Self-Assembly of Strain-Adaptable Surface-Enhanced Raman Scattering Substrate on Polydimethylsiloxane Nanowrinkles.

Flexible surface-enhanced Raman scattering (SERS) substrates adaptable to strains enable effective sampling from irregular surfaces, but the preparation of highly stable and sensitive flexible SERS substrates is still challenging. This paper reports a method to fabricate a high-performance strain-adaptable SERS substrate by self-assembly of Au nanoparticles (AuNPs) on polydimethylsiloxane (PDMS) nanowrinkles. Nanowrinkles are created on prestrained PDMS slabs by plasma-induced oxidation followed by the release of the prestrain, and self-assembled AuNPs are transferred onto the nanowrinkles to construct the high-performance SERS substrate. The results show that the nanowrinkled structure can improve the surface roughness and enhance the SERS signals by ∼4 times compared to that of the SERS substrate prepared on flat PDMS substrates. The proposed SERS substrate also shows good adaptability to dynamic bending up to ∼|0.4| 1/cm with excellent testing reproducibility. Phenolic pollutants, including aniline and catechol, were quantitatively tested by the SERS substrate. The self-assembled flexible SERS substrate proposed here provides a powerful tool for chemical analysis in the fields of environmental monitoring and food safety inspection.

Modification of the electrokinetic motion of microalgae through light illumination for viability assessment.

The viability detection of microalgae with the electrokinetic (EK) technique shows vast applications in the biology and maritime industry. However, due to the slight variations in the EK properties between alive and dead microalgae cells, the accuracy and practicability of this technique is limited. In this paper, the light illumination pretreatment was conducted to modify the EK velocity of microalgae for enhancing the EK difference. The effects of the illumination time and light color on the EK velocities of Chlorella vulgaris and Isochrysis galbana were systematically measured, and the EK differences between alive and dead cells were calculated and compared. The results indicate that under light illumination, the photosynthesis of the alive cells leads to the amplification of the zeta potential, leading toward increase in the EK difference along with the illumination time. By using light with different color spectra to treat the microalgae, it was found that the EK difference changes with the light color according to the following order: white light > red light > blue light > green light. The difference in EK potential with exposure to white light treatment surpasses over 10-fold in comparison to those without such treatment. The light pretreatment technique, as illustrated in this study, offers an advantageous strategy to enhance the EK difference between living and dead cells, proving beneficial in the field of microalgae biotechnology.

Elasto-inertial instabilities in the merging flow of viscoelastic fluids.

Many engineering and natural phenomena involve the merging of two fluid streams through a T-junction. Previous studies of such merging flows have been focused primarily upon Newtonian fluids. We observed in our recent experiment with five different polymer solutions a direct change from an undisturbed to either a steady vortical or unsteady three-dimensional flow at the T-junction with increasing inertia. The transition state(s) in between these two types of merging flow patterns is, however, yet to be known. We present here a systematic experimental study of the merging flow of polyethylene oxide (PEO) solutions with varying polymer concentrations and molecular weights. Two new paths of flow development are identified with the increase of Reynolds number: one is the transition in very weakly viscoelastic fluids first to steady vortical flow and then to a juxtaposition state with an unsteady elastic eddy zone in the middle and a steady inertial vortex on each side, and the other is the transition in weakly viscoelastic fluids first to a steady vortical and/or a juxtaposition state and then to a fully unsteady flow. Interestingly, the threshold Reynolds number for the onset of elastic instabilities in the merging flow is not a monotonic function of the elasticity number, but instead follows a power-law dependence on the polymer concentration relative to its overlap value. Such a dependence turns out qualitatively consistent with the prediction of the McKinley-Pakdel criterion.

Recent advances in multimode microfluidic separation of particles and cells.

