ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING Asia-Pac. J. Chem. Eng. 2008; 3: 255–268 Published online in Wiley InterScience (www.interscience.wiley.com) DOI:10.1002/apj.144 Research Article The forces at work in colloidal self-assembly: a review on fundamental interactions between colloidal particles Qin Li, 1,2 *Ulrich Jonas, 2,3 *X. S. Zhao 4 and Michael Kappl 2 1 Department of Chemical Engineering, Curtin University of Technology, Perth, WA 6845, Australia 2 Max Planck Institute for Polymer Research, Mainz 55128, Germany 3 FORTH/IESL, Voutes Str., 71110 Heraklion, Crete, Greece 4 Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore Received 10 December 2007; Accepted 1 February 2008 ABSTRACT: Colloidal particles with well-defined sizes can self-assemble into ordered, crystalline structures under non-equilibrium conditions. This phenomenon originates from the various forces acting upon them. In this article, we provide an overview on the forces at work in a colloidal system, in particular, the roles of these forces at various stages in colloidal self-assembly. Van der Waals, electrostatic, hydrodynamic, and capillary forces, as well as Brownian motions, are extensively discussed, whereas other types of interactions are briefly introduced and summarized.  2008 Curtin University of Technology and John Wiley & Sons, Ltd. KEYWORDS: colloidal forces; colloidal self-assembly INTRODUCTION Colloidal dispersions cover an extremely broad range of systems in which the kinetic units being dispersed throughout a liquid are of a size ranging from nanome- tres to micrometres. They have long been recognized as an important class of materials. Owing to their small sizes and the forces exerted upon them through the dis- persion media, [1–3] colloidal particles have the ability to self-assemble into ordered, crystalline structures under non-equilibrium conditions and for well-defined particle size distributions, e.g. mono-, [4] binary, [5] and ternary dispersions. [6] Three-dimensional (3D) colloidal crys- tals can be formed by drop-casting, [7] vertical lifting deposition, sedimentation, [8] and electrode position, [9] to name but a few. The most common type of colloidal crystals formed by self-assembly of mono-disperse par- ticles has a face-centred cubic (fcc) lattice symmetry with the highest crystalline packing density of 74% vol- ume filling, as shown by the scanning electron micro- scope (SEM) image in Fig. 1, which was fabricated by flow-controlled vertical deposition. Self-assembly takes place at all scales from atoms and molecules up to meso- and macroscopic objects. [11] Colloids belong to the group of mesoscale objects, and *Correspondence to : Qin Li, Max Planck Institute for Polymer Research, Mainz 55128, Germany. E-mail: Q.Li@curtin.edu.au Ulrich Jonas, Max Planck Institute for Polymer Research, Mainz 55128, Germany. E-mail: jona@mpip-mainz.mpg.de colloidal self-assembly is a dynamic reverse process towards thermodynamic minimum-energy structure. Irrespective of the self-assembly techniques employed, an aqua-based or organic-solvent-based suspension of nano/microspheres is the prerequisite to grow colloidal crystals because the particles must be able to move with respect to each other. In the self-assembly of col- loidal crystal, the particles experience a sequence of phases, namely suspension, migration (towards the crys- tallization front), deposition/crystallization, and dry- ing/fixation. Various types of interactions take place simultaneously and the resultant structure of the colloid system is a complex balance of the attractive and repul- sive interactions. In each phase, the dominant mech- anisms of colloidal interactions (particle–particle, and particle–boundaries) vary. Since the colloidal forces underpin the suspension stability, the particle deposition efficiency, and the self-assembled structure quality, a structured and clear understanding of the contributing interactions is essen- tial for analyzing experimental facts and data as well as designing better and novel experiments and processes. Although there is a large amount of literature dedicated to the colloidal forces, a concise, albeit comprehensive, review on the relevance of colloidal forces to colloidal self-assembly is yet unavailable. This review intends to fill this gap by illustrating the mechanisms under- lying the colloidal self-assembly. It may serve as a concise summary of the current theories of colloidal forces and a short compendium for colloidal scientists  2008 Curtin University of Technology and John Wiley & Sons, Ltd.