Orientation of discontinuous fillers in polymer composites: modelling, characterization, control and applications

•A framework on processing-structure–property relationships of filler orientation in composites is proposed.•Future research & development perspectives on filler orientation are suggested.•An insight into modelling and characterization approaches of filler orientation is provided.•Orientation mechanisms and selection criteria of controlling strategies are presented.•Dependences of composite properties upon filler orientation are unraveled. Polymer composites have progressively found applications in sectors such as automotive, aerospace and energy storage. Their high performance can be mainly attributed to their compositions (e.g. the types of fillers and matrices) and structures (e.g. the orientation and dimension of fillers). Filler orientation has long been a crucial subject for the composite community. While the emerging discontinuous fillers offer new attributes to the next generation composites and new composite manufacturing techniques such as additive manufacturing, their orientation, however, has been much less understood and controlled as compared to that of the continuous fillers, mainly due to their small scales. Focusing on the role of filler orientation in the processing-structure–property relationships of polymer composites, the present Review critically discusses recent progress on the modelling, characterization, control and applications of the orientation of discontinuous fillers, with the advantages and disadvantages of the representative approaches analyzed. Their correlations are revealed under a proposed theoretical framework based on the rotational degree of freedom and translational degree of freedom of the fillers. Some selection criteria are proposed to guide future selection of composite manufacturing techniques for customized filler orientation. Finally, unaddressed fundamental issues and future perspectives on research and development in this field are highlighted.