Microfluidic separation of particles and cells is crucial to lab-on-a-chip applications in the fields of science, engineering, and industry. The continuous-flow separation methods can be classified as active or passive depending on whether the force involved in the process is externally imposed or internally induced. The majority of current separations have been realized using only one of the active or passive methods. Such a single-mode process is usually limited to one-parameter separation, which often becomes less effective or even ineffective when dealing with real samples because of their inherent heterogeneity. Integrating two or more separation methods of either type has been demonstrated to offer several advantages like improved specificity, resolution, and throughput. This article reviews the recent advances of such multimode particle and cell separations in microfluidic devices, including the serial-mode prefocused separation, serial-mode multistage separation, and parallel-mode force-tuned separation.

Electrokinetics in antimicrobial resistance analysis: A review.

Infections caused by antimicrobial resistance are a serious problem in the world. Currently, commercial devices for antimicrobial susceptibility testing and resistant bacteria identification are time-consuming. There is an urgent need to develop fast and accurate methods, especially in the process of sample pretreatment. Electrokinetic (EK) is a family of electric-field-based kinetic phenomena of fluid or embedded objects, and EK applications have been found in various fields. In this paper, EK bacteria manipulation, including enrichment and separation, is reviewed. Focus is given to the rapid electric-based minimum inhibitory concentration measurement. The future directions and major challenges in this field are also outlined.

Electrokinetic transport phenomena in nanofluidics and their applications.

Much progress has been made in the electrokinetic phenomena inside nanochannels in the last decades. As the dimensions of the nanochannels are compatible to that of the electric double layer (EDL), the electrokinetics inside nanochannels indicate many unexpected behaviors, which show great potential in the fields of material science, biology, and chemistry. This review summarizes the recent development of nanofluidic electrokinetics in both fundamental and applied research. First, the techniques for constructing nanochannels are introduced to give a guideline for choosing the optimal fabrication technique based on the specific feature of the nanochannel. Then, the theories and experimental investigations of the EDL, electroosmotic flow, and electrophoresis of nanoparticles inside the nanochannels are discussed. Furthermore, the applications of nanofluidic electrokinetics in iontronics, sensing, and biomolecule separation fields are summarized. In Section 5, some critical challenges and the perspective on the future development of nanofluidic electrokinetics are briefly proposed.

Electrokinetic motion and viability assessment of algae with a polyethylene glycol-dextran interface.

At present, there is still limited report on the electrokinetic (EK) behavior of bioparticles at the interface of an aqueous two-phase system. In this paper, the EK motion and viability assessment of live algae mixed with the NaClO treated dead algae were carried out at the interface formed by polyethylene glycol (PEG)-rich phase and dextran (DEX)-rich phase in a straight microchannel. The experimental results show that both the live and dead algae at the PEG-DEX interface migrate from the negative electrode to the positive electrode, and the EK velocity of live algae at the interface is slightly larger than that of the dead ones with similar diameters. For either live or dead algae, the EK velocity at the interface decreases with the increase in diameter. A size-velocity curve was used to evaluate the viability of the algae. As most of the microorganisms in ballast water are algae, the method in this paper provides a promising way to detect and evaluate the live microorganism in treated ballast water of a ship.

Electrokinetic Effect of a Two-Liquid Interface within a Slit Microchannel.

This paper presents an investigation of the electrokinetic effect at a two-liquid (immiscible liquid-aqueous solution) interface within a slit microchannel using a three-dimensional (3D) numerical model, with a particular focus on the impact of the surface ζ-potential and liquid phase height on the interface electrokinetic velocity. The findings indicate that the direction of the interface movement depends on the ζ-potentials at the two-liquid interface and the microchannel wall. When the absolute value of the negative ζ-potential at the interface is smaller than that at the wall, the interface moves toward the negative pole of the applied direct current (DC) electric field; conversely, it moves in the opposite direction. The velocity of interface motion decreases as the height of the aqueous phase and the dynamic viscosity ratio between the immiscible liquid and the aqueous solution increase. Conversely, the velocity increases with an elevation in the height of the immiscible liquid phase and the DC electric field intensity. This study holds significant importance in elucidating the patterns of change in fluid interface electrokinetic effects and their potential applications in manipulating and separating particulate pollutants within water systems.

An experimental study of the merging flow of polymer solutions in a T-shaped microchannel.

