Binary mixtures have been a long historical topic due to their much richer phase diagram, structures and dynamics properties than the single component ones, and its application in food industry and alloys productions. It has been studied in many different system such as colloids [2,33], polymer blends [31, 84] and alloys [77].
Recently, complex plasmas with different types of dust-particles (or multi-component complex plasmas) are gaining more and more attention [53, 72, 119, 161, 162, 179], and physicists in this field realized that binary complex plasma,i.e., complex plasma consisting of mixtures of two different sized dust particles can also serve as a model system to study various phenomena for binary mixtures such as phase separation, lane formation, etc.
Recent studies with binary complex plasmas under microgravity conditions [72, 119,
161, 162, 179] have demonstrated their promising prospects, as so many interesting phe- nomena have been observed, such as classic tunneling by Ref. [119], lane formation as in Fig. 1.7a by Ref. [161,162] and phase separation as in Fig. 1.7b by Ref. [179]. In chapter 3.
1.3 Binary complex plasmas 17
2 mm
(a) (b)
Figure 1.7: Phenomena in binary complex plasma: (a) Lane formation in complex plasmas. A short burst of small (3.4µm) particles is injected into a cloud of big (9.2µm) background particles. From [161, 162]; and, (b) Phase separation in binary complex plasmas. The figure illustrates one experiment performed under microgravity conditions in the PK-3 Plus chamber (in argon discharge at a pressure of 30 Pa), with particles of 9.2 and 3.4 µm diameter. Small particles (colored in red) were injected into a stationary cloud of big particles (colored in green) and formed a spheroidal droplet which moved slowly towards the center of the chamber (to the right). From [179].
1.3.1
Phase separation
Phase separation, in which different types of particles tend to separate from each other, is a ubiquitous phenomenon in many different systems of multi-component mixtures, such as molecular fluids and colloidal suspensions, and has been a long-standing research topic in physics, because of both its fundamental and practical importance. It can be stimulated by either the interplay between individual particles, such as the interaction non-additivity or external perturbations, such as shear flow, temperature gradient and electric field. The phase separation is a scaling phenomenon with the average domain size, L, following a power law and is sequenced to a series of domain growth regimes with different growth exponents peculiar to each regime:
L∝ t1/3 :Diffusive regime; t :Viscous regime; t2/3 :Inertial regime. (1.19)
The diffusive regime obeys the power law with growth exponent of 1/3 [99], where diffusion is the dominant mechanism driving like particles to accumulate in the formation of tiny clusters. After the formation of clear interfaces between unlike particles, the minimization of interfacial energy becomes the dominant mechanism. This regime is termed as viscous regime with domain growth follows a linear scaling law [153]. The last regime with growth exponent of 2/3 is known as inertial regime [78], where the segregation is dominated by inertia due to the increase of the Reynolds number.
In complex plasmas, phase separation was first reported in experiment by Morfill et al. [119] and later by S¨utterlin et al. [161], both under microgravity conditions. Ivlev et al.
[72] has recently shown a theoretical model in a binary complex plasma; the inter-particle interaction is always asymmetrical, i.e., for point particles of type “1” and “2”, the 1-2 (inter-species interaction) is more repulsive/attractive than the geometric mean of 1-1 and 2-2 interactions. This asymmetry in the mutual interaction between different species is called “interaction non-additivity”, and in the case of complex plasma, one has always a positive non-additivity. According to the theory, such an interaction non-additivity leads to a spinodal region, where the fluid phase separation (spinodal decomposition) could happen, and this region overlaps with typical experimental conditions of complex plasmas in laboratory. This explains well the phase separation phenomena observed in recent experiments and also makes binary complex plasma a promising model system for studying the kinetics of phase separation.
1.3.2
Lane formation
The formation of lanes is a ubiquitous phenomenon occurring in nature when two species are moving toward and penetrating into each other. Couzin [27] has found that the move- ment rules of individual ants on trails can lead to a collective choice of direction and the formation of distinct traffic lanes that minimize congestion. This phenomenon can also be found in human beings walking on the street that they occupy specific region where lanes are formed and traffic flow is maximized. Lane formation also draws considerable interest in different branches of physics. When two different species of particles are driven into each other, like-driven particles form “stream lines” and move collectively in lanes, which depends on the details of the particle interactions and their dynamics [133]. Typically, the lanes exhibit a considerable anisotropic structural order accompanied by an enhancement of their (unidirectional) mobility. The phenomenon is commonly known from pedestrian dynamics in highly crowded pedestrian zones [60], where people heading in the same di- rections form lanes to pass the zebra crossings. It also occurs in different systems of driven particles, such as colloidal dispersions [33, 96], lattice gases [141], and molecular ions [123]. In other words, it is a ubiquitous generic process of considerable interest in different branches of physics. It is also a genuine nonequilibrium transition [141] which depends on the details of the particle interactions and their dynamics [133]. Recently, particle laning was also observed in complex plasmas [160–162] as shown in Fig.1.7a.