Ignition and subsequent flame spread over a thin cellulosic material

Both ignition and flame spread on solid fuels are processes that not only are of considerable scientific interest but that also have important fire safety applications. Both types of processes, ignition and flame spread, are complicated by strong coupling between chemical reactions and transport processes, not only in the gas phase but also in the condensed phase. In most previous studies, ignition and flame spread were studied separately with the result that there has been little understanding of the transition from ignition to flame spread. In fire safety applications this transition is crucial to determine whether a fire will be limited to a localized, temporary burn or will transition into a growth mode with a potential to become a large fire. In order to understand this transition, the transient mechanisms of ignition and subsequent flame spread must be studied. However, there have been no definitive experimental or modeling studies, because of the complexity of the flow motion generated by buoyancy near the heated sample surface. One must solve the full Navier-Stokes equations over an extended region to represent accurately the highly unstable buoyant plume and entrainment of surrounding gas from far away. In order to avoid the complicated nature of the starting plume problem under normal gravity, previous detailed radiative ignition models were assumed to be one-dimensional or were applied at a stagnation point. Thus, these models cannot be extended to include the transition to flame spread. The mismatch between experimental and calculated geometries means that theories cannot be compared directly with experimental results in normal gravity. To overcome the above difficulty, theoretical results obtained without buoyancy can be directly compared with experimental data measured in a microgravity environment. Thus, the objective of this study is to develop a theoretical model for ignition and the transition to flame spread and to make predictions using the thermal and chemical characteristics of a cellulosic material which are measured in normal gravity

Velocity measurements and flow structure visualizations of a self-sustained oscillating jet

The purpose of this study is the experimental investigation on self-sustained oscillating jet characteristics. Main aim was to describe vertical structures of turbulent air jet issuing from the nozzle of special configuration modified by the controlled oscillations in free jet setup. In the present experiments was used so-called "whistler nozzle", a simple-structured device capable to induce self-sustained excitations with controllable frequencies depending on the nozzle geometrical configuration. The frequency of the excitation measured with a far-field condenser microphone probe was around 1-2 kHz. The jet Reynolds number was in the range 48,000-95,000 in all experimental conditions presented in this paper. Flow field velocity measurements were provided in the free jet setup, with and without self-sustained excitations. The images of both free and impinging jets were taken with a high-speed digital video camera. The flow field and structure of the jet were found to be extremely sensitive to the excitation and dependent on the excitation conditions. This fact can lead to the conclusion that the local heat transfer characteristics of jet impingement are also remarkably dependent on the jet excitation

Two-dimensional fluid viscosity measurement in microchannel flow using fluorescence polarization imaging

This study describes the development of a noncontact and two-dimensional fluid viscosity measurement technique based on fluorescence polarization microscopy. This technique exploits fluorescence depolarization due to rotational Brownian motion of fluorophores and determines fluid viscosity in microchannel flow by measuring steady-state fluorescence polarization. The main advantage of the technique is that planar distributions of fluid viscosity can be visualized by noncontact optical measurement, while commonly-used mechanical viscometers measure the viscosity of bulk liquids. Moreover, steady-state polarization measurements are realized using a simpler experimental setup compared to other noncontact techniques such as time-resolved fluorescence lifetime/polarization measurements. The relationship between the fluid viscosity (μ) and the fluorescence polarization degree () was experimentally obtained using casein molecules labeled with fluorescein isothiocyanate as a fluorescent probe. The fluid viscosity was controlled within the range of 0.7-3.0 mPa s, which is the range often encountered in biological materials, by mixing sucrose or glucose with the solution. The fluid temperature was maintained uniform at 30 °C during the measurement. The calibration result showed that 1/ linearly increased with 1/μ which qualitatively agreed well with the theoretical prediction. The measurement uncertainty was 7.5%-9.5% based on the slope of the calibration curve. The viscosity gradient generated by the mass diffusion between the two solutions co-flowing in the Y-shaped microchannel was clearly visualized under uniform temperature conditions by applying the calibration curve. Finally, the influence of the temperature change on was experimentally evaluated. The results supported the applicability of the present technique for visualization of the viscosity distribution induced by temperature change. These results confirmed the feasibility of the present technique for analyzing microscale viscosity fields associated with mass transport or temperature change

Extreme deformability of insect cell membranes is governed by phospholipid scrambling

昆虫の細胞は柔らかい！ --細胞膜を柔らかくするタンパク質を発見--. 京都大学プレスリリース. 2021-06-09.Organization of dynamic cellular structure is crucial for a variety of cellular functions. In this study, we report that Drosophila and Aedes have highly elastic cell membranes with extremely low membrane tension and high resistance to mechanical stress. In contrast to other eukaryotic cells, phospholipids are symmetrically distributed between the bilayer leaflets of the insect plasma membrane, where phospholipid scramblase (XKR) that disrupts the lipid asymmetry is constitutively active. We also demonstrate that XKR-facilitated phospholipid scrambling promotes the deformability of cell membranes by regulating both actin cortex dynamics and mechanical properties of the phospholipid bilayer. Moreover, XKR-mediated construction of elastic cell membranes is essential for hemocyte circulation in the Drosophila cardiovascular system. Deformation of mammalian cells is also enhanced by the expression of Aedes XKR, and thus phospholipid scrambling may contribute to formation of highly deformable cell membranes in a variety of living eukaryotic cells

