fluidic resistance. The optimal scaling laws established in present work can be used for the optimization design of the fractal rectangular microchannel network for EOF to reach maximum transport efficiency.

Optimal fractal tree-like microchannel networks with slip for laminar-flow-modified Murray's law.

The fractal tree-like branched network is an effective channel design structure to reduce the hydraulic resistance as compared with the conventional parallel channel network. In order for a laminar flow to achieve minimum hydraulic resistance, it is believed that the optimal fractal tree-like channel network obeys the well-accepted Murray's law of ßm = N-1/3 (ßm is the optimal diameter ratio between the daughter channel and the parent channel and N is the branching number at every level), which is obtained under the assumption of no-slip conditions at the channel wall-liquid interface. However, at the microscale, the no-slip condition is not always reasonable; the slip condition should indeed be considered at some solid-liquid interfaces for the optimal design of the fractal tree-like channel network. The present work reinvestigates Murray's law for laminar flow in a fractal tree-like microchannel network considering slip condition. It is found that the slip increases the complexity of the optimal design of the fractal tree-like microchannel network to achieve the minimum hydraulic resistance. The optimal diameter ratio to achieve minimum hydraulic resistance is not only dependent on the branching number, as stated by Murray's law, but also dependent on the slip length, the level number, the length ratio between the daughter channel and the parent channel, and the diameter of the channel. The optimal diameter ratio decreases with the increasing slip length, the increasing level number and the increasing length ratio between the daughter channel and the parent channel, and decreases with decreasing channel diameter. These complicated relations were found to become relaxed and simplified to Murray's law when the ratio between the slip length and the diameter of the channel is small enough.

Effective Boundary Slip Induced by Surface Roughness and Their Coupled Effect on Convective Heat Transfer of Liquid Flow.

As a significant interfacial property for micro/nano fluidic system, the effective boundary slip can be induced by the surface roughness. However, the effect of surface roughness on the effective slip is still not clear, both increased and decreased effective boundary slip were found with increased roughness. The present work develops a simplified model to study the effect of surface roughness on the effective boundary slip. In the created rough models, the reference position of the rough surfaces to determinate effective boundary slip was set based on ISO/ASME standard and the surface roughness parameters including Ra (arithmetical mean deviation of the assessed profile), Rsm (mean width of the assessed profile elements) and shape of the texture varied to form different surface roughness. Then, the effective boundary slip of fluid flow through the rough surface was analyzed by using COMSOL 5.3. The results show that the effective boundary slip induced by surface roughness of fully wetted rough surface keeps negative and further decreases with increasing Ra or decreasing Rsm. Different shape of roughness texture also results in different effective slip. A simplified corrected method for the measured effective boundary slip was developed and proved to be efficient when the Rsm is no larger than 200 nm. Another important finding in the present work is that the convective heat transfer firstly increases followed by an unobvious change with increasing Ra, while the effective boundary slip keeps decreasing. It is believed that the increasing Ra enlarges the area of solid-liquid interface for convective heat transfer, however, when Ra is large enough, the decreasing roughness-induced effective boundary slip counteracts the enhancement effect of roughness itself on the convective heat transfer.

Thermal Characteristics of Staggered Double-Layer Microchannel Heat Sink.

The present work numerically studies the thermal characteristics of a staggered double-layer microchannel heat sink (DLMCHS) with an offset between the upper layer of microchannels and lower layer of microchannels in the width direction, and investigates effects of inlet velocity and geometric parameters including the offset of the two layers of microchannels, vertical rib thickness and microchannel aspect ratio on the thermal resistance of the staggered DLMCHS. The present work found that the thermal resistance of the staggered DLMCHS increases with the increasing offset value when the vertical rib thickness is small, but decreases firstly and then increases as the offset value increases when the vertical rib thickness is large enough. Furthermore, the thermal resistance of the staggered DLMCHS decreases with the increasing offset when the aspect ratio is small, but increases with the increasing offset when the aspect ratio is large enough. Thus, for the DLMCHS with a small microchannel aspect ratio and large vertical rib thickness, the offset between the upper layer of microchannels and the lower layer of microchannels in the width direction is a potential method to reduce thermal resistance and improve the thermal performance of the DLMCHS.

Numerical investigation of the tribological performance of micro-dimple textured surfaces under hydrodynamic lubrication.

