69 research outputs found
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 and the corresponding
dimensionless parameter are systematically studied. A power-law
relationship between the linear term coefficient and 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
Modelling the Vegetation Response to Climate Changes in the Yarlung Zangbo River Basin Using Random Forest
Vegetation coverage variation may influence watershed water balance and water resource availability. Yarlung Zangbo River, the longest river on the Tibetan Plateau, has high spatial heterogeneity in vegetation coverage and is the main freshwater resource of local residents and downstream countries. In this study, we proposed a model based on random forest (RF) to predict the Normalized Difference Vegetation Index (NDVI) of the Yarlung Zangbo River Basin and explore its relationship with climatic factors. High-resolution datasets of NDVI and monthly meteorological observation data from 2000 to 2015 were used to calibrate and validate the proposed model. The proposed model was then compared with artificial neural network and support vector machine models, and principal component analysis and partial correlation analysis were also used for predictor selection of artificial neural network and support vector machine models for comparative study. The results show that RF had the highest model efficiency among the compared models. The Nash–Sutcliffe coefficients of the proposed model in the calibration period and verification period were all higher than 0.8 for the five subzones; this indicated that the proposed model can successfully simulate the relationship between the NDVI and climatic factors. By using built-in variable importance evaluation, RF chose appropriate predictor combinations without principle component analysis or partial correlation analysis. Our research is valuable because it can be integrated into water resource management and elucidates ecological processes in Yarlung Zangbo River Basin
Multiple time scale analysis of sediment and runoff changes in the Lower Yellow River
Sediment and runoff changes of seven hydrological
stations along the Lower Yellow River (LYR) (Huayuankou Station, Jiahetan
Station, Gaocun Station, Sunkou Station, Ai Shan Station, Qikou Station and
Lijin Station) from 1980 to 2003 were alanyzed at multiple time scale. The
maximum value of monthly, daily and hourly sediment load and runoff
conservations were also analyzed with the annually mean value. Mann–Kendall
non-parametric mathematics correlation test and Hurst coefficient method
were adopted in the study. Research results indicate that (1)Â the runoff of
seven hydrological stations was significantly reduced in the study period at
different time scales. However, the trends of sediment load in these
stations were not obvious. The sediment load of Huayuankou, Jiahetan and
Aishan stations even slightly increased with the runoff decrease. (2)Â The
trends of the sediment load with different time scale showed differences at
Luokou and Lijin stations. Although the annually and monthly sediment load
were broadly flat, the maximum hourly sediment load showed decrease trend.
(3)Â According to the Hurst coefficients, the trend of sediment and runoff
will be continue without taking measures, which proved the necessary of
runoff-sediment regulation scheme
Multifocal laser direct writing through spatial light modulation guided by scalable vector graphics
Multifocal laser direct writing (LDW) based on phase-only spatial light
modulator (SLM) can realize flexible and parallel nanofabrication with high
throughput potential. In this investigation, a novel approach of combining
two-photon absorption, SLM and vector path guided by scalable vector graphics
(SVG) has been developed and tested preliminarily, for fast, flexible and
parallel nanofabrication. Three laser focuses are independently controlled with
different paths, which are according to SVG, to optimize fabrication and
promote time efficiency. The minimum structure width can be as low as 74 nm.
Accompanied with a translation stage, a carp structure of 18.16 m by 24.35
m has been fabricated. This method shows the possibility of developing LDW
techniques towards full-electrical system, and provides a potential way to
efficiently engrave complex structures on nanoscales
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
() separating the two statuses is determined and is discovered by
defining a generalized scaling law with respect to the convection velocity
() and AC frequency () as ~. 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
Transition routes of electrokinetic flow in a divergent microchannel with bending walls
Electrokinetic flow can be generated as a highly coupled phenomenon among
velocity field, electric conductivity field and electric field. It can exhibit
different responses to AC electric fields in different frequency regimes,
according to different instability/receptivity mechanisms. In this
investigation, by both flow visualization and single-point laser-induced
fluorescence (LIF) method, the response of AC electrokinetic flow and the
transition routes towards chaos and turbulence have been experimentally
investigated. It is found, when the AC frequency Hz, the interface
responds at both the neutral frequency of the basic flow and the AC frequency.
However, when Hz, the interface responds only at the neutral
frequency of the basic flow. Both periodic doubling and subcritical
bifurcations have been observed in the transition of AC electrokinetic flow. We
hope the current investigation can promote our current understanding on the
ultrafast transition process of electrokinetic flow from laminar state to
turbulence
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
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