Numerical and Experimental Study on the Development of Electric Sensor as for Measurement of Red Blood Cell Deformability in Microchannels

A microsensor that can continuously measure the deformability of a single red blood cell (RBC) in its microchannels using microelectrodes is described in this paper. The time series of the electric resistance is measured using an AC current vs. voltage method as the RBC passes between counter-electrode-type micro-membrane sensors attached to the bottom wall of the microchannel. The RBC is deformed by the shear flow created in the microchannel; the degree of deformation depends on the elastic modulus of the RBC. The resistance distribution, which is unique to the shape of the RBC, is analyzed to obtain the deformability of each cell. First, a numerical simulation of the electric field around the electrodes and RBC is carried out to evaluate the influences of the RBC height position, channel height, distance between the electrodes, electrode width, and RBC shape on the sensor sensitivity. Then, a microsensor was designed and fabricated on the basis of the numerical results. Resistance measurement was carried out using samples of normal RBCs and rigidified (Ca^[2+]-A23186 treated) RBCs. Visualization measurement of the cells’ behavior was carried out using a high-speed camera, and the results were compared with those obtained above to evaluate the performance of the sensor

Particle and rigidized red blood cell concentration distributions in microchannel flows

The motion and concentration distribution of particles and cells in flow are important factors which affect the fluid properties, flow structure, and mass transfer of biological and chemical species in blood vessels and channels. In this study, number density distributions of particles and rigidized red blood cells (RBCs) in a microchannel whose size is comparable to the sizes of the particle and RBCs are measured. Measurements were conducted at several streamwise locations for suspensions of particles and RBCs with hematocrits of the order of 10% and particle sizes of 5 and 8 µm. Analysis of the migration and resulting concentration distribution of the particles and RBCs was conducted using a model that considers the particle–particle collision and fluid dynamic force. As the size of the microchannel is small, the wall effect on the collision and migration of the particles and RBCs was significant. The wall reduced the overlapping area of the particles in collision and their displacement after collision (mobility), which varied the number, location, and magnitude of the maximum peaks observed in the number density distribution. Furthermore, the rotational motion of the rigidized RBCs in the channel flow reduced the effective lengths of the overlapping area and displacement, whereas it produced additional migration at the wall. With these terms added in the model, the number density distributions of the particles and RBCs showed reasonable agreement with those of the measurement. Especially, the number of peaks and their location for the maximum values in the model and measurement matched well

Timing and Spacing Control in Microchannel Flow by Applying Periodic Force over Space and Time

This study develops a technique to control the timing, spacing (interval), and velocity of particles in a microchannel flow by periodically exerting forces on the particles over space and time. The periodic force was produced by dielectrophoretic force using boxcar-shaped electrodes on the channel wall. We could define the timing, interval, and velocity of the particles by configuring the on–off cycles of the applied voltage. Controlling the particle spacing and timing when it crosses a position in the channel and the focusing effect in the cross-sectional position could improve the performance and throughput of microfluidics, particularly for sensing, active sorting, and encapsulation of particles and cells. The proposed technique was first evaluated by a one-dimensional analysis based on a perturbation theory. We conducted a numerical simulation to solve the dielectrophoretic force distribution and the equation of motion of the particles to understand the relationship between the force and the particle motion in the boxcar-electrode region. We measured the velocity and position of the micro-particles flowing over the boxcar-electrode region in the microchannel and demonstrated the performance and accuracy of the proposed technique for alignment and timing control. The probability density functions (PDFs) of the period between the particles, particle velocity, and timing, concentrated at the target value with minimal variation. Furthermore, the measurement of particles with diameters of 8, 10, and 12 μm resulted in the same PDFs, which showed the applicability to a reasonable variation of particle diameters

Electrical classification of single red blood cell deformability in high-shear microchannel flows

A sensor that can efficiently and sequentially measure the deformability of individual red blood cell (RBC) flowing along a microchannel is described. Counter-electrode-type microsensors are attached to the channel bottom wall, and as RBCs pass between the electrodes, the time series of the electric resistance is measured. An RBC is deformed by the high shear flow to a degree dependent upon its elastic modulus. Hence, the profile of the resistance, which is unique to the shape of the RBC, can be analyzed to obtain the deformability of each cell. First, theoretical and experimental analyses were conducted to identify the specific AC frequency at which the effect of the electric double layer formed on the electrode surface is minimized. Measurements were then conducted upon samples of normal human RBCs and glutaraldehyde-treated (rigidified) RBCs to evaluate the feasibility of the present method. In addition, simultaneous visualization of RBC deformation was performed using a high-speed camera. Normal RBCs were observed to have a degree of deformation index (DI) of around 0.57, whereas the rigidified RBCs was DI = 0 in the microchannel. The experimental measurements showed a strong correlation between the half-width of the maximum of the resistance distribution and the DI of the RBC