Dynamics of a self-diffusiophoretic particle in shear flow

Shape-controlled movement of Zn/SU-8 micromotors

Creating micromotors (MMs) that will have the highest possible velocities has become one of the main focuses in the field of autonomous microdevices research. The importance of velocity stems from various autonomous microdevices applications, ranging from faster drug delivery to the eradication of various bacterial biofilms using only mechanical movement. To investigate how different shapes affect the velocity of Zn/SU-8 micromotors in acid solution, we fabricated micromotors with various geometries (Zn/SU-8/Cylindrical, Zn/SU-8/Rectangular cuboid, Zn/SU-8/Triangular prism, Zn/SU-8/Pentagonal prism and Zn/SU-8/Pentagrammic prism MMs). This is the first comparative study where shape has been isolated as the critical factor influencing micromotor velocity under the same catalytic surface conditions. Our results demonstrate that Zn/SU-8/Rectangular cuboid and Zn/SU-8/Triangular prism MMs exhibit significantly higher average velocities compared to the other studied MMs. The shape-optimized Zn/SU-8 micromotors, characterized by their simple synthesis process and low cost, offer significant potential to enhance efficiency and navigation in both environmental and medical applications through precise movement control.

Diffusiophoresis-enhanced Turing patterns

Turing patterns are fundamental in biophysics, emerging from short-range activation and long-range inhibition processes. However, their paradigm is based on diffusive transport processes that yield patterns with shallower gradients than those observed in nature. A complete physical description of this discrepancy remains unknown. We propose a solution to this phenomenon by investigating the role of diffusiophoresis, which is the propulsion of colloids by a chemical gradient, in Turing patterns. Diffusiophoresis enables robust patterning of colloidal particles with substantially finer length scales than the accompanying chemical Turing patterns. A scaling analysis and a comparison to recent experiments indicate that chromatophores, ubiquitous in biological pattern formation, are likely diffusiophoretic and the colloidal Péclet number controls the pattern enhancement. This discovery suggests that important features of biological pattern formation can be explained with a universal mechanism that is quantified straightforwardly from the fundamental physics of colloids.

Microfluidics for Microswimmers: Engineering Novel Swimmers and Constructing Swimming Lanes on the Microscale, a Tutorial Review

This paper provides an updated review of recent advances in microfluidics applied to artificial and biohybrid microswimmers. Sharing the common regime of low Reynolds number, the two fields have been brought together to take advantage of the fluid characteristics at the microscale, benefitting microswimmer research multifold. First, microfluidics offer simple and relatively low-cost devices for high-fidelity production of microswimmers made of organic and inorganic materials in a variety of shapes and sizes. Microscale confinement and the corresponding fluid properties have demonstrated differential microswimmer behaviors in microchannels or in the presence of various types of physical or chemical stimuli. Custom environments to study these behaviors have been designed in large part with the help of microfluidics. Evaluating microswimmers in increasingly complex lab environments such as microfluidic systems can ensure more effective implementation for in-field applications. The benefits of microfluidics for the fabrication and evaluation of microswimmers are balanced by the potential use of microswimmers for sample manipulation and processing in microfluidic systems, a large obstacle in diagnostic and other testing platforms. In this review various ways in which these two complementary technology fields will enhance microswimmer development and implementation in various fields are introduced.

Diffusiophoresis: from dilute to concentrated electrolytes

Electrolytic diffusiophoresis is the movement of colloidal particles in response to a concentration gradient of an electrolyte. The diffusiophoretic velocity v_DP is typically predicted through the relation v_DP = D_DP ∇log c_s, where D_DP is the diffusiophoretic mobility and c_s is the concentration of the electrolyte. The logarithmic dependence of v_DP on c_s may suggest that the strength of diffusiophoretic motion is insensitive to the magnitude of the electrolyte concentration. In this article, we emphasize that D_DP is intimately coupled with c_s for all electrolyte concentrations. For dilute electrolytes, the finite double layer thickness effects are significant such that D_DP decreases with a decrease in c_s. In contrast, for concentrated electrolytes, charge screening could result in a decrease in D_DP with an increase in c_s. Therefore, we predict a maximum in D_DP with c_s for moderate electrolyte concentrations. We also show that for typical colloids and electrolytes , where D_s is the solute ambipolar diffusivity. To validate our model, we conduct microfluidic experiments with a wide range of electrolyte concentrations. The experimental data also reveals a maximum in D_DP with c_s, in agreement with our predictions. Our results have important implications in the broad areas of electrokinetics, lab-on-a-chip, active colloidal transport and biophysics.

