Complex (dusty) plasmas are composed of a weakly ionized gas and charged microparticles and represent the plasma state of soft matter. Complex plasmas have several remarkable features: Dynamical time scales associated with microparticles are ``stretched'' to tens of milliseconds, yet the microparticles themselves can be easily visualized individually. Furthermore, since the background gas is dilute, the particle dynamics in strongly coupled complex plasmas is virtually undamped, which provides a direct analogy to regular liquids and solids in terms of the atomistic dynamics. Finally, complex plasmas can be easily manipulated in different ways---also at the level of individual particles. Altogether, this gives us a unique opportunity to go beyond the limits of continuous media and study---at the kinetic level---various generic processes occurring in liquids or solids, in regimes ranging from the onset of cooperative phenomena to large strongly coupled systems. In the first part of the review some of the basic and new physics are highlighted which complex plasmas enable us to study, and in the second (major) part strong coupling phenomena in an interdisciplinary context are examined. The connections with complex fluids are emphasized and a number of generic liquid and solid-state issues are addressed. In summary, application oriented research is discussed.

Complex plasmas: An interdisciplinary research field

Tuned, driven, and active soft matter

Simple models for complex nonequilibrium fluids

▪ Abstract Recent progress in modeling and simulation of the flow of nematic liquid crystals is presented. The Leslie-Ericksen (LE) theory has been successful in elucidating the flow of low molar-mass nematics. The theoretical framework for the flow of polymeric nematic liquid crystals is still evolving; extensions of the Doi theory capture qualitative features of the flow of polymeric nematics in simple geometries, but these theories have not been shown to predict texture development in flow. Mesoscopic theories for textured materials based on spatial averaging capture only some qualitative features of nonrectilinear liquid-crystalline polymer flow. Interfacial effects in liquid-crystalline systems have begun to receive attention in the context of interfacial viscoelasticity and the dynamics of dispersed liquid-crystalline polymers in immiscible blends.

Dynamical phenomena in liquid-crystalline materials

The problem of the inception of cavitation is formulated in terms of a comparison of the breaking strength or cavitation threshold at each point in a liquid sample with the principal stresses there. A criterion of maximum tension is proposed which unifies the theory of cavitation, the theory of maximum tensile strength of liquid filaments and the theory of fracture of amorphous solids. Liquids at atmospheric pressure which cannot withstand tension will cavitate when and where tensile stresses due to motion exceed one atmosphere. A cavity will open in the direction of the maximum tensile stress which is 45° from the plane of shearing in pure shear of a Newtonian fluid. Experiments which support these ideas are discussed and some new experiments are proposed.

Cavitation and the state of stress in a flowing liquid

In start-up of steady sheafing flow of two viscous unen- tangled liquids, namely low-molecu- lar-weight polystyrene and a-D-glu- cose, the shear stress catastrophi- cally collapses if the shear rate is raised above a value corresponding to a critical initial shear stress of around 0.1-0.3 MPa. The time de- pendence of the shear stress during this process is similar for the two liquids, but visualization of samples in situ and after quenching reveals significant differences. For a-D-glu- cose, the stress collapse evidently results from debonding of the sam- ple from the rheometer tool, while in polystyrene, bubbles open up within the sample, as occurs in cavi- tation. Some similarities are pointed out between these phenomena and that of "lubrication failure" reported in the tribology literature.

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Fracture phenomena in shearing flow of viscous liquids

The results of an experimental study to measure the tumbling parameter, λ, for various small-molecule liquid crystals and their mixtures are presented. The methods used include textural observations (twist walls), a direct method, a rheological method, and the oscillatory method developed by Mather, Pearson, and Burghardt in 1995. The single-component results are compared with a molecular theory derived in 1995 by Archer and Larson as well as Kroger and Sellers, which predicts the temperature dependence of λ, while the results from the binary mixtures are compared to a continuum theory derived by Rey in 1996, giving the concentration dependence of λ. The results from the four experimental methods agree with each other for single-component liquid crystals, but not for mixtures. This suggests a failure of the single director Leslie-Ericksen theory to describe the rheology of liquid crystal mixtures.

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Flow-aligning and tumbling in small-molecule liquid crystals: pure components and mixtures

In this work we attempt to determine the origin of damped stress oscillations upon flow start-up of a nematic liquid crystalline monodomain. These damped stress oscillations were first observed by Gu et al. (1993) in the cone-plate flow cell and have since also been observed by Mather et al. (1997) in the parallel disk cell. Although Mather's work explained the cause of the stress oscillation damping in the torsional flow cell, the origin of the damping in the cone-plate device remains a mystery. Here we report finding similar damped stress oscillations in the cylindrical Couette cell and combined with the optical experiments reported earlier by Cladis and Torza (1975, 1976) we are able to propose an explanation for the damping in this geometry. We also report new optical experiments using the cone-plate cell in hopes of determining a cause to the damping in the cone-plate cell.

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Dennis J. Ternet

Papers

Fracture phenomena in shearing flow of viscous liquids

Flow-aligning and tumbling in small-molecule liquid crystals: pure components and mixtures

Transient director patterns upon flow start-up of nematic liquid crystals (an explanation for stress oscillation damping)