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The shape memory effect and pseudoelastic effect of NiTi shape memory alloys occur within respectively a narrow temperature and stress window associated with its martensitic transformation. This renders difficulty in accurate and reliable control for actuation applications using these properties. One approach to improving controllability of an actuation component is to design a geometrically graded shape memory structure to create a nonuniform transformation field within the structure. This paper presents analytical modelling and experimental evaluation of geometrically graded NiTi structures with parallel design configuration. Closed-form solutions are obtained to describe the stress-strain relationship of such structures under tensile loading conditions, which can be used as an engineering tool for optimizing shape memory performances of such components. The geometrically graded structures exhibited partial stress gradient over stress-induced transformation. A maximum stress window of 420 MPa was achieved over transformation stage, giving enlarged stress interval for shape memory actuation control. Image 1 • Closed-form stress-strain relationships were obtained for optimizing shape memory performance. • Transformation propagation in parallel design configuration was perpendicular to the loading axis. • Geometrically graded NiTi demonstrated partial stress gradient over transformation. • Widened transformation stress window for actuation application was created. [ABSTRACT FROM AUTHOR]

Abstract Shape memory alloys exhibit unique thermomechanical properties, e.g., the shape memory effect and the pseudoelasticity. By proper geometrical gradient design, these alloys can be made to exhibit different and more intricate thermomechanical behaviour to enable innovative applications. This paper reports the design of geometrically gradient NiTi and characterisation of its complex and nonuniform transformation and deformation fields. The study investigated two designs, with one using multiple pseudoelastic NiTi strips of different lengths in parallel arrangement to create a discrete geometrical gradient in the direction perpendicular to the loading axis and the other a tapered plate to give a continuous length gradient in the lateral direction for comparison with the former. The geometrically gradient structures exhibited partial stress gradient during stress-induced transformation. A maximum stress window of 520 MPa was achieved, giving an expanded stress interval for shape memory actuation control. Finite element modelling was applied to characterise the deformation behaviour of such structures under tensile loading and to reveal the complex propagation of the stress-induced transformation in such structures. Graphical abstract Image 1 Highlights • Nonuniform transformation field was created in parallel geometrical gradient design. • Geometrically graded NiTi demonstrated partial stress gradient over transformation. • Widened transformation stress window for actuation application was created. • Transformation propagation in discrete design was perpendicular to the loading axis. [ABSTRACT FROM AUTHOR]

Owing to geometrical non-uniformity, geometrically graded shape memory alloy (SMA) structures by design have the ability to exhibit different and novel thermal and mechanical behaviors compared to geometrically uniform conventional SMAs. This paper reports a study of the pseudoelastic behavior of geometrically graded NiTi plates. This geometrical gradient creates partial stress gradient over stress-induced martensitic transformation, providing enlarged stress controlling interval for shape memory actuation. Finite element modeling framework has been established to predict the deformation behavior of such structures in tensile loading cycles, which was validated by experiments. The modeling results show that the transformation mostly propagates along the gradient direction as the loading level increases. [ABSTRACT FROM AUTHOR]

Directional liquid transportation is important for a variety of biological processes and technical applications. Although surface engineering through asymmetric chemical modification or geometrical patterning facilitates effective liquid manipulation and enables water droplet self-transportation on synthetic surfaces, self-transportation of oil droplets poses a major challenge because of their low surface tension. We report oil droplet self-transportation on oleophobic surfaces that are microtextured with radial arrays of undercut stripes. More significantly, we observe three modes of oil motion on various sample surfaces, namely, inward transportation, pinned, and outward spreading, which can be switched by the structure parameters, including stripe intersection angle and width. Accompanying theoretical modeling provides an in-depth mechanistic understanding of the structure-droplet motion relationship. Finally, we reveal how to optimize the texture parameters to maximize oil droplet self-transportation capability and demonstrate spontaneous droplet movement for liquids down to a surface tension of 22.4 mN/m. The surfaces presented here open up new avenues for power-free liquid transportation and oil contamination self-removal applications in various analytical and fluidic devices.