Computational Study of the Magnetic Structure of Na2_2IrO3_3

The magnetic structure of honeycomb iridate Na$_2$IrO$_3$ is of paramount importance to its exotic properties. The magnetic order is established experimentally to be zigzag antiferromagnetic. However, the previous assignment of ordered moment to the $\bm{a}$-axis is tentative. We examine the magnetic structure of Na$_{2}$IrO$_{3}$ using first-principles methods. Our calculations reveal that total energy is minimized when the zigzag antiferromagnetic order is magnetized along $\bm{g}\approx\bm{a}+\bm{c}$. Such a magnetic configuration is explained by adding anisotropic interactions to the nearest-neighbor Kitaev-Heisenberg model. Spin-wave spectrum is also calculated, where the calculated spin gap of $10.4$ meV can in principle be measured by future inelastic neutron scattering experiments. Finally we emphasize that our proposal is consistent with all known experimental evidence, including the most relevant resonant x-ray magnetic scattering measurements [X. Liu \emph{et al.} {Phys. Rev. B} \textbf{83}, 220403(R) (2011)].Comment: 18 pages, 7 figure

Transition of AC electroosmotic flow from linear to nonlinear state in different pH environment

Electroosmotic flow (EOF) exists widely at the solid-liquid interface in the presence of external electric field. However, the EOF driven by an alternating current (AC) electric field in diverse chemical environments was far from being well understood due to limited experimental investigations. In this investigation, through the high-resolution laser-induced fluorescent photobleaching anemometer (LIFPA), the transient velocity according to the AC EOF on the electric double layer (EDL) far from the electrodes has been experimentally characterized, by means of time series and power spectra. With analyzing the transient velocity, the transition of AC EOF from linear to nonlinear behavior is observed in a broad parameter space, e.g. mean flow velocity, the frequency and intensity of the AC electric field, and the pH value of the bulk fluid. To take all these parameters into account, an electro-inertial velocity has been applied as the characteristic velocity, instead of the commonly applied Helmholtz-Smouluchowski velocity. Then, the transitional electric field intensity $E_{A,C}$ and the corresponding dimensionless parameter $Z_{nlc}$ are systematically studied. A power-law relationship between the linear term coefficient $Z_l$ and $Z_{nlc}$ has been established, with the scaling exponents determined by the pH value of the electrolyte solution. We hope the current investigation can provide a deeper understanding of the transition of AC EOF and the instantaneous response of EOFs in other forms. It also provides a simple model to understand the coupling between electric field and fluid flow, in both linear and nonlinear status

Onset of nonlinear electroosmotic flow under AC electric field

Nonlinearity of electroosmotic flows (EOFs) is ubiquitous and plays a crucial role in the mass and energy transfer in ion transport, specimen mixing, electrochemistry reaction, and electric energy storage and utilizing. When and how the transition from a linear regime to a nonlinear one is essential for understanding, prohibiting or utilizing nonlinear EOF. However, suffers the lacking of reliable experimental instruments with high spatial and temporal resolutions, the investigation of the onset of nonlinear EOF still stays in theory. Herein, we experimentally studied the velocity fluctuations of EOFs driven by AC electric field via ultra-sensitive fluorescent blinking tricks. The linear and nonlinear AC EOFs are successfully identified from both the time trace and energy spectra of velocity fluctuations. The critical electric field ($E_{A,C}$) separating the two statuses is determined and is discovered by defining a generalized scaling law with respect to the convection velocity ($U$) and AC frequency ($f_f$) as $E_{A,C}$~${f_f}^{0.48-0.027U}$. The universal control parameters are determined with surprising accuracy for governing the status of AC EOFs. We hope the current investigation could be essential in the development of both theory and applications of nonlinear EOF

Large-Scale Flow in Micro Electrokinetic Turbulent Mixer

In the present work, we studied the three-dimensional (3D) mean flow field in a micro electrokinetic (μEK) turbulence based micromixer by micro particle imaging velocimetry (μPIV) with stereoscopic method. A large-scale solenoid-type 3D mean flow field has been observed. The extraordinarily fast mixing process of the μEK turbulent mixer can be primarily attributed to two steps. First, under the strong velocity fluctuations generated by μEK mechanism, the two fluids with different conductivity are highly mixed near the entrance, primarily at the low electric conductivity sides and bias to the bottom wall. Then, the well-mixed fluid in the local region convects to the rest regions of the micromixer by the large-scale solenoid-type 3D mean flow. The mechanism of the large-scale 3D mean flow could be attributed to the unbalanced electroosmotic flows (EOFs) due to the high and low electric conductivity on both the bottom and top surface