Diffusiophoresis in Polymer and Nanoparticle Gradients

Diffusiophoresis is the movement of the colloidal particles in response to a concentration gradient and can be observed for both electrolyte (e.g., salt) and nonelectrolyte (e.g., glucose) solutes. Here, we investigated the diffusiophoretic behavior of polystyrene (PS–carboxylate surface) microparticles in nonadsorbing charged and uncharged solute gradients [sodium polystyrenesulfonate (NaPSS), polyethylene glycol (PEG), and nanoscale colloidal silica (SiO2)] using a dead-end channel setup. We compared the diffusiophoretic motion in these gradient types with each other and to the case of using a monovalent salt gradient. In each of the nonadsorbing gradient systems (NaPSS, PEG, and SiO2 nanoparticles), the PS particles migrated toward the lower solute concentration. The exclusion distance values (from the initial position) of particles were recorded within the dead-end channel, and it was found that an increase in solute concentration increases exclusion from the main channel. In the polyelectrolyte case, the motion of PS microparticles was reduced by the addition of a background salt due to reduced electrostatic interaction, whereas it remained constant when using the neutral polymer. Particle diffusiophoresis in gradients of polyelectrolytes (charged macromolecules) is quite similar to the behavior when using a PEG gradient (uncharged macromolecule) in the presence of a background electrolyte. Moreover, we observed PS microparticles under different concentrations and molecular weights of PEG gradients. By combining the simulations, we estimated the exclusion length, which was previously proposed to be the order of the polymer radius. Furthermore, the movement of PS microparticles was analyzed in the gradient of silica nanoparticles. The exclusion distance was higher in silica nanoparticle gradients compared to similar-size PEG gradients because silica nanoparticles are charged. The diffusiophoretic transport of the PS microparticles could be simulated by considering the interaction between the PS microparticles and silica nanoparticles

Direct Measurements of Colloidal Solvophoresis under Imposed Solvent and Solute Gradients

We describe a microfluidic system that enables direct visualization and measurement of diffusiophoretic migration of colloids in response to imposed solution gradients. Such measurements have proven difficult or impossible in macroscopic systems due to difficulties in establishing solution gradients that are sufficiently strong yet hydrodynamically stable. We validate the system with measurements of the concentration-dependent diffusiophoretic mobility of polystyrene colloids in NaCl gradients, confirming that diffusiophoretic migration velocities are proportional to gradients in the logarithm of electrolyte concentration. We then perform the first direct measurement of the concentration-dependent "solvophoretic" mobility of colloids in ethanol-water gradients, whose dependence on concentration and gradient strength was not known either theoretically or experimentally, but which our measurements reveal to be proportional to the gradient in the logarithm of ethanol mole fraction. Finally, we examine solvophoretic migration under a variety of qualitatively distinct chemical gradients, including solvents that are miscible or have finite solubility with water, an electrolyte for which diffusiophoresis proceeds down concentration gradients (unlike for most electrolytes), and a nonelectrolyte (sugar). Our technique enables the direct characterization of diffusiophoretic mobilities of various colloids under various solvent and solute gradients, analogous to the electrophoretic ζ-potential measurements that are routinely used to characterize suspensions. We anticipate that such measurements will provide the feedback required to test and develop theories for solvophoretic and diffusiophoretic migration and ultimately to the conceptual design and engineering of particles that respond in a desired way to their chemical environments

The study of vesicle phoresis in a concentration gradient

The targeted delivery of drugs to specific diseased sites within the body is one of the major issues in the development of drug delivery today. This is particularly relevant in addressing central nervous system (CNS) disorders and diseases, diseases localised in the brain and spinal cord, where the delivery of the therapeutics is limited by the brain barriers. Consequently, the design of drug delivery systems able to independently navigate within the body to specifically deliver the cargo into the brain is desirable. Inspired by nature, scientists have created artificial self-propelling micro and nanoparticles, known as micro- or nano- swimmers. This thesis focuses on the self-assembly and migration of vesicular nanoparticles within a glucose gradient as potential nanoswimmers. Two types of nanoparticles are explored: a) bi-component PMPC-PDPA:PEO-PBO polymersomes and b) α-hemolysine-porated liposomes. First, a methodology to self-assemble the PMPC-PDPA:PEO-PBO polymersomes is optimised to attain a monodisperse, vesicular sample. This was followed by the incorporation of glucose oxidase enzyme into both the polymersomes and liposomes with an average of 6 and 16 enzymes per nanoparticle respectively. The diffusioosmotic drift and the background convection of the fluid within the glucose gradient in presence of the nanoparticles was then investigated. It is found that velocities of the fluid significantly vary by the mere introduction of a nanoparticle into the gradient. Velocity profiles of each nanoparticle in the glucose gradient were developed isolating the diffusioosmotic drifts. Finally, the diffusiophoretic and chemotactic components of the nanoparticles were quantified and isolated. It was found that the polymersomes experience higher mean-square displacement with directed self-propulsion velocities towards the region of higher glucose concentration. The liposomes experience a strong diffusioosmotic drift away from the glucose gradient, nevertheless, a slight reduction in these velocities was observed suggesting self-propulsion of the active liposomes towards the glucose incapable of overcoming the diffusioosmotic drift