Surface texturing is an important approach for controlling the tribological behavior of friction pairs used in mechanical and biological engineering. In this study, by utilizing the method of three-dimensional computational fluid dynamics (CFD) simulation, the lubrication model of a friction pair with micro-dimple array was established based on the Navier-Stokes equations. The typical pressure distribution of the lubricant film was analyzed. It was found that a positive hydrodynamic pressure is generated in the convergent part of the micro-dimple, while a negative hydrodynamic pressure is generated in the divergent part. With suitable parameters, the total integration of the pressure is positive, which can increase the load-carrying capacity of a friction pair. The effects of the micro-dimple parameters as well as fluid properties on tribological performance were investigated. It was concluded that under the condition of hydrodynamic lubrication, the main mechanism for the improvement in the tribological performance is the combined effects of wedging and recirculation. Within the range of parameters investigated in this study, the optimum texture density is 13%, while the optimum aspect ratio varies with the Reynolds number. For a given Reynolds number, there exists a combination of texture density and aspect ratio at which the optimum tribological performance could be obtained. Conclusions from this study could be helpful for the design of texture parameters in mechanical friction components and even in artificial joints.

The effect of the electrical double layer on hydrodynamic lubrication: a non-monotonic trend with increasing zeta potential.

In the present study, a modified Reynolds equation including the electrical double layer (EDL)-induced electroviscous effect of lubricant is established to investigate the effect of the EDL on the hydrodynamic lubrication of a 1D slider bearing. The theoretical model is based on the nonlinear Poisson-Boltzmann equation without the use of the Debye-Hückel approximation. Furthermore, the variation in the bulk electrical conductivity of the lubricant under the influence of the EDL is also considered during the theoretical analysis of hydrodynamic lubrication. The results show that the EDL can increase the hydrodynamic load capacity of the lubricant in a 1D slider bearing. More importantly, the hydrodynamic load capacity of the lubricant under the influence of the EDL shows a non-monotonic trend, changing from enhancement to attenuation with a gradual increase in the absolute value of the zeta potential. This non-monotonic hydrodynamic lubrication is dependent on the non-monotonic electroviscous effect of the lubricant generated by the EDL, which is dominated by the non-monotonic electrical field strength and non-monotonic electrical body force on the lubricant. The subject of the paper is the theoretical modeling and the corresponding analysis.

Effect of Surface Charge on the Nanofriction and Its Velocity Dependence in an Electrolyte Based on Lateral Force Microscopy.

The nanofriction between a silicon nitride probe and both a silicon wafer and an octadecyltrichlorosilane (OTS)-coated surface is studied in saline solution by using lateral force microscopy (LFM). The effects of surface charge on the nanofriction in an electrolyte as well as its velocity dependence are studied, while the surface charge at the solid-liquid interface is adjusted by changing the pH value of the electrolyte. The results show that the nanofrictional behavior between the probe and the samples in an electrolyte depends strongly on the surface charge at the solid-liquid interface. When the probe and the sample in the electrolyte are charged with the same sign, a repulsive electrostatic interaction between the probe and the sample is produced, leading to a reduction in nanofriction. In contrast, when the two surfaces are charged with the opposite sign, nanofriction is enhanced by the attractive electrostatic interaction between the probe and the sample. The velocity dependence of nanofriction in an electrolyte is believed to be tied to charge regulation referring to a decreasing trend in surface charge densities for the two approaching charged surfaces in an electrolyte. When the probe slides on the sample at a low velocity, charge regulation occurs and weakens the electrostatic interaction between the probe and the sample. As a result, nanofriction is reduced for surfaces charged with the opposite sign, and it is enhanced for surfaces charged with the same sign. When the sliding velocity between the probe and the sample is high, there is insufficient time for charge regulation to occur. Thus, the friction pair shows a larger nanofriction when the surfaces are charged with the opposite sign and a smaller nanofriction when the surfaces are charged with the same sign when compared to the case of a lower sliding velocity.

Study of the Relationship between Boundary Slip and Nanobubbles on a Smooth Hydrophobic Surface.

Surface nanobubbles, which are nanoscopic or microscopic gaseous domains forming at the solid/liquid interface, have a strong impact on the interface by changing the two-phase contact to a three-phase contact. Therefore, they are believed to affect the boundary condition and liquid flow. However, there are still disputes in the theoretical studies as to whether the nanobubbles can increase the slip length effectively. Furthermore, there are still no direct experimental studies to support either side. Therefore, an intensive study on the effective slip length for flows over bare surfaces with nanobubbles is essential for establishing the relation between nanobubbles and slip length. Here, we study the effect of nanobubbles on the slippage experimentally and theoretically. Our experimental results reveal an increase from 8 to 512 nm in slip length by increasing the surface coverage of nanobubbles from 1.7 to 50.8% and by decreasing the contact angle of nanobubbles from 42.8 to 16.6°. This is in good agreement with theoretical results. Our results indicate that nanobubbles could always act as a lubricant and significantly increase the slip length. The surface coverage, height, and contact angle are key factors for nanobubbles to reduce wall friction.

