Junemo Koo

Forces Acting on a Single Particle in an Evaporating Sessile Droplet on a Hydrophilic Surface

The evaporating sessile droplet of a mono/didisperse colloid on a plate is a very useful and handy technique in micro/nano/bioapplications to separate, pattern, and control the particles. Although the fundamental nature of the evaporation phenomena and its applications have been extensively proposed, the crucial forces affecting a single particle motion in an evaporating droplet are not reported yet. To elucidate the impact of various forces including the drag, electrostatic, van der Waals, and surface tension forces on the particle motion in suspension, the magnitudes of them are compared using the scale analysis. In the early stage of the evaporation, in which the contact line is fixed, the motion of a single particle suspended in liquid are mostly affected by drag force. Later, with the incidence of the contact line recession, the surface tension force takes over the control of the single particle motion.

A nonlinear effective thermal conductivity model for carbon nanotube and nanofiber suspensions

It has been experimentally demonstrated that suspensions of carbon nanotubes (CNTs) and nanofibers (CNFs) significantly increase the thermal conductivity of nanofluids; however, a physically sound theory of the underlying phenomenon is still missing. In this study, the nonlinear nature of the effective thermal conductivity enhancement with the particle concentration of CNT and CNF nanofluids is explained physically using the excluded volume concept. Specifically, the number of contacting CNTs and CNFs could be calculated by using the excluded volume concept, where the distance for heat to travel in a cylinder between the contacting cylinders in the thermal network of percolating CNTs and CNFs increased with the excluded volume. In contrast to the effective thermal conductivity model of Sastry et al (2008 Nanotechnology 19 055704) the present revised model could reproduce the nonlinear increase of the thermal conductivity with particle concentration, as well as the dependence on the diameter and aspect ratio of the CNTs and CNFs. It was found that the alignment of CNTs and CNFs due to the long range repulsion force decreases the excluded volume, leading to both the convex and concave nonlinear as well as linear increase of the thermal conductivity with particle concentration. The difference between various carrier fluids of the suspensions could be explained as the result of the change in the excluded volume in different base fluids.

PARAMETRIC STUDY ON TRANSIENT HOT-WIRE METHOD TO MEASURE NANOFLUID CONDUCTIVITIES

Recently, nanofluid is a hot research topic among thermal engineers to enhance the heat transfer. However, there are deviations between research groups in the effective thermal conductivity measurements by the transient hot-wire method. Since there has been no report for the researchers to select a proper data range of temperature to estimate the thermal conductivities, the deviation could be partly attributed to the poor selection of the temperature data range. In this study, the impacts of the data range selection, the power supplier response delay, the thermal coefficient of resistors, the hot-wire type and test section size on the thermal conductivity measurement using the transient hot-wire method are analyzed. The proper selection of the temperature data range to be used to estimate the thermal conductivities of fluids using transient hot-wire method is suggested by investigating the impact of it on the measurement by both computationally and experimentally. The thermal coefficient of resistor is found to affect the measurement seriously whereas the type of hot-wire does not. The test section size of the measuring device is compared with the thermal penetration depth.