Manipulating Particles for Micro- and Nano-Fluidics Via Floating Electrodes and Diffusiophoresis

The ability to accurately control micro- and nano-particles in a liquid is fundamentally useful for many applications in biology, medicine, pharmacology, tissue engineering, and microelectronics. Therefore, first particle manipulations are experimentally studied using electrodes attached to the bottom of a straight microchannel under an imposed DC or AC electric field. In contrast to a dielectric microchannel possessing a nearly-uniform surface charge, a floating electrode is polarized under the imposed electric field. The purpose is to create a non-uniform distribution of the induced surface charge, with a zero-net-surface charge along the floating electrode\u27s surface. Such a field, in turn, generates an induced-charge electro-osmotic (ICED) flow near the metal strip. The demonstrations by using single and multiple floating electrodes at the bottom of a straight microchannel, with induced DC electric field, include particle enrichment, movement, trapping, reversal of motion, separation, and particle focusing. A flexible strategy for the on-demand control of the particle enrichment and positioning is also proposed and demonstrated by using a locally-controlled floating metal electrode. Then, under an externally imposed AC electric field, the particle deposition onto a floating electrode, which is placed in a closed circular cavity, has been experimentally investigated. In the second part of the study, another particle manipulation method was computationally investigated. The diffusiophoretic and electrodiffusiophoretic motion of a charged spherical particle in a nanopore is subjected to an axial electrolyte concentration gradient. The charged particle experiences electrophoresis because of the imposed electric field and the diffusiophoresis is caused solely by the imposed concentration gradient. Depending on the magnitude and direction of the imposed concentration gradient, the particle\u27s electrophoretic motion can be accelerated, decelerated, and even reversed in a nanopore by the superimposed diffusiophoresis. Based on the results demonstrated in the present study, it is entirely conceivable to extend the development to design devices for the following objectives: (1) to enrich the concentration of, say, DNA or RNA, and to increase their concentrations at a desired location. (2) to act as a filtration device, wherin the filtration can be achieved without blocking the microfluidic channel and without any porous material. (3) to act as a microfluidic valve, where the particles can be locally trapped in any desired location and the direction can be switched as desired. (4) to create nanocomposite material formation or even a thin nanocomposite film formation on the floating electrode. (5) to create a continuous concentration-gradient-generator nanofluidic device that may be obtained for nanoparticle translocation process. This may achieve nanometer-scale spatial accuracy sample sequencing by simultaneously controlling the electric field and concentration gradient

Steering particles via micro-actuation of chemical gradients using model predictive control

Biological systems rely on chemical gradients to direct motion through both chemotaxis and signaling, but synthetic approaches for doing the same are still relatively naïve. Consequently, we present a novel method for using chemical gradients to manipulate the position and velocity of colloidal particles in a microfluidic device. Specifically, we show that a set of spatially localized chemical reactions that are sufficiently controllable can be used to steer colloidal particles via diffusiophoresis along an arbitrary trajectory. To accomplish this, we develop a control method for steering colloidal particles with chemical gradients using nonlinear model predictive control with a model based on the unsteady Green’s function solution of the diffusion equation. We illustrate the effectiveness of our approach using Brownian dynamics simulations that steer single particles along paths, such as circle, square, and figure-eight. We subsequently compare our results with published techniques for steering colloids using electric fields, and we provide an analysis of the physical parameter space where our approach is useful. Based on these findings, we conclude that it is theoretically possible to explicitly steer particles via chemical gradients in a microfluidics paradigm

Technology Roadmap of Micro/Nanorobots

Inspired by Richard Feynman?s 1959 lecture and the 1966 film Fantastic Voyage, the field of micro/nanorobots has evolved from science fiction to reality, with significant advancements in biomedical and environmental applications. Despite the rapid progress, the deployment of functional micro/nanorobots remains limited. This review of the technology roadmap identifies key challenges hindering their widespread use, focusing on propulsion mechanisms, fundamental theoretical aspects, collective behavior, material design, and embodied intelligence. We explore the current state of micro/nanorobot technology, with an emphasis on applications in biomedicine, environmental remediation, analytical sensing, and other industrial technological aspects. Additionally, we analyze issues related to scaling up production, commercialization, and regulatory frameworks that are crucial for transitioning from research to practical applications. We also emphasize the need for interdisciplinary collaboration to address both technical and nontechnical challenges, such as sustainability, ethics, and business considerations. Finally, we propose a roadmap for future research to accelerate the development of micro/nanorobots, positioning them as essential tools for addressing grand challenges and enhancing the quality of life