Measurement and mitigation of free convection in microfluidic gradient generators

A Cautionary Perspective on Hydrogel-Induced Concentration Gradient Generation for Studying Chemotaxis

The achievement of consistent and static chemical gradients is critically important in the study of diffusion and chemotaxis at the micro- and nanoscales. In this context, a number of groups have reported on hydrogel-based systems for generating concentration gradients. Here, we analyze the behavior of agarose and gelatin-based hydrogels in hybridization chambers of different heights. Our focus is on the issues that are caused by the presence of robust bulk fluid flows in such systems due to the solutes present in the hydrogel and/or the surrounding fluid. We describe the key insights derived from these experiments, offering practical guidelines for establishing gradients using hydrogel-based systems and make the community aware of different variables that can make the experiments nonreproducible and prone to misinterpretations.

Unidirectional drying of a suspension of diffusiophoretic colloids under gravity

Recent experiments (K. Inoue and S. Inasawa, RSC Adv., 2020, 10, 15763-15768) and simulations (J.-B. Salmon and F. Doumenc, Phys. Rev. Fluids, 2020, 5, 024201) demonstrated the significant impact of gravity on unidirectional drying of a colloidal suspension. However, under gravity, the role of colloid transport induced by an electrolyte concentration gradient, a mechanism known as diffusiophoresis, is unexplored to date. In this work, we employ direct numerical simulations and develop a macrotransport theory to analyze the advective-diffusive transport of an electrolyte-colloid suspension in a unidirectional drying cell under the influence of gravity and diffusiophoresis. We report three key findings. First, drying a suspension of solute-attracted diffusiophoretic colloids causes the strongest phase separation and generates the thinnest colloidal layer compared to non-diffusiophoretic or solute-repelled colloids. Second, when colloids are strongly solute-repelled, diffusiophoresis prevents the formation of colloid concentration gradient and hence gravity has a negligible effect on colloidal layer formation. Third, our macrotransport theory predicts new scalings for the growth of the colloidal layer. The scalings match with direct numerical simulations and indicate that the colloidal layer produced by solute-repelled diffusiophoretic colloids could be an order of magnitude thicker compared to non-diffusiophoretic or solute-attracted colloids. Our results enable tailoring the separation of colloid-electrolyte suspensions by tuning the interactions between the solvent, electrolyte, and colloids under Earth's or microgravity, which is central to ground-based and in-space applications.

Convection Confounds Measurements of Osmophoresis for Lipid Vesicles in Solute Gradients

Lipid vesicles immersed in solute gradients are predicted to migrate from regions of high to low solute concentration due to osmotic flows induced across their semipermeable membranes. This process─known as osmophoresis─is potentially relevant to biological processes such as vesicle trafficking and cell migration; however, there exist significant discrepancies (several orders of magnitude) between experimental observations and theoretical predictions for the vesicle speed. Here, we seek to reconcile predictions of osmophoresis with observations of vesicle motion in osmotic gradients. We prepare quasi-steady solute gradients in a microfluidic chamber using density-matched solutions of sucrose and glucose to eliminate buoyancy-driven flows. We quantify the motions of giant DLPC vesicles and Brownian tracer particles in such gradients using Bayesian analysis of particle tracking data. Despite efforts to mitigate convective flows, we observe directed motion of both lipid vesicles and tracer particles in a common direction at comparable speeds of order 10 nm/s. These observations are not inconsistent with models of osmophoresis, which predict slower motion at ca. 1 nm/s; however, experimental uncertainty and the confounding effects of fluid convection prohibit a quantitative comparison. In contrast to previous reports, we find no evidence for anomalously fast osmophoresis of lipid vesicles when fluid convection is mitigated and quantified. We discuss strategies for enhancing the speed of osmophoresis using high permeability membranes and geometric confinement.

Bacterial chemotaxis in static gradients quantified in a biopolymer membrane-integrated microfluidic platform

