Dielectrophoretic Colloidal Levitation by Electrode Polarization in Oscillating Electric Fields

Colloidal ionogels: Controlled assembly and self-propulsion upon tunable swelling

Active colloids driven out of thermal equilibrium serve as building blocks for smart materials with tunable structures and functions. Using chemical energy to drive colloids is advantageous but requires precise control over chemical release. To address this, we developed colloidal ionogels-polymer microspheres infused with ionic liquids-that show controlled assembly and self-propulsion upon tunable swelling. For example, we synthesized microspheres of polymethylmethacrylate loaded with ionic liquid [Bmim][PF6], which were released from the colloidal ionogel upon swelling in alcohol-water mixtures and dissociated into cations and anions of different diffusivities. The resulting electric field leads to four types of pair-wise colloidal interactions via ionic diffusiophoresis and diffusioosmosis, giving rise to four types of self-assembled superstructures. These interactions were precisely modulated by altering the swelling conditions and the ionic liquids used. Additionally, partially blocking the ionogel's surface induces anisotropic swelling and asymmetric ion release, turning the colloidal ionogel into a self-propelled Janus colloidal motor powered by ionic self-diffusiophoresis, reaching speeds of several µm/s and lasting about 100 s. These findings indicate that colloidal ionogels are smart colloidal building blocks with highly tunable pair-wise interactions, self-assembled structures, and self-propulsion, offering potential applications in biomedical sensing, environmental monitoring, and photonics.

Continuous Focusing of Particles by AC-Electroosmosis and Induced Dipole Interactions

Continuous particle focusing by using microfluidics is an effective method for separating particles, cells, or droplets for analytical purposes. Previously, it was shown that an alternating current across rectangular microchannels with slightly deformed side walls results in vortex flow patterns caused by alternating current electroosmosis (AC-EOF) and could lead to particle focusing. In this work, we explore this mechanism by experimentally studying the particle focusing behavior for various fluid flow velocities through a microchannel. Since it is unlikely that the particles are kept in their focused position solely by convection, a theoretical force balance between the hydrodynamic and the induced dipole force was determined. In our experiments, it was found that there is no substantial effect of the pressure-driven fluid velocity on the particle focusing velocity within the studied range. From the theoretical force balance calculations, it was determined that while the addition of the induced dipole force can still not completely describe the experimentally observed particle focusing, the induced dipole can be strong enough to overcome the hydrodynamic force. Finally, it is hypothesized that under specific circumstances, including a repulsive electrostatic force between a particle and electrode wall can complete the theoretical particle focusing force balance. Alternative phenomena that could also play a role in particle focusing are proposed.

Asymmetric Rectified Electric Fields for Symmetric Electrolytes

In this paper, building upon the numerical discovery of asymmetric rectified electric fields (AREFs), we explore the generation of AREF by applying a sawtooth-like voltage to 1:1 electrolytes with equal diffusion coefficients confined between two planar blocking electrodes. This differs from an earlier approach based on a sinusoidal AC voltage applied to 1:1 electrolytes with unequal diffusion coefficients. By numerically solving the full Poisson-Nernst-Planck equations, we demonstrate that AREF can be generated by a slow rise and a fast drop of the potential (or vice versa), even for electrolytes with equal diffusion coefficients of the cations and anions. We employ an analytically constructed equivalent electric circuit to explain the underlying physical mechanism. Importantly, we find that the strength of AREF can be effectively tuned from zero to its maximal value by only manipulating the time dependence of the driving voltage, eliminating the necessity to modify the electrolyte composition between experiments. This provides valuable insights to control the manipulation of AREF, which facilitates enhanced applications in diverse electrochemical systems.

Recent advances in nano/microfluidics-based cell isolation techniques for cancer diagnosis and treatments

Miniaturization has improved significantly in the recent decade, which has enabled the development of numerous microfluidic systems. Microfluidic technologies have shown great potential for separating desired cells from heterogeneous samples, as they offer benefits such as low sample consumption, easy operation, and high separation accuracy. Microfluidic cell separation approaches can be classified into physical (label-free) and biological (labeled) methods based on their working principles. Each method has remarkable and feasible benefits for the purposes of cancer detection and therapy, as well as the challenges that we have discussed in this article. In this review, we present the recent advances in microfluidic cell sorting techniques that incorporate both physical and biological aspects, with an emphasis on the methods by which the cells are separated. We first introduce and discuss the biological cell sorting techniques, followed by the physical cell sorting techniques. Additionally, we explore the role of microfluidics in drug screening, drug delivery, and lab-on-chip (LOC) therapy. In addition, we discuss the challenges and future prospects of integrated microfluidics for cell sorting.

Asymmetric rectified electric fields: nonlinearities and equivalent circuits

Recent experiments [S. H. Hashemi et al., Phys. Rev. Lett., 2018, 121, 185504] have shown that a long-ranged steady electric field emerges when applying an oscillating voltage over an electrolyte with unequal mobilities of cations and anions confined between two planar blocking electrodes. To explain this effect we analyse full numerical calculations based on the Poisson-Nernst-Planck equations by means of analytically constructed equivalent electric circuits. Surprisingly, the resulting equivalent circuit has two capacitive elements, rather than one, which introduces a new timescale for electrolyte dynamics. We find a good qualitative agreement between the numerical results and our simple analytic model, which shows that the long-range steady electric field emerges from the different charging rates of cations and anions in the electric double layers.