### Session P3: Emergent Behavior in Particle Systems Subjected to Time-Dependent Fields

Chair: Jack Douglass, National Institute of Standards and Technology
Room: Oregon Ballroom 203

 Wednesday, March 17, 2010 8:00AM - 8:36AM P3.00001: Colloidal dispersions in external fields: from equilibrium to non-equilibrium Invited Speaker: Hartmut Lowen Dispersions of colloidal particles are excellent model systems of classical statistical mechanics in order to understand the principles of self-organization processes. Using an external field (e.g. electric or magnetic field) the effective interaction between the colloidal particles can be tailored and the system can be brought into non-equilibrium in a controlled way. Glass formation after an ultrafast quench in a two-dimensional superparamagnetic binary colloidal mixture [1,2] will be discussed as well as lane [3,4,5,6,7] and band [8] formation in mixtures of charged suspensions and dusty plasmas driven by an electric field. \\[4pt] References:\\[0pt] [1] L. Assoud, F. Ebert, P. Keim, R. Messina, G. Maret, H. Lowen, Phys. Rev. Letters 102, 238301 (2009). \\[0pt] [2] L. Assoud, F. Ebert, P. Keim, R. Messina, G. Maret, H. Lowen, J. Phys.: Condensed Matter 21, 464114 (2009). \\[0pt] [3] J. Dzubiella, G. P. Hoffmann, H. Lowen, Phys. Rev. E 65, 021402 (1-8) (2002). \\[0pt] [4] M. E. Leunissen, C. G. Christova, A. P. Hynninen, C. P. Royall, A. I. Campbell, A. Imhof, M. Dijkstra, R. van Roij, A. van Blaaderen, Nature 437, 235 (2005). \\[0pt] [5] M. Rex, H. Lowen, Phys. Rev. E 75, 051402 (2007). \\[0pt] [6] M. Rex, C. P. Royall, A. van Blaaderen, H. Lowen, Lane formation in driven colloidal mixtures: is it continuous or discontinuous?, http://arxiv.org/abs/0812.0908 \\[0pt] [7] K. R. Sutterlin, A. Wysocki, A. V. Ivlev, C. Rath, H. M. Thomas, M. Rubin-Zuzic, W. J. Goedheer, V. E. Fortov, A. M. Lipaev, V. I. Molotkov, O. F. Petrov, G. E. Morfill, H. Lowen, Phys. Rev. Letters 102, 085003 (2009). \\[0pt] [8] A. Wysocki, H. Lowen, Phys. Rev. E 79, 041408 (2009). Wednesday, March 17, 2010 8:36AM - 9:12AM P3.00002: Dynamics of particle suspensions subjected to biaxial and triaxial magnetic fields: vortex mixing and isothermal magnetic advection Invited Speaker: James Martin We have developed several new magnetic methods for stimulating functional fluid flows. These methods depend on adding magnetic particles to the fluids and subjecting them to spatially uniform, time-dependent magnetic fields. The key aspect is the nature of the particles and the way in which the direction and magnitude of the magnetic field changes with time. The first of these new methods, which we call \textit{vortex field mixing}, gives rise to vigorous fluid mixing that occurs uniformly throughout the sample volume, eliminating the stagnation regions that plague standard methods. This method is ideally suited for microfluidic devices, but can used for mixing at any scale. The second method involves the stimulation of organized fluid flow fields that can efficiently transfer heat and mass along any desired direction. This \textit{isothermal magnetic advection} has the functionality of natural convection, but because the effect does not depend on gravity or the existence of a thermal gradient, it can be used to stimulate flow where natural convection fails. It is possible to cool under or beside a hot object, in the microgravity environments of space, and without any concern over the magnitude of the thermal gradient. Wednesday, March 17, 2010 9:12AM - 9:48AM P3.00003: Emergent phenomena in ferrofluids: Solitary spikes and self-propelled streams Invited Speaker: Reinhard Richter Wednesday, March 17, 2010 9:48AM - 10:24AM P3.00004: Far-from-equilibrium magnetic granular layers: dynamic patterns, magnetic order and self-assembled swimmers Invited Speaker: Alexey Snezhko Ensembles of interacting particles subject to an external periodic forcing often develop nontrivial collective behavior and self-assembled dynamic patterns. We study emergent phenomena in magnetic granular ensembles suspended at a liquid-air and liquid-liquid interfaces and subjected to a transversal alternating magnetic field. Experiments reveal a new type of nontrivially ordered dynamic self-assembled structures (in particular, magnetic snakes'', asters'', clams'') emerging in such systems in a certain range of excitation parameters. These non-equilibrium dynamic structures emerge as a result of the competition between magnetic and hydrodynamic forces and have complex magnetic ordering. Transition between different self-assembled phases with parameters of external driving magnetic field is observed. I will show that above some frequency threshold magnetic snakes spontaneously break the symmetry of the self-induced surface flows (symmetry breaking instability) and turn into swimmers. Self-induced surface flows symmetry can be also broken in a controlled fashion by introduction of a large bead to a magnetic snake (bead-snake hybrid), that transforms it into a robust self-locomoting entity. Some features of the self-localized structures can be understood in the framework of an amplitude equation for parametric waves coupled to the conservation law equation describing the evolution of the magnetic particle density and the Navier-Stokes equation for hydrodynamic flows. Wednesday, March 17, 2010 10:24AM - 11:00AM P3.00005: Novel forms of colloidal self-organization in temporally and spatially varying external fields: from low-density network-forming fluids to spincoated crystals Invited Speaker: Anand Yethiraj External fields affect self-organization in Brownian colloidal suspensions in many different ways [1]. High-frequency time varying a.c. electric fields can induce effectively quasi-static dipolar inter-particle interactions. While dipolar interactions can provide access to multiple open equilibrium crystal structures [2] whose origin is now reasonably well understood, they can also give rise to competing interactions on short and long length scales that produce unexpected low-density ordered phases [3]. Farther from equilibrium, competing external fields are active in colloid spincoating. Drying colloidal suspensions on a spinning substrate produces a perfect polycrystal'' - tiny polycrystalline domains that exhibit long-range inter-domain orientational order [4] with resultant spectacular optical effects that are decoupled from single-crystallinity. High-speed movies of drying crystals yield insights into mechanisms of structure formation. Phenomena arising from multiple spatially- and temporally-varying external fields can give rise to further control of order and disorder, with potential application as patterned (photonic and magnetic) materials. \\[4pt] [1] A. Yethiraj, Soft Matter 3, 1099 (2007). \newline [2] A. Yethiraj, A. van Blaaderen, Nature 421, 513 (2003). \newline [3] A.K. Agarwal, A. Yethiraj, Phys. Rev. Lett ,102, 198301 (2009). \newline [4] C. Arcos, K. Kumar, W. Gonz\'{a}lez-Vi\~{n}as, R. Sirera, K. Poduska, A. Yethiraj, Phys. Rev. E ,77, 050402(R) (2008).