Surface charge-induced EDL interaction on the contact angle of surface nanobubbles.

The contact angle (CA) of surface nanobubbles is believed to affect the stability of nanobubbles and fluid drag in micro/nanofluidic systems. The CA of nanobubbles is dependent on size and is believed to be affected by the surface charge-induced electrical double layer (EDL). However, neither of these of attributes are well understood. In this paper, by introducing an EDL-induced electrostatic wetting tension, a theoretical model is first established to study the effect of EDLs formed near the solid-liquid interface and the liquid-nanobubble interface on the gas phase CA of nanobubbles. The size-dependence of this EDL interaction is studied as well. Next, by using atomic force microscopy (AFM), the effect of the EDL on nanobubbles' gas phase CA is studied with variable electrical potential at the solid-liquid interface, which is adjusted by an applied voltage. Both the theoretical and the experimental results show that the EDLs formed near the solid-liquid interface and the liquid-nanobubble interface lead to a reduction of gas phase CA of the surface nanobubbles because of an electrostatic wetting tension on the nanobubble due to the attractive electrostatic interaction between the liquid and nanobubble within the EDL, which is in the nanobubbles' outward direction. An EDL with a larger zeta potential magnitude leads to a larger gas phase CA reduction. Furthermore, the effect of EDL on the nanobubbles' gas phase CA shows a significant size-dependence considering the size dependence of the electrostatic wetting tension. The gas phase CA reduction due to the EDL decreases with increasing nanobubble height and increases with the nanobubble's increasing curvature radius, indicating that a surface charge-induced EDL could possibly explain the size dependence of the gas phase CA of nanobubbles.

The coupling of surface charge and boundary slip at the solid-liquid interface and their combined effect on fluid drag: A review.

Fluid drag of micro/nano fluidic systems has inspired wide scientific interest. Surface charge and boundary slip at the solid-liquid interface are believed to affect fluid drag. This review summarizes the recent studies on the coupling of surface charge and slip, and their combined effect on fluid drag at micro/nano scale. The effect of pH on surface charge of borosilicate glass and silica surfaces in deionized (DI) water and saline solution is discussed using a method based on colloidal probe atomic force microscopy (AFM). The boundary slip of various oil-solid interfaces are discussed for samples with different degrees of oleophobicity prepared by nanoparticle-binder system. By changing the pH of solution or applying an electric field, effect of surface charge on slip of a smooth hydrophobic octadecyltrichlorosilane (OTS) in DI water and saline solution is studied. A theoretical model incorporating the coupling relationship between surface charge and slip is used to discuss the combined effect of surface charge-induced electric double layer (EDL) and slip on fluid drag of pressure-driven flow in a one-dimensional parallel-plates microchannel. A theoretical method is used to reduce the fluid drag. The studies show that the increasing magnitude of surface charge density leads to a decrease in slip length. The surface charge results in a larger fluid drag, and the coupling of surface charge and slip can further increase the fluid drag. Surface charge-induced EDLs with asymmetric zeta potentials can effectively reduce the fluid drag.

Electroviscous effect on fluid drag in a microchannel with large zeta potential.

The electroviscous effect has been widely studied to investigate the effect of surface charge-induced electric double layers (EDL) on the pressure-driven flow in a micro/nano channel. EDL has been reported to reduce the velocity of fluid flow and increase the fluid drag. Nevertheless, the study on the combined effect of EDL with large zeta potential up to several hundred millivolts and surface charge depenedent-slip on the micro/nano flow is still needed. In this paper, the nonlinear Poisson-Boltzmann equation for electrical potential and ion distribution in non-overlapping EDL is first analytically solved. Then, the modified Navier-Stokes equation for the flow considering the effect of surface charge on the electrical conductivity of the electrolyte and slip length is analytically solved. This analysis is used to study the effect of non-overlapping EDL with large zeta potential on the pressure-driven flow in a microchannel with no-slip and charge-dependent slip conditions. The results show that the EDL leads to an increase in the fluid drag, but that slip can reduce the fluid drag. When the zeta potential is large enough, the electroviscous effect disappears for flow in the microchannel under a no-slip condition. However, the retardation of EDL on the flow and the enhancement of slip on the flow counteract each other under a slip condition. The underlying mechanisms of the effect of EDL with large zeta potential on fluid drag are the high net ionic concentration near the channel wall and the fast decay of electrical potential in the EDL when the zeta potential is large enough.

Coalescence and stability analysis of surface nanobubbles on the polystyrene/water interface.

