AsianScientist (Jan. 18, 2018) – In a study published in the journal Nature, scientists at the Center for Soft and Living Matter within the Institute for Basic Science (IBS) in South Korea have made a surfactant, based on nanoparticle dimers, which is responsive to multiple stimuli. The nanosurfactant combines several characteristics of each ‘active’ molecular surfactant, which allows tremendous flexibility in the manipulation of liquid droplets.
Molecular surfactants, a union of hydrophilic and hydrophobic molecular groups, naturally perch between oil and water. A hydrophilic group sticks to water while a hydrophobic group is immersed in oil. If there is enough surfactant, water would retain the shape of droplets in oil, and vice versa. Previous studies have demonstrated that the molecular structure of surfactants can be modified to make them responsive. For instance, there were surfactants addressable by temperature, electric or magnetic field, light and chemical agents.
Combing nanoscience and surfactant research, the researchers devised a new class of surfactants consisting of nanoscopic domains within a two-component nanoparticle. One domain is functionized with hydrophilic organic ligands and the other with hydrophobic ones. Such composite nanosurfactants and the droplets they cover are responsive to light, magnets and electrostatic forces.
With the introduction of a laser, the droplets move towards the beam and make close-packed structures within seconds, later scattering when the light source is turned off. Turning the laser on and off repeats assembly and scattering, indicating that the process is reversible and repeatable. When the light is guided to the edge of a droplet, it begins to rotate. If the droplet is located in a packed group, they function as mechanical gears as the rotation of one rotates the others. The speed and direction of rotation can be adjusted by the angle and intensity of the laser.
“We can locally induce conductive flows to move particles in the droplets. Just using light, a non-contact force, we can manipulate or drive dynamic assembly of the droplets,” stated Dr. Yang Zhijie of IBS, who is the co-first author of the study.
The droplets are also responsive to magnetic fields because the surfactant is made with nanoparticles comprising a magnetic domain. The sensitivity to magnetism can be adjusted by mixing in non-magnetic nanoparticles. If magnetic and non-magnetic droplets are fused together, they create a nanoscopic compass.
Whereas the magnetic and optical fields manipulate the position and movement of the droplet, the introduction of an electric field ‘welds’ them into non-spherical or other specific-shaped liquid droplets. Applying a quick, strong electric field pulse to the point where two or more droplets are touching causes the surfactant to swirl up and down instantaneously, creating permanent channels through the gaps on the ‘equator,’ or even merging the droplets into one larger droplet. Opening a channel between the droplets allow for the mixing of their individual liquid contents.
This research opens a variety of potential possibilities, including light-guided 3D printing, coating living cells in nanosurfactants, or virtual microfluid channels with elements which would dissolve traditional channels.
The article can be found at: Yang et al. (2018) Systems of Mechanized and Reactive Droplets Powered by Multi-responsive Surfactants.
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Source: Institute for Basic Science; Photo: Pixabay.
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