Improving the capturing ability of swirl-based microfluidic chip by introducing baffle wall

Microfluidics technology is promising in developing microparticle manipulation technology due to its nondestructive control and notable adaptability. The manipulation of microparticle based on swirling stagnation point is one of the feasible microfluidics biotechnologies. Aiming to improve the regulation and control of microparticle, baffle wall is introduced into the 2-microchannel flow field. The theory of wall attachment jet is employed to elucidate the effect of baffle wall. Subsequently, finite volume method simulation is conducted by modeling the swirling flow region (SFR), and the swirling strength is calculated to characterize the SFR's particle-capturing ability. Experimental validation of the modeling and simulation methods is performed using a printed microfluidic chip, which has demonstrated exceptional reliability. Simulation results show that the baffle wall makes considerable influence on the SFR. Strikingly, a global range adjustment of stagnation point is realized when the baffle wall is configured with a convex shape, which has remarkably outperformed our previous work, where the stagnation point could only move within half range of the field. This work significantly contributes to advanced flow field structure and provides insight into better regulation of stagnation point as well as microparticles. These findings have potential applications in the analysis of the effect of bio/chemical substances on single cell.

Superstructures of water-dispersive hydrophobic nanocrystals: specific properties

Here, we describe water-soluble superstructures of hydrophobic nanocrystals that have been developed in recent years. We will also report on some of their properties which are still in their infancy. One of these structures, called "cluster structures", consists of hydrophobic 3D superlattices of Co or Au nanocrystals, covered with organic molecules acting like parachutes. The magnetic properties of Co "cluster structures" a retained when the superstructures is dispersed in aqueous solution. With Au "cluster structures", the longer wavelength optical scattered spectra are very broad and red-shifted, while at shorter wavelengths the localized surface plasmonic resonance of the scattered nanocrystals is retained. Moreover, the maximum of the long-wavelength signal spectra is linearly dependent on the increase in assembly size. The second superstructure was based on liquid-liquid instabilities favoring the formation of Fe3O4 nanocrystal shells (colloidosomes) filled or unfilled with Au 3D superlattices and also spherical solid crystal structures are called supraballs. Colloidosomes and supraballs in contact with cancer cells increase the density of nanocrystals in lysosomes and near the lysosomal membrane. Importantly, the structure of their organization is maintained in lysosomes for up to 8 days after internalization, while the initially dispersed hydrophilic nanocrystals are randomly aggregated. These two structures act as nanoheaters. Indeed, due to the dilution of the metallic phase, the penetration depth of visible light is much greater than that of homogeneous metallic nanoparticles of similar size. This allows for a high average heat load overall. Thus, the organic matrix acts as an internal reservoir for efficient energy accumulation within a few hundred picoseconds. A similar behavior was observed with colloidosomes, supraballs and "egg" structures, making these superstructures universal nanoheaters, and the same behavior is not observed when they are not dispersed in water (dried and deposited on a substrate). Note that colloidosomes and supraballs trigger local photothermal damage inaccessible to isolated nanocrystals and not predicted by global temperature measurements.

Microparticle Manipulation Performed on a Swirl-Based Microfluidic Chip Featured by Dual-Stagnation Points

Stagnation-based microfluidics technology is promising for microparticle control due to its noncontact and low cost. However, the current research is still hindered by insufficient pose regulating ability and soft control. Based on our previous work on controlling single particles by generating a swirling flow region (SFR) with a stagnation point in the designed flow field, a new 3-microchannel structure is herein proposed for simultaneous control of two microparticles. It is addressed as the dual-stagnation model because there are two SFRs generated for particle capturing and manipulation. Simulation study is conducted to optimize the fluid field structure and explore the regulation of the two SFRs by adjusting velocities of microchannel inlets. Experiments are carried out on a 3D-printed microfluidic chip to validate the feasibility of the dual-stagnation model and the predicting capacity of the simulations. It is demonstrated that two SFRs with stagnation points are successfully formed in specific locations, indicating that two microparticles can be concurrently captured and controlled. Significantly, the results of simulation and experimental studies agree well with each other referring to flow streamlines and stagnation point regulation. During experiments, it is confirmed that microparticles with different shapes and varied sizes can be captured. Besides, the deviation between the positions of microparticles and the generated stagnation points is characterized to reveal the trapping stability of this microfluidic chip. This work contributes to an advanced flow field structure for swirl-based microfluidic chips and provides insights into soft contact and flexible manipulation of multiple microparticles for revealing the interaction between two bio-/chemical microparticles.