The merging flow through a T-junction is relevant to sample mixing and particle manipulation in microfluidic devices. It has been extensively studied for Newtonian fluids, particularly in the high inertial regime where flow bifurcation takes place for enhanced mixing. However, the effects of fluid rheological properties on the merging flow have remained largely unexplored. We investigate here the flow of five types of polymer solutions along with water in a planar T-shaped microchannel over a wide range of flow rates for a systematic understanding of the effects of fluid shear thinning and elasticity. It is found that the merging flow near the stagnation point of the T-junction can either be vortex dominated or have unsteady streamlines, depending on the strength of elasticity and shear thinning present in the fluid. Moreover, the shear thinning effect is found to induce a symmetric unsteady flow in comparison to the asymmetric unsteady flow in the viscoelastic fluids, the latter of which exhibits greater interfacial fluctuations.

Surface charging and electrophoretic behavior of conductive polymer micro-droplets in conductive polymer liquid solutions.

This study investigates the surface charging and electrophoretic motion of polyethylene glycol-rich (PEG-rich) micro-droplets in dextran-rich solutions or dextran-rich micro-droplets in PEG-rich solutions. The electrophoretic velocities of the droplets were measured in a centimeter-sized chamber under an optical microscope. It was found that the direction of electrophoretic motion of both the PEG-rich droplets and dextran-rich droplets is opposite to the applied electric field, meaning that both the PEG-rich droplets and dextran-rich droplets are negatively charged. The electrophoretic velocity is independent of droplet size but proportional to the electric field strength. Increasing the NaCl concentration reduces the electrophoretic velocity of PEG-rich droplets and increases it for dextran-rich droplets, suggesting different surface charge changes due to ion affinity. The charge densities and velocities are affected by the PEG and dextran mass fractions. Physical models for droplet surface charging under different conditions were proposed to explain the experimental results.

Marine Structural Health Monitoring with Optical Fiber Sensors: A Review.

Real-time monitoring of large marine structures' health, including drilling platforms, submarine pipelines, dams, and ship hulls, is greatly needed. Among the various kinds of monitoring methods, optical fiber sensors (OFS) have gained a lot of concerns and showed several distinct advantages, such as small size, high flexibility and durability, anti-electromagnetic interference, and high transmission rate. In this paper, three types of OFS used for marine structural health monitoring (SHM), including point sensing, quasi-distributed sensing, and distributed sensing, are reviewed. Emphases are given to the applicability of each type of the sensors by analyzing the operating principles and characteristics of the OFSs. The merits and demerits of different sensing schemes are discussed, as well as the challenges and future developments in OFSs for the marine SHM field.

Size-dependent electrophoretic motion of polystyrene particles at polyethylene glycol-dextran interfaces.

Currently, there is very limited information on the electrophoretic behavior of particles at a liquid-liquid interface formed by two conducting liquid solutions. Here, electrophoretic velocities of polystyrene particles at a polyethylene glycol (PEG)-dextran (DEX) interface were investigated in this paper. Experimental results show that the particle at the interface moves in the opposite direction to the applied electric field, with a velocity much lower than that in the PEG-rich phase and a litter larger than that in the DEX-rich phase. Similarly to the movement in Newtonian fluids, the velocity increases linearly with the increase in the applied electric field. Different to particle electrophoresis in Newtonian fluids, the velocities of the particles at the PEG-DEX interface increase linearly with the decrease in particle's diameters, implying a possible size-based particle differentiation at an interface.

Fluid rheological effects on streaming dielectrophoresis in a post-array microchannel.

Recent studies have demonstrated the strong influences of fluid rheological properties on insulator-based dielectrophoresis (iDEP) in single-constriction microchannels. However, it is yet to be understood how iDEP in non-Newtonian fluids depends on the geometry of insulating structures. We report in this work an experimental study of fluid rheological effects on streaming DEP in a post-array microchannel that presents multiple contractions and expansions. The iDEP focusing and trapping of particles in a viscoelastic polyethylene oxide solution are comparable to those in a Newtonian buffer, which is consistent with the observations in a single-constriction microchannel. Similarly, the insignificant iDEP effects in a shear-thinning xanthan gum solution also agree with those in the single-constriction channel except that gel-like structures are observed to only form in the post-array microchannel under large DC electric fields. In contrast, the iDEP effects in both viscoelastic and shear-thinning polyacrylamide solution are significantly weaker than in the single-constriction channel. Moreover, instabilities occur in the electroosmotic flow and appear to be only dependent on the DC electric field. These phenomena may be associated with the dynamics of polymers as they are electrokinetically advected around and through the posts.