Chemotaxis is a fundamental bacterial response mechanism to changes in chemical gradients of specific molecules known as chemoattractant or chemorepellent. The advancement of biological platforms for bacterial chemotaxis research is of significant interest for a wide range of biological and environmental studies. Many microfluidic devices have been developed for its study, but challenges still remain that can obscure analysis. For example, cell migration can be compromised by flow-induced shear stress, and bacterial motility can be impaired by nonspecific cell adhesion to microchannels. Also, devices can be complicated, expensive, and hard to assemble. We address these issues with a three-channel microfluidic platform integrated with natural biopolymer membranes that are assembled in situ. This provides several unique attributes. First, a static, steady and robust chemoattractant gradient was generated and maintained. Second, because the assembly incorporates assembly pillars, the assembled membrane arrays connecting nearby pillars can be created longer than the viewing window, enabling a wide 2D area for study. Third, the in situ assembled biopolymer membranes minimize pressure and/or chemiosmotic gradients that could induce flow and obscure chemotaxis study. Finally, nonspecific cell adhesion is avoided by priming the polydimethylsiloxane (PDMS) microchannel surfaces with Pluronic F-127. We demonstrated chemotactic migration of Escherichia coli as well as Pseudomonas aeruginosa under well-controlled easy-to-assemble glucose gradients. We characterized motility using the chemotaxis partition coefficient (CPC) and chemotaxis migration coefficient (CMC) and found our results consistent with other reports. Further, random walk trajectories of individual cells in simple bright field images were conveniently tracked and presented in rose plots. Velocities were calculated, again in agreement with previous literature. We believe the biopolymer membrane-integrated platform represents a facile and convenient system for robust quantitative assessment of cellular motility in response to various chemical cues.

Temperature dependence of diffusiophoresis via a novel microfluidic approach

The temperature dependence of the diffusiophoretic mobility (D_DP) is investigated experimentally and compared with theoretical predictions. These systematic measurements were made possible by a new microfluidic approach that enables truly steady state gradients to be imposed, and direct and repeatable measurements of diffusiophoretic migration to be made over hours-long time scales. Diffusiophoretic mobilities were measured for fluorescent, negatively charged polystyrene particles under NaCl gradients, at temperatures ranging from 20 °C to 70 °C. Measured D_DP values were found to increase monotonically with temperature, and to agree, both qualitatively and relatively quantitatively, with theoretical predictions based on electrophoretically-measured zeta potentials. These results provide confidence that existing diffusiophoresis theories can accurately predict DP mobilities over a range of temperatures. More broadly, we anticipate our new microfluidic approach will facilitate and enable new tests of diffusiophoretic phenomena under a wide range of physical and chemical conditions.

Mixing Performance of a Passive Micro-Mixer with Mixing Units Stacked in Cross Flow Direction

A new passive micro-mixer with mixing units stacked in the cross flow direction was proposed, and its performance was evaluated numerically. The present micro-mixer consisted of eight mixing units. Each mixing unit had four baffles, and they were arranged alternatively in the cross flow and transverse direction. The mixing units were stacked in four different ways: one step, two step, four step, and eight step stacking. A numerical study was carried out for the Reynolds numbers from 0.5 to 50. The corresponding volume flow rate ranged from 6.33 μL/min to 633 μL/min. The mixing performance was analyzed in terms of the degree of mixing (DOM) and relative mixing energy cost (MEC). The numerical results showed a noticeable enhancement of the mixing performance compared with other micromixers. The mixing enhancement was achieved by two flow characteristics: baffle wall impingement by a stream of high concentration and swirl motion within the mixing unit. The baffle wall impingement by a stream of high concentration was observed throughout all Reynolds numbers. The swirl motion inside the mixing unit was observed in the cross flow direction, and became significant as the Reynolds number increased to larger than about five. The eight step stacking showed the best performance for Reynolds numbers larger than about two, while the two step stacking was better for Reynolds numbers less than about two.

Combined Effects of Zeta-potential and Temperature of Nanopores on Diffusioosmotic Ion Transport

Diffusioosmosis (DO) results from ion transport near charged surfaces in the presence of electrolyte gradients and is critical in nanofluidic systems. However, DO has not yet been comprehensively studied because nanofabrication materials have limitations of low throughput and difficult quantification. Herein, we describe a self-assembled particle membrane (SAPM)-integrated microfluidic platform that can modulate the material properties (e.g., zeta-potential) and transport flux of nanopores. We quantify the effect of the zeta-potential on DO by measuring the electrical signals across three different nanopores/nanochannels of the SAPM. We then empirically quantify the effects of the temperature and ionic strength of the electrolytes on DO and reveal a nonlinear relationship with DO-driven ion transport; the ionic strengths govern the DO- or diffusion-effective ion transport phenomena. Finally, we demonstrate DO-driven electric power generation with enhanced performance as a potential application under optimized experimental conditions.

