Small droplets in emulsions can merge much faster in the presence of an alternating electric field. The phenomenon is called electrocoalescence and is of essential importance for the operation of advanced microfluidic devices, allowing one to carry out chemical reactions in microliter volume or less. A recent study carried out at the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) allows for improved control of electrocoalescence and for optimisation of the course of the process.
Electrocoalescence has been known for over 100 years. Originally, it has been used in the oil industry. Crude oil extracted from the sea floor contains significant amount of water in a form of droplets. Engineers have noticed that in the presence of an alternating electric field these droplets merge quickly and fall on the tank bottom from where the water can be easily removed. "Electrocoalescence has started its career untypically, from large scale applications in the oil industry. In microfluidics the phenomenon has been applied to merging of droplets only within the last few years," says Dr Piotr Garstecki from the IPC PAS.
Microfluidic devices are miniaturised chemical reactors resembling a credit card in size. Chemical reactions take place inside the droplets that are suspended in a neutral liquid flowing through appropriately designed microchannels. The droplets can be very small: from a fraction of a microliter even down to nano- or picoliters.
"The primary challenge in microfluidic techniques is to combine the stability of droplets with the ability to merge them. Stabilization requires covering the interface with a surfactant, i.e., a surface active agent," explains Tomasz Szymborski, a PhD student at the Institute of Physical Chemistry of the PAS. Surfactant molecules are composed of a hydrophilic (water-loving) and a hydrophobic (water-hating) part. If a water droplet is placed in oil, the surfactant covers it so that the hydrophilic parts are immersed in the droplet, whereas the hydrophobic ones remain in oil. "From outside, the droplets covered with surfactant resemble rolled up hedgehogs and have no chance to touch each other. Their stability is increased due to the fact that surfactant molecules repel each other via electrostatic and entropic forces," explains Szymborski.
Problems appear when precisely selected droplets of different reagents are to be merged in a microfluidic device to carry out a chemical reaction. Since recently electric field has been used to induce merging. Electrocoalescence is known for its macro scale industrial applications, yet how the mechanism and efficiency of the process depends on the parameters of the electric field was largely an open question.
The researchers at the IPC PAS observed merging of water microdroplets in a carried liquid, hexadecane. The rate of droplet merging increased in line with applied voltage and the frequency of electric field oscillations. For each voltage there was a limiting frequency, above which the droplets became stable again. "We showed that the merging proceeded at its maximum rate when the electric field oscillated with a frequency close to a threshold one and we found a simple function allowing to estimate the value of the threshold quickly," says Szymborski.
Rapid merging of droplets is related, i.a., to a periodic movement of ions contained in the droplets that are stimulated by an alternating electric field. The ions separate in the oil-droplet interface while charging it electrically. The droplets with opposite charges attract strongly, which results in droplet merging in spite of the presence of stabilizing surfactants. The results of the study suggest that there is a simple relation between the nanoscopic electrostatic screening length and the optimum frequency for merging of droplets.
The data collected at the IPC PAS will be helpful for practical optimisation of processes involving electrocoalescence, both in microfluidic devices and industrial plants. The results will also facilitate formulation of universal laws describing the efficiency of electrocoalescence in non-equilibrium systems such as flowing liquids.
The research has been financed from a TEAM grant from the Foundation for Polish Science and the Iuventus-Plus Programme of the Polish Ministry of Science and Higher Education.
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