Lensless computational imaging with a hybrid framework of holographic propagation and deep learning.

Lensless imaging has attracted attention as it avoids the bulky optical lens. Lensless holographic imaging is a type of a lensless imaging technique. Recently, deep learning has also shown tremendous potential in lensless holographic imaging. A labeled complex field including real and imaginary components of the samples is usually used as a training dataset. However, obtaining such a holographic dataset is challenging. In this Letter, we propose a lensless computational imaging technique with a hybrid framework of holographic propagation and deep learning. The proposed framework takes recorded holograms as input instead of complex fields, and compares the input and regenerated holograms. Compared to previous supervised learning schemes with a labeled complex field, our method does not require this supervision. Furthermore, we use the generative adversarial network to constrain the proposed framework and tackle the trivial solution. We demonstrate high-quality reconstruction with the proposed framework compared to previous deep learning methods.

Effects of vertical confinement on the flow of polymer solutions in planar constriction microchannels.

The flow of polymer solutions under extensional conditions is frequently encountered in numerous engineering fields. Planar contraction and/or expansion microchannels have been a subject of interest for many studies in that regard, which, however, have mostly focused on shallow channel structures. We investigate here the effect of changing the depth of contraction-expansion microchannels on the flow responses of three types of polymer solutions and water. The flow of viscoelastic polyethylene oxide (PEO) solution is found to become more stable with suppressed vortex formation and growth in the contraction part while being less stable in the expansion part with the increase of the channel depth. These opposing trends in the contraction and expansion flows are noted to have similarities with our recent findings of constriction length-dependent instabilities in the same PEO solution (M. K. Raihan, S. Wu, Y. Song and X. Xuan, Soft Matter, 2021, 17, 9198-9209), where the contraction flow gets stabilized while the expansion flow becomes destabilized with the increase of the constriction length. In contrast, the entire flow becomes less stable in deeper channels for the shear-thinning xanthan gum (XG) solution as well as the shear thinning and viscoelastic polyacrylamide (PAA) solution. This observation aligns with that of water flow, which is attributed to the reduced top/bottom wall stabilizing effects.

Conductivity-difference-enhanced DC dielectrophoretic particle separation in a microfluidic chip.

A conductivity-difference-based method for increasing dielectrophoretic (DEP) force for particle separation in a microfluidic chip is presented in this paper. By applying a direct-current (DC) voltage across two immiscible electrolyte solutions with a conductivity difference, an enhanced electric field gradient is generated at the liquid-liquid interface. Theoretical analysis based on equivalent circuit theory found that the gradient of the electric field squared increases with the decrease in the conductivity ratio of the two liquids (main channel to the side channel). As a result, the particle separation distance (an indicator of DEP force) increases with the decrease in the conductivity ratio, which is both numerically predicted and experimentally verified. Numerical simulations also show that the separation distance increases with the increase in the magnitude of the electric field and the decrease in the width of the orifice. The method presented in this paper is simple and advantageous for increasing DEP force without applying higher DC voltages or fabricating smaller orifices.

Living algae detection with a PDMS-liquid chlorophyll fluorescence microfluidic chip filter and a smartphone.

