The Application of Spectral Induced Polarization to Determination of Hydraulic Conductivity

<p>Spectral Induced Polarization (SIP) is a geophysical technique that measures the frequency dependence of the electrical conductivity of a material. This thesis is an attempt to investigate the potential of using SIP as a proxy to predict the hydraulic conductivity of New Zealand shallow coastal aquifers. SIP measurements were made on sand samples that are typical of New Zealand coastal aquifers with a custom built impedance spectrometer and sample holder allowing the measurement of a phase difference as small a milliradian. Even though the relaxation time shows a small dependence on pore fluid conductivity, especially at lower pore fluid conductivities, this variation is not serious enough to affect the hydraulic conductivity estimation at the field scale, but could be significant in the investigation of mechanisms that cause polarization in porous media. Measurements on sieved fractions of sand established that there is an excellent correlation between the Cole-Cole relaxation time constant and grain size. The Cole-Cole relaxation time constant is very sensitive to the grain size distribution. Hydraulic conductivity predictions were attempted using various existing models. While the results are encouraging, it looks like there may not be a single universal model to predict hydraulic conductivity using SIP response. When a correction term in the form of a multiplication constant is used, all the tested models seem to make very good predictions. But the constants calculated by fitting to the measured data could be applicable only to the type of materials studied. The dependence of the existing models on quantities like counterion diffusion coefficient, electrical formation factor and porosity makes hydraulic conductivity prediction challenging as these quantities are difficult to measure accurately in a field setting. Nevertheless it is concluded that SIP can be successfully applied to study hydraulic conductivity of New Zealand shallow coastal aquifers.</p>

The Application of Spectral Induced Polarization to Determination of Hydraulic Conductivity

<p>Spectral Induced Polarization (SIP) is a geophysical technique that measures the frequency dependence of the electrical conductivity of a material. This thesis is an attempt to investigate the potential of using SIP as a proxy to predict the hydraulic conductivity of New Zealand shallow coastal aquifers. SIP measurements were made on sand samples that are typical of New Zealand coastal aquifers with a custom built impedance spectrometer and sample holder allowing the measurement of a phase difference as small a milliradian. Even though the relaxation time shows a small dependence on pore fluid conductivity, especially at lower pore fluid conductivities, this variation is not serious enough to affect the hydraulic conductivity estimation at the field scale, but could be significant in the investigation of mechanisms that cause polarization in porous media. Measurements on sieved fractions of sand established that there is an excellent correlation between the Cole-Cole relaxation time constant and grain size. The Cole-Cole relaxation time constant is very sensitive to the grain size distribution. Hydraulic conductivity predictions were attempted using various existing models. While the results are encouraging, it looks like there may not be a single universal model to predict hydraulic conductivity using SIP response. When a correction term in the form of a multiplication constant is used, all the tested models seem to make very good predictions. But the constants calculated by fitting to the measured data could be applicable only to the type of materials studied. The dependence of the existing models on quantities like counterion diffusion coefficient, electrical formation factor and porosity makes hydraulic conductivity prediction challenging as these quantities are difficult to measure accurately in a field setting. Nevertheless it is concluded that SIP can be successfully applied to study hydraulic conductivity of New Zealand shallow coastal aquifers.</p>

Stress relaxation amplitude of hydrogels determines migration, proliferation, and morphology of cells in 3-D

The viscoelastic behavior of hydrogel matrices sensitively influences the cell behavior in 3D culture and biofabricated tissue model systems. Previous reports have demonstrated that cells tend to adhere, spread, migrate and proliferate better in hydrogels with pronounced stress relaxation. However, it is currently unknown if cells respond more sensitively to the amplitude of stress relaxation, or to the relaxation time constant. To test this, we compare the behavior of fibroblasts cultured for up to 10 days in alginate and oxidized alginate hydrogels with similar Young's moduli but diverging stress relaxation behavior. We find that fibroblasts elongate, migrate and proliferate better in hydrogels that display a higher stress relaxation amplitude. By contrast, the cells' response to the relaxation time constant was less pronounced and less consistent. Together, these data suggest that it is foremost the stress relaxation amplitude of the matrix that determines the ability of cells to locally penetrate and remodel the matrix, which subsequently leads to better spreading, faster migration, and higher cell proliferation. We conclude that the stress relaxation amplitude is a central design parameter for optimizing cell behavior in 3-D hydrogels.

