Novel forms of colloidal self-organization in temporally and spatially varying external fields: from low-density network-forming fluids to spincoated crystals

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).