Building an optical filtration function into a microfluidic chip is a promising way of simplifying the optical detection system of a microfluidic device. In this paper, a PDMS microfluidic chip filter that is capable of transmitting chlorophyll fluorescence and blocking interfering light in the visible wavelength range was developed for living algae detection with a smartphone. The chip was fabricated by sealing a layer of crystal violet solution in a PDMS layer on the top of the Sudan II-doped PDMS slab, which has a straight microchannel. Optimum dye concentrations and thicknesses for the crystal violet solution layer and Sudan II-doped PDMS slab were investigated and determined by spectrum measurements. It was found that the cut-on range of this integrated microfluidic chip is extended to about 625 nm and the transmittance in the chlorophyll fluorescence range (650 nm to 710 nm) is as high as 95%, when 25 mg L-1 Sudan II-doped PDMS slab (with a 3 mm thickness) and 2 mg L-1 crystal violet solution (with a 0.3 mm thickness) were used. Living algae detection using this chlorophyll-fluorescence-filtering PDMS microfluidic chip and a smartphone-based imaging platform was achieved, and the results compared favorably with those using a commercial filter.

Mechanism and performance of ionic diodes fabricated from 2D trapezoidal-shaped nanochannels.

Bioinspired asymmetric two-dimensional (2D) nanochannels with ionic diode behavior are highly desirable, as they can be constructed and modified easily. However, the knowledge about the rectification mechanism of the nanochannels is still very limited. In this paper, the ionic current rectification (ICR) of the 2D trapezoidal-shaped nanochannels was studied both numerically and experimentally. A multi-physics model, considering the electric field, the ion concentration field, and the flow field, was built for simulating the ion transportation inside the nanochannels. With a limited channel height, the 2D nanochannels are counter-ion selective; therefore, under an external electric field, the accumulation of co-ions takes place at one end of the nanochannels. By introducing shape asymmetry to the nanochannels, the ICR was achieved due to the asymmetric ion concentration polarization at two ends of the nanochannels under opposite electric fields. The structure of the nanochannels, the surface charge density of the nanochannel walls, and the ionic strength of the working fluids affect the ICR of the ionic diodes by changing the ion concentration polarization at two ends of the nanochannels. In the experiment, the current-voltage curves of the nanochannel arrays fabricated by assembling graphene oxide nanosheets were measured, which are in accordance with the numerical results. This paper provides a comprehensive understanding of the mechanism of the 2D trapezoidal-shaped ionic diodes, which may act as a guideline for the design and optimization of ionic diodes.

Insulator-based dielectrophoretic focusing and trapping of particles in non-Newtonian fluids.

Insulator-based dielectrophoretic (iDEP) microdevices have been limited to work with Newtonian fluids. We report an experimental study of the fluid rheological effects on iDEP focusing and trapping of polystyrene particles in polyethylene oxide, xanthan gum, and polyacrylamide solutions through a constricted microchannel. Particle focusing and trapping in the mildly viscoelastic polyethylene oxide solution are slightly weaker than in the Newtonian buffer. They are, however, significantly improved in the strongly viscoelastic and shear thinning polyacrylamide solution. These observed particle focusing behaviors exhibit a similar trend with respect to electric field, consistent with a revised theoretical analysis for iDEP focusing in non-Newtonian fluids. No apparent focusing of particles is achieved in the xanthan gum solution, though the iDEP trapping can take place under a much larger electric field than the other fluids. This is attributed to the strong shear thinning-induced influences on both the electroosmotic flow and electrokinetic/dielectrophoretic motions.

Joule heating-enabled electrothermal enrichment of nanoparticles in insulator-based dielectrophoretic microdevices.

Insulator-based dielectrophoresis (iDEP) exploits the electric field gradients formed around insulating structures to manipulate particles for diverse microfluidic applications. Compared to the traditional electrode-based dielectrophoresis, iDEP microdevices have the advantages of easy fabrication, free of water electrolysis, and robust structure, etc. However, the presence of in-channel insulators may cause thermal effects because of the locally amplified Joule heating of the fluid. The resulting electrothermal flow circulations are exploited in this work to trap and concentrate nanoscale particles (of 100 nm diameter and less) in a ratchet-based iDEP microdevice. Such Joule heating-enabled electrothermal enrichment of nanoparticles are found to grow with the increase of alternating current or direct current electric field. It also becomes more effective for larger particles and in a microchannel with symmetric ratchets. Moreover, a depth-averaged numerical model is developed to understand and simulate the various parametric effects, which is found to predict the experimental observations with a good agreement.