Diffusiophoresis in Multivalent Electrolytes

Diffusiophoresis is the spontaneous movement of colloidal particles in a concentration gradient of solutes. As a small-scale phenomenon that harnesses energy from concentration gradients, diffusiophoresis may prove useful for passively manipulating particles in lab-on-a-chip applications as well as configurations involving interfaces. Though naturally occurring ions are often multivalent, experimental studies of diffusiophoresis have been mostly limited to monovalent electrolytes. In this work, we investigate the motion of negatively charged polystyrene particles in one-dimensional salt gradients for a variety of multivalent electrolytes. We develop a one-dimensional model and obtain good agreement between our experimental and modeling results with no fitting parameters. Our results indicate that the ambipolar diffusivity, which is dependent on the valence combination of cations and anions, dictates the speed of the diffusiophoretic motion of the particles by controlling the time scale at which the electrolyte concentration evolves. In addition, the ion valences also modify the electrophoretic and chemiphoretic contributions to the diffusiophoretic mobility of the particles. Our results are applicable to systems where the chemical concentration gradient is comprised of multivalent ions, and motivate future research to manipulate particles by exploiting ion valence.

Catalytic Locomotion of Core-Shell Nanowire Motors

We report Au/Ru core-shell nanowire motors. These nanowires are fabricated using our previously developed electrodeposition-based technique, and their catalytic locomotion in the presence of H₂O₂ is investigated. Unlike conventional bimetallic nanowires that are self-electroosmotically propelled, our open-ended Au/Ru core-shell nanowires show both a noticeable decrease in rotational diffusivity and increase in motor speed with increasing nanowire length. Numerical modeling based on self-electroosmosis attributes decreases in rotational diffusivity to the formation of toroidal vortices at the nanowire tail, but fails to explain the speed increase with length. To reconcile this inconsistency, we propose a combined mechanism of self-diffusiophoresis and electroosmosis based on the oxygen gradient produced by catalytic shells. This mechanism successfully explains not only the speed increase of Au/Ru core-shell nanomotors with increasing length, but also the large variation in speed among Au/Ru, Au/Rh, and Rh/Au core-shell nanomotors. The possible contribution of diffusiophoresis to an otherwise well-established electroosmotic mechanism sheds light on future designs of nanomotors, at the same time highlighting the complex nature of nanoscale propulsion.

Self-propelled autonomous nanomotors meet microfluidics

Self-propelled autonomous nano/micromotors are in the forefront of current materials science and technology research. These small machines convert chemical energy from the environment into propulsion, and they can move autonomously in the environment and are capable of chemotaxis or magnetotaxis. They can be used for drug delivery, microsurgeries or environmental remediation. It is of immense interest from a future biomedical application point of view to understand the motion of the nano/micromotors in microfluidic channels. In this minireview, we review the progress on the use of nano/micromotors in microfluidic channels and lab-on-chip devices.

Tilting and Tumbling of Janus Nanoparticles at Sheared Interfaces

We investigate the response of a single Janus nanoparticle adsorbed at an oil-water interface to imposed shear flows using molecular dynamics simulations. We consider particles of different geometry, including spheres, cylinders, and discs, and tune their degree of amphiphilicity by controlling the affinity of their two sides to the fluid phases. We observe that depending on the shape, amphiphilicity, and the applied shear rate, two modes of rotational dynamics takes place: a smooth tilt or a tumbling motion. We demonstrate that irrespective of this dynamic behavior, a steady-state orientation is eventually achieved as a result of the balance between the shear- and capillary-induced torques, which can be tuned by controlling the surface property and flow parameters. Our findings provide insight on using flow fields to tune particle orientation at an interface and to utilize it to direct their assembly into ordered monolayers.

Two-dimensional motion of Brownian swimmers in linear flows

The motion of viruses and bacteria and even synthetic microswimmers can be affected by thermal fluctuations and by external flows. In this work, we study the effect of linear external flows and thermal fluctuations on the diffusion of those swimmers modeled as spherical active (self-propelled) particles moving in two dimensions. General formulae for their mean-square displacement under a general linear flow are presented. We also provide, at short and long times, explicit expressions for the mean-square displacement of a swimmer immersed in three canonical flows, namely, solid-body rotation, shear and extensional flows. These expressions can now be used to estimate the effect of external flows on the displacement of Brownian microswimmers. Finally, our theoretical results are validated by using Brownian dynamics simulations.

Rheotaxis of spherical active particles near a planar wall

For active particles the interplay between the self-generated hydrodynamic flow and an external shear flow, especially near bounding surfaces, can result in a rich behavior of the particles not easily foreseen from the consideration of the active and external driving mechanisms in isolation. For instance, under certain conditions, the particles exhibit "rheotaxis", i.e., they align their direction of motion with the plane of shear spanned by the direction of the flow and the normal of the bounding surface and move with or against the flow. To date, studies of rheotaxis have focused on elongated particles (e.g., spermatozoa), for which rheotaxis can be understood intuitively in terms of a "weather vane" mechanism. Here we investigate the possibility that spherical active particles, for which the "weather vane" mechanism is excluded due to the symmetry of the shape, may nevertheless exhibit rheotaxis. Combining analytical and numerical calculations, we show that, for a broad class of spherical active particles, rheotactic behavior may emerge via a mechanism which involves "self-trapping" near a hard wall owing to the active propulsion of the particles, combined with their rotation, alignment, and "locking" of the direction of motion into the shear plane. In this state, the particles move solely up- or downstream at a steady height and orientation.