Particle actuation by rotating magnetic fields in microchannels: a numerical study

Magnetic particles confined in microchannels can be actuated to perform translation motion using a rotating magnetic field, but their actuation in such a situation is not yet well understood. Here, the actuation of a ferromagnetic particle confined in square microchannels is studied using immersed-boundary lattice Boltzmann simulations. In wide channels, when a sphere is positioned close to a side wall but away from channel corners, it experiences a modest hydrodynamic actuation force parallel to the channel walls. This force decreases as the sphere is shifted toward the bottom wall but the opposite trend is found when the channel is narrow. When the sphere is positioned midway between the top and bottom channel walls, the actuation force decreases as the channel width decreases and can reverse its direction. These phenomena are elucidated by studying the flow and pressure fields in the channel-particle system and by analyzing the viscous and pressure components of the hydrodynamic force acting on different parts of the sphere.

Discretized Motion of Surface Walker under a Nonuniform AC Magnetic Field

The motion of peanut-shaped magnetic microrods (PSMRs) with different magnetic moment (M_s) orientations φ_M under a nonuniform AC magnetic field has been investigated systematically. When gradually changing φ_M from 90° (perpendicular to the long axis of the PSMR) to 0°, the motion of the PSMR evolves from rolling to precession, then to tumbling. Systematic investigations on the translational velocity v_p versus the magnitude of the applied magnetic field B and the angular velocity ω_B show that the overall motion of the PSMRs can be divided into four different zones: Brownian motion zone, synchronized zone, asynchronized zone, and oscillation zone. The v_p-ω_B relationship can be rescaled by a critical frequency ω_c, which is determined by M_s, B, and a hydrodynamic term. An intrinsic quality factor q_m for the translational motion of a magnetically driven micro-/nanomotor is defined and is found to range from 0.73 to 13.65 T^-1 in the literature, while the Fe PSMRs in the current work give the highest q_m (= 25.48 T^-1). High speed movies reveal that both the tumbling and precession motions of the PSMRs have a discretized nature. At the instances when the magnetic field changes direction, the PSMR performs an instantaneous rotation and the strong hydrodynamic wall effect would impose a driving force to move the PSMR translationally, and about more than 60% of the time, the PSMR neither rotates nor moves translationally. Based on this discretized motion nature, an analytic expression for q_m is found to be determined by the shape of the surface walker, the hydrodynamics near a wall, and the magnetic properties of the surface walker. This work can help us to better understand the motion of magnetic surface walkers and gain insight into designing better micro-/nanomotors.

Magnetic Actuation of Surface Walkers: The Effects of Confinement and Inertia

Driven by a magnetic field, the rotation of a particle near a wall can be rectified into a net translation. The particles thus actuated, or surface walkers, are a kind of active colloid that finds application in biology and microfluidics. Here, we investigate the motion of spherical surface walkers confined between two walls using simulations based on the immersed-boundary lattice Boltzmann method. The degree of confinement and the nature of the confining walls (slip vs no-slip) significantly affect a particle's translational speed and can even reverse its translational direction. When the rotational Reynolds number Re_ω is larger than 1, inertia effects reduce the critical frequency of the magnetic field, beyond which the sphere can no longer follow the external rotating field. The reduction of the critical frequency is especially pronounced when the sphere is confined near a no-slip wall. As Re_ω increases beyond 1, even when the sphere can still rotate in the synchronous regime, its translational Reynolds number Re_T no longer increases linearly with Re_ω and even decreases when Re_ω exceeds ∼10.