Flow-assembled chitosan membranes in microfluidics: recent advances and applications

The integration of membranes in microfluidic devices has been extensively exploited for various chemical engineering and bioengineering applications over the past few decades. To augment the applicability of membrane-integrated microfluidic platforms for biomedical and tissue engineering studies, a biologically friendly fabrication process with naturally occurring materials is highly desired. The in situ preparation of membranes involving interfacial reactions between parallel laminar flows in microfluidic networks, known as the flow-assembly technique, is one of the most biocompatible approaches. Membranes of many types with flexible geometries have been successfully assembled inside complex microchannels using this facile and versatile flow-assembly approach. Chitosan is a naturally abundant polysaccharide known for its pronounced biocompatibility, biodegradability, good mechanical stability, ease of modification and processing, and film-forming ability under near-physiological conditions. Chitosan membranes assembled by flows in microfluidics are freestanding, robust, semipermeable, and well-aligned in microstructure, and show high affinity to bioactive reagents and biological components (e.g. biomolecules, nanoparticles, or cells) that provide facile biological functionalization of microdevices. Here, we discuss the recent developments and optimizations in the flow-assembly of chitosan membranes and chitosan-based membranes in microfluidics. Furthermore, we recapitulate the applications of the chitosan membrane-integrated microfluidic platforms dedicated to biology, biochemistry, and drug release fields, and envision the future developments of this important platform with versatile functions.

Diffusioosmotic and convective flows induced by a nonelectrolyte concentration gradient

Glucose is an important energy source in our bodies, and its consumption results in gradients over length scales ranging from the subcellular to entire organs. Concentration gradients can drive material transport through both diffusioosmosis and convection. Convection arises because concentration gradients are mass density gradients. Diffusioosmosis is fluid flow induced by the interaction between a solute and a solid surface. A concentration gradient parallel to a surface creates an osmotic pressure gradient near the surface, resulting in flow. Diffusioosmosis is well understood for electrolyte solutes, but is more poorly characterized for nonelectrolytes such as glucose. We measure fluid flow in glucose gradients formed in a millimeter-long thin channel and find that increasing the gradient causes a crossover from diffusioosmosis-dominated to convection-dominated flow. We cannot explain this with established theories of these phenomena which predict that both scale linearly. In our system, the convection speed is linear in the gradient, but the diffusioosmotic speed has a much weaker concentration dependence and is large even for dilute solutions. We develop existing models and show that a strong surface-solute interaction, a heterogeneous surface, and accounting for a concentration-dependent solution viscosity can explain our data. This demonstrates how sensitive nonelectrolyte diffusioosmosis is to surface and solution properties and to surface-solute interactions. A comprehensive understanding of this sensitivity is required to understand transport in biological systems on length scales from micrometers to millimeters where surfaces are invariably complex and heterogeneous.

Interfacial Electrofabrication of Freestanding Biopolymer Membranes with Distal Electrodes

Using electrical signals to guide materials' deposition has a long-standing history in metal coating, microchip fabrication, and the integration of organics with devices. In electrodeposition, however, the conductive materials can be deposited only onto the electrode surfaces. Here, an innovative process is presented to electrofabricate freestanding biopolymer membranes at the interface of electrolytes without any supporting electrodes at the fabrication site. Chitosan, a derivative from the naturally abundant biopolymer chitin, has been broadly explored in electrodeposition for integrating biological entities onto microfabricated devices. It is widely believed that the pH gradients generated at the cathode deprotonate the positively charged chitosan chains into a film on the cathode surface. The interfacial electrofabrication with pH indicators, however, demonstrated that the membrane growth was driven by the instantaneous flow of hydroxyl ions from the ambient alginate solution, rather than the slow propagation of pH gradients from the cathode surface. This interfacial electrofabrication produces freestanding membrane structures and can be expanded to other materials, which presents a new direction in using electrical signals for manufacturing.

Collective diffusion coefficient of a charged colloidal dispersion: interferometric measurements in a drying drop

In the present work, we use Mach-Zehnder interferometry to thoroughly investigate the drying dynamics of a 2D confined drop of a charged colloidal dispersion. This technique makes it possible to measure the colloid concentration field during the drying of the drop at a high accuracy (about 0.5%) and with a high temporal and spatial resolution (about 1 frame per s and 5 μm per pixel). These features allow us to probe mass transport of the charged dispersion in this out-of-equilibrium situation. In particular, our experiments provide the evidence that mass transport within the drop can be described by a purely diffusive process for some range of parameters for which the buoyancy-driven convection is negligible. We are then able to extract from these experiments the collective diffusion coefficient of the dispersion D(φ) over a wide concentration range φ = 0.24-0.5, i.e. from the liquid dispersed state to the solid glass regime, with a high accuracy. The measured values of D(φ) ≃ 5-12D0 are significantly larger than the simple estimate D0 given by the Stokes-Einstein relation, thus highlighting the important role played by the colloidal interactions in such dispersions.