# Effects of Tube Radius and Surface Tension on Capillary Rise Dynamics of Water/Butanol Mixtures
###### Abstract  
Capillary-driven action is an important phenomenon which aids the development of high-performance heat transfer devices, such as microscale heat pipes. This study examines the capillary rise dynamics of n-butanol/water mixture in a single vertical capillary tube with different radii (0.4, 0.6, and 0.85 mm). For liquids, distilled water, n-butanol, and their blends with varying concentrations of butanol (0.3, 0.5, and 0.7 wt.%) were used. The results show that the height and velocity of the capillary rise were dependent on the tube radius and liquid surface tension. The larger the radius and the higher the surface tension, the lower was the equilibrium height (\(h_{\mathrm{e}}\)) and the velocity of rise. The process of capillary rise was segregated into three characteristic regions: purely inertial, inertial + viscous, and purely viscous regions. The early stages (purely inertial and inertial + viscous) represented the characteristic heights \(h_{1}\) and \(h_{2}\), which were dominant in the capillary rise process. There were linear correlations between the characteristic heights (\(h_{1}\), \(h_{2}\), and \(h_{\mathrm{e}}\)), tube radius, and surface tension. Based on these correlations, a linear function was established between each of the three characteristic heights and the consolidated value of tube radius and surface tension (\(\varepsilon L/2\pi r^{2}\)). capillary tube; surface tension; butanol/water mixture; liquid rise; capillarity; meniscus 4.0.0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-00000-00000-00000-00000-00000-00000-00000-00000-00000-00000-00000-000000-000000-000000-000000-000000-000000-000000-00  
and require electrical power to operate in microscale heat pipes. This implies that capillary flow does not require any additional power-consuming actuation devices [8; 9]. [PERSON] et al. [10] investigated the capillary pumping performance of a porous wick for a looped heat pipe with water and acetone as the working fluid, and they found that the capillary pumping rate increased with increasing porosity of the wicks. [PERSON] et al. [11] developed a novel stainless steel fiber-powder composite wick with high capillary pressure and good permeability for use in stainless steel heat pipes. Their work showed that the powder particle size has a significant influence on the capillary performance of the wick. [PERSON] et al. [12] showed that wicks using working fluids with lower viscosity, larger surface tension, and higher density demonstrated better capillary performance by comparing the results of three different working fluids: water, acetone, and ethanol. Generally, micro heat pipes have a wickless configuration, and hence, the capillary dynamics in the micro-pipes is utilized to drive the working fluids. Moreover, the operation of the micro-heat pipe is governed by transportation of the working fluids and aided by the capillaries or microscale pipes, and its overall thermal performance is predominantly determined by the thermal properties of the working fluids and the size of the capillaries. Thus, the selection of the type of working fluid and the physical size of the system, as well as their impacts on the capillary dynamics are of technological interest. Therefore, the capillary rise dynamics of mixtures with different surface tension and capillary tubes of several hundred micrometers were investigated. [PERSON] et al. [13] investigated the dynamics of water/ethanol mixtures in capillary tubes and found that surface tension plays an important role in the travelling speed inside the capillary tubes. [PERSON] et al. [14] investigated the capillary rise dynamics of glycerol and aqueous glycerol solutions in vertical glass capillaries of various radii. [PERSON] and [PERSON] [15] measured the forces and energies generated by the rising liquids such as ethylene glycol, glycerol, and silicone oil in capillary tubes with diameters ranging from 0.5 to 1.4 mm.