In this article, we have studied the surface nanobubbles on polystyrene (PS)/water interfaces using tapping mode atomic force microscopy (TM-AFM). Detailed bubble coalescence phenomenon of differently sized surface nanobubbles (with lateral size up to about ∼10 µm) was obtained. The quantity of gas molecules, before and after coalescence, was calculated. It was found that after coalescence the quantity of gas molecules was increased by approximately 112.5%. The possible reasons for this phenomenon were analyzed and discussed. Our analysis shows that a reasonable explanation should be an influx of gas into the bubble caused by the depinning of the contact line and the decrease in the inner pressure during bubble coalescence. The factors affecting the coalescence speed of surface bubbles were also discussed. It was found that the coalescence speed of larger bubbles is usually slower than that of the smaller ones. We also noticed that it is uncertain whether a larger or smaller bubble will move first to merge into others. This is due to the combined effects of the contact line and the surface properties. Furthermore, the temporal evolution of surface bubbles was studied. The three-phase contact line of bubbles kept the pinning within the incubation time. This was consistent with the contact line pinning theory, based on which the theoretical lifetime of the surface bubbles in our experiments was calculated to be t(b) ≈ 6.9 h. This value is close to the experimental results. Meanwhile, the faster gas diffusion from the oversized bubbles after 12 h of incubation was observed and analyzed. Our results indicate that a viable stability mechanism for surface nanobubbles would be favored simultaneously by the contact line pinning, gas influx near the contact line from an interfacial gas enrichment (IGE), a thin "contaminant film" around the gas/liquid interface, and even the electrostatic effect.

Boundary slip of superoleophilic, oleophobic, and superoleophobic surfaces immersed in deionized water, hexadecane, and ethylene glycol.

The boundary slip condition is an important property, and its existence can reduce fluid drag in micro/nanofluidic systems. The boundary slip on various surfaces immersed in water and various electrolytes has been widely studied. For the surfaces immersed in oil, the boundary slip on superoleophilic and oleophilic surfaces has been studied, but there is no data on oleophobic and superoleophobic surfaces. In this paper, experiments are carried out to study electrostatic force and boundary slip on superoleophilic, oleophobic, and superoleophobic surfaces immersed in deionized (DI) water, hexadecane, and ethylene glycol. In addition, the surface charge density of the samples immersed in DI water is quantified. Results show that the electrostatic force and the absolute value of the surface charge density of an octadecyltrichlorosilane surface are larger than that of a polystyrene surface, and the electrostatic force and the absolute value of surface charge density of a superoleophilic surface are larger than that of oleophobic and superoleophobic surfaces. For the same liquid, the larger contact angle leads to a larger slip length at the solid-liquid interface. For the same surface, the larger liquid viscosity leads to a larger slip length. The relevant mechanisms are discussed in this paper.

Quantification of surface charge density and its effect on boundary slip.

Reduction of fluid drag is important in the micro-/nanofluidic systems. Surface charge and boundary slip can affect the fluid drag, and surface charge is also believed to affect boundary slip. The quantification of surface charge and boundary slip at a solid-liquid interface has been widely studied, but there is a lack of understanding of the effect of surface charge on boundary slip. In this paper, the surface charge density of borosilicate glass and octadecyltrichlorosilane (OTS) surfaces immersed in saline solutions with two ionic concentrations and deionized (DI) water with different pH values and electric field values is quantified by fitting experimental atomic force microscopy (AFM) electrostatic force data using a theoretical model relating the surface charge density and electrostatic force. Results show that pH and electric field can affect the surface charge density of glass and OTS surfaces immersed in saline solutions and DI water. The mechanisms of the effect of pH and electric field on the surface charge density are discussed. The slip length of the OTS surface immersed in saline solutions with two ionic concentrations and DI water with different pH values and electric field values is measured, and their effects on the slip length are analyzed from the point of surface charge. Results show that a larger absolute value of surface charge density leads to a smaller slip length for the OTS surface.

Effect of boundary slip and surface charge on the pressure-driven flow.

Drag reduction in micro/nanofluidic systems is an important issue. The effect of boundary slip and electrical double layer (EDL) induced by surface charge on the pressure-driven flow in a micro/nanochannel has been widely studied. However, change in electrical conductivity as a result of ionic redistribution caused by surface charge, which can affect the EDL-induced electrical force exerted on the flow, is often neglected. In addition, the effect of surface charge on the slip length is not considered. In this work, a model incorporating the effect of surface charge on electrical conductivity and slip length was developed to investigate the effect of boundary slip and EDL induced by surface charge on the volumetric flow rate and skin friction coefficient. The underlying mechanisms for the results regarding the effect of slip and surface charge on the flow were analyzed.