Surface drops is caused by surface tension. “This

Surface tension is the attractive force acting on the surface of a liquid as the molecules beneath draw the surface molecules into bulk and tries to form a shape with the least surface area, which is “similar to the tendency of liquid surface to form minimum area.”(Burdon, 2014). Surface tension occurs because of the Van der Waals forces, which is the various intermolecular forces drawing the liquid particles together along the surface and also to the rest of the liquid. Surface tension is related to physiological, ranging from cloud formation, lung functioning, nature, etc. An example of water surface tension is a water droplet. When using a water dropper, the droplet forms a nearly spherical shape. The shape of the drops is caused by surface tension. “This phenomenon is  caused by only a few molecule thick layer” (Burdon, 2014).However, the ones mentioned above are all the cohesion force of water. The most common and important use of surface tension is the use of adhesion forces on plants. Water is essential to the survival and growth of plants, and water is uptaken through the plant’s roots. The uptake of water can occur because of adhesion. Adhesion is when water molecules are attracted to substances other than water because of an opposite molecular charge. It can cause water to stick to the inner surface of the roots and allow water to “climb” upwards. When water is going up the roots, capillary action takes place. The three main variables that determine whether a liquid can allow capillary action to happen are cohesive force, surface tension, and adhesive force. Scientists studied the uptake of roots and developed the capillary method to measure the liquid’s surface tension. They concluded that “concentration of liquid and wettability of capillary tube cause liquid to rise above water level.” (Udeagbara, 2010) “Surface tension depends on the liquid’s properties, environment, and temperature.”(Udeagbara, 2010) Different liquids have a different surface tension, and it can change the rate and ability of uptake of plant roots, thus affect plant health. From the research of Andrew M. Prpich and Yuebiao Sheng, they studied water vapor has any role with liquid’s surface tension. They used a method called “Axisymmetric Drop Shape Analysis-Profile” method to measure the surfactant concentration under different conditions. When the two solutions (drop solution and environment solution) both had different surfactant concentrations, molecular exchange across the vapor/liquid interface would cause a change in surface tension of the drop because “when a new interface is formed, the surfactant is drawn toward the interface to reach a thermodynamically more favorable state to diminish free energy of the system, and surface tension is decreased.”(Prpich, Sheng, Wang, Biswas, & Chen 2009)From results, under the positive concentration-difference condition, it shows that the surface tension increments after the induction period. Under the negative concentration-difference condition, the surface tension decreases after the period. Under the zero concentration-difference condition, the surface tension remains unchanged because there is no driving force for molecular transfer. Results suggest that surface tension is controlled by liquid and vapor phase concentrations, and that the “final state of the surface tension is determined by the vapor phase.”(Prpich et al., 2009)The experiment concluded that fluid in a vapourized state plays a exceptionally imperative part on surface tension even though it has a much lower density of surfactant in the vapor phase. Therefore, a different in concentration can change the surface tension of a liquid. From the book written by Burdon, R. S., he suggested that surface tension is similar to mentioned that the phenomenon of surface tension seemed like a elastic and stretchy film. However, the stretching of water is actually the formation of new a layer of water , not by the elasticity of water. One of the measuring unit for surface tension is dyne. He discovered that when water surface area was increased or decreased by any square centimeter, the surface would ” perform y ergs to work”(Burdon, 2014). Surface tension of a liquid is equal to the free energy of its surface per unit area. “Surface energy is the energy formed due to the molecule’s superficial position, as the molecules must move to the surface against unbalance power.” (Burdon, 2014)When the liquid have more than one kind of molecule, equilibrium will reach when the free energy of the surface reached its minimum. Therefore,  a liquid can take days for its surface tension to reach its final value. Moreover, the surface tension of a fresh formed solution would be higher than the surface tension of the same solution that has been sitting at rest because of the absorption of the surface active-solute, or diffusion, and also by the aging of the liquid’s surface. The longer a solution sits, the lower the surface tension, which also cause a solution days to weeks of time to reach its final equilibrium state.From the paper published by Idris et al., they aimed to measure the surface tension of an alkaline or basic monoethanolamine solution. In this paper, a goniometer and DROPimage Advanced v2.4 software were used to measure the concentration of the monoethanolamine solution and measure the size of the water droplet at a temperature between 30 and 65 degrees celsius. Also Idris also mentioned that for “every surface tension experiment precise temperature and pressure control is required.”(Idris, Han, Jayarathna, & Eimer, 2017) For example in the capillary-rise technique, the height of the solution inside an immersed capillary is measured. The results obtained by Idris showed that “as the mole fraction of MEA increases, surface tension decreases”(Idris et al., 2017) because the increase of ratio of moles of monoethanolamine to the total number of moles, or mole fraction cause more molecules concentrate  at the solution-air interface,  thus lowering the surface tension of solution in comparison to water. Also, with the increase of temperature, the molecules obtained higher thermal motion. Because of the increase of motion, the intermolecular attraction decreases so as the surface tension. From the book “Effect of Temperature and Impurities on Surface Tension of Crude Oil”, the author, Stephen Gekwu Udeagbara, wanted to investigate the effect of temperature and impurities to crude oil, water, and detergent surface tension. From a result, he found out that the rise of temperature cause the surface tension to decrease but salt and bentonite concentration increased. He further concluded that “inorganic substances can increase surface tension by lowering the concentration of salt”(Udeagbara, 2010), and excess amount of  concentration of soap would reduce surface tension. Therefore, temperature affects surface tension, and impurities would decrease surface tension. However, some compound has a different effect on surface tension. “Inorganic salt would increase a liquid’s surface tension,” (Udeagbara, 2010) and sugar has no effect on surface tension. To measure the surface tension, there are six static methods including the capillary rise, sessile drop, pendant drop, drop weight, maximum bubble pressure, and Wilhelmy plate. They are all suitable to measure the surface tension of pure substance. However, as the liquid reaches equilibrium slowly, the author recommended dynamic methods like the capillary waves method or the unstable jets method. From the experiment on capillary rise from Barozzia et al., they measure the surface tension of water using capillary tubes placed in different angles in the water. The angle varied from 0 degrees to 88 degrees.The selection of the capillaries to be used in the experiment was controlled by “the necessity of finding tubes of adequate length, so as to allow high inclination angles, and correspondingly long capillary extensions.”(Barozzia et al., 2014) The selection of the capillaries must be chosen with caution to suit the container length and height, so as the allowance of high inclination angles. To obtain data, a camera was used. The capillary tube was placed between the light source and the camera on the same level. In each of the measurement with a different degree, the camera was used to enhance and capture the place of meniscus within the tube. A digital surface gauge was then used to measure the height of the meniscus vertex. Therefore, the capillary rise value was obtained twice with a different measured level. Results were then “presented from an experiment on capillary rise of water in inclined small-bore cylindrical tubes”(Barozzia & Angeli, 2014). The final results showed that capillary rise rate reduces significantly with the increase of the degree being placed into the water.


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