Effects of salinity and pH on the spectral induced polarization signals of graphite particles

SUMMARY The electrical property of micrometre-sized graphite particles was investigated under different particle concentration, particle size, fluid conductivity and pH conditions. Due to its large internal electronic conductivity and ability to polarize under external potential field, significant enhancement of its spectral induced polarization (SIP) responses is observed when graphite is included in sand mixtures. While a small amount of graphite inclusion significantly increases the SIP response of its mixtures with sand, further concentration increase does not necessarily lead to a proportional increase of the SIP response. This is shown to be related to the formation of graphite aggregates at higher concentrations. Changes of fluid salinity have a significant effect on graphite's SIP behaviour. This includes a positive impact on normalized chargeability and imaginary conductivity, but a negative impact on chargeability and relaxation time constant. The effect of pH on the SIP response of graphite is small but shows consistent trend, where pH increase leads to a decrease of both the chargeability and relaxation time constant. The underlying cause of this effect is not clear.

Negative Differential Resistance in Bidirectional Threshold Switching of Ag/HfOx/Pt Device

In this work, the dependence of negative differential resistance (NDR) on compliance current (Icc) was investigated based on Ag/HfOx/Pt resistive memory device. Tunable conversion from bidirectional threshold switching (TS) to memory switching (MS) were achieved through enhancing Icc. NDR can be observed in TS as Icc is below 800μA but vanishes in MS. The switching voltages and readout windows of TS evolve with Icc. Furthermore, the dynamic conductance (dI/dV) as a function of time in NDR can be well illustrated by capacitor-like relaxation equation, and the relaxation time constant is significantly dependent on Icc. These phenomena were elucidated from viewpoint of nanofilament evolution controlled by Icc as well as nanocapacitor effects originated from nanofilament gap. The Icc-dependent NDR as well as conversion between TS and MS on Ag/HfOx/Pt resistive memory device indicates its potential application as a multifunctional electronic device.

P1510 Assessment of left ventricular relaxation time constant by noninvasive left ventricular pressure-strain loop: invasive validation

Introduction Left ventricular (LV) relaxation time constant, Tau (τ), is one of the best indexes to evaluate left ventricular diastolic function and is usually assessed invasively. Tau is the time constant of the exponential regression Pt =(P0-P∞)e-t/τ+P∞ that expresses left ventricular isovolumic pressure decay, where Pt is LV pressure (LVP) at time t, P0 is LVP at dP/dtmin and P∞ is the asymptotic pressure, to which relaxation would lead if completed without LV filling. Several noninvasive methods were developed to calculate Tau, however, they are time-consuming and complicated. Recently, a simple method regional left ventricular pressure–strain loops and myocardial work was introduced and validated. We hypothesize that left ventricular relaxation time constant can be derived and calculated from the LV pressure–strain loops. Objective To calculate noninvasively Tau using LV pressure–strain loops and compare it to invasive Tau assessment. Methods The study includes patients with preserved LV systolic function without significant valvular disease that were scheduled to elective coronary catheterization. During catheterization, a fluid-filled catheter was placed in the LV to measure pressure. Echocardiography was performed simultaneously with LV pressure recording. Three standard apical views were acquired and subsequently 2D strain analysis was performed using commercially available GE software and LV pressure–strain loops were calculated. Doppler signal was used for timing of valvular events. Tau was calculated by the equation τ =P/(-dP/dt) that is a derivative of the ventricular pressure decline P = P0e-t/τ+PB with respect to time. The pressure between peak negative dP/dt and the lowest LV pressure shortly after mitral valve opening was used for this calculation. The study was approved by the institutional ethics committee. Results Forty patients, (mean age 65.1 ± 10.9 years, 27 male, BSA1.93 ± 0.18 m2) were included in the study. Heart rate and the mean blood pressure at the time of catheterization were 69.9 ± 11.8 min-1 and 85.2 ± 18.8 mmHg, respectively. The mean LV end diastolic diameter was 44 ± 4 mm, the LV mass was 86.7 ± 25.2 g/m2, LVEF 58.4 ± 6.6% and GLS 21.0 ± 3.2%. Tau, calculated noninvasively using derivative method was significantly lower than invasively derived measurement (40.1 ± 13.4 vs 49.8 ± 7.7 msec, p = 0.002). However, a significant positive correlation was observed between the two methods (r =0.67, p &lt;0.001, Figure). Conclusions This preliminary study demonstrates that Tau estimated by a noninvasive method using LV pressure–strain loops has a good correlation with Tau measured invasively. Therefore, Tau can be estimated noninvasively using novel left ventricular pressure–strain loop method. Abstract P1510 Figure.