Active matter contains self-propelled
units able to convert stored
or ambient free energy into motion. Such systems demonstrate amazing
features related to the phenomenon of self-organization and phase
transitions and can be used for the development of artificial materials
and machines that operate away from equilibrium. Significant advances
in the fabrication of active matter were achieved when studying low-density
gas and small crystallites. However, the technique of preparation
of active matter, where one can observe the formation of stable crystals,
is extremely challenging. Here, we describe the novel method to obtain
a stable 2D crystal in the active octane-in-water emulsion during
the process of heterogeneous crystallization. Active motion is driven
by the Marangoni flow emerging at the interface of the droplet. It
is established that the crystal volume increases linearly in time
in the process of crystallization. Moreover, the dependence of the
crystal growth rate on the average velocity of droplets motion in
the emulsion has a maximum. The kinetics of crystal growth is defined
by a competition between the processes of attachment and detachment
of droplets from the crystal surface. Crystallization proceeds via
condensation of droplets from the gas phase through the formation
of liquid as an intermediate phase, which covers the crystal surface
with a thin layer. Inside the liquid layer the bond-orientational
order of droplets decreases from the crystal surface toward the gas
phase. We anticipate our study to be a starting point for the development
of new materials and technologies on the basis of nonequilibrium droplet
systems.
The
locomotion of droplets in emulsions is of practical significance
for fields related to medicine and chemical engineering, which can
be done with a magnetic field to move droplets containing magnetic
materials. Here, we demonstrate a new method of droplet locomotion
in the oil-in-water emulsion with the help of a nonuniform magnetic
field in the case where magnetic nanoparticles (MNPs) are dispersed
in the continuous phase of the emulsion. The paper analyses the motion
of the droplets in a liquid film and in a capillary for various diameters
of droplets, their number density, and viscosity of the continuous
phase of the emulsion. It is established that the mechanism of droplet
locomotion in the emulsion largely depends on the wettability of MNPs.
Hydrophobic nanoparticles are adsorbed on the droplet surfaces, forming
the agglomerates of MNPs with the droplets. Such agglomerates move
at much higher velocities than passive droplets. Hydrophilic nanoparticles
are not adsorbed at the surfaces of the droplets but form mobile magnetic
clusters dispersed in the continuous phase of the emulsion. Mobile
magnetic clusters set the surrounding liquid and droplets in motion.
The results obtained in this paper can be used in drug delivery.
Numerous living organisms as well as artificially created self-propelled objects can form dissipative structures due to the nonlinear effects and nonequilibrium of the system. Here we present an active oil-in-water emulsion in which the oil droplets take part in the reciprocating motion under the action of Marangoni flow near the air−water interface. The droplet dynamics in the emulsion is governed by the chemical reaction proceeding between quiescent copper particles and ammonia and by the convective mixing of a surfactant. We established that the reciprocating motion of droplets in the emulsion arises as a result of a periodic change in the Marangoni flow direction at the air−water interface. The feature of the considered system is that the reciprocating motion of droplets is realized only when the surface area fraction of droplets in the emulsion is close to the density of a twodimensional colloid crystal. Oscillations degenerate under the reduction in surface area fraction to the critical value of ∼50% since the existence of oscillations in the emulsion requires a suppression of the surfactant convective mixing between the inner layers of liquid film and the air−water interface.
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