Highly Ordered Arrays of Femtoliter Surface Droplets.

exploited to manipulate the average droplet size on homogeneous substrates. [ 25,26 ] In particular, it has been demonstrated that the average droplet size can be tuned through global modulation of the concentration gradient in the solutions to produce droplet heights on the order 0.01–1 μm (and hence the term “mesoscale droplets”) and droplet volumes in the 0.5–50 femtoliter (fL) range. [ 27 ] Nevertheless, the limitations to date with the solvent-exchange method has been the large distribution in the droplet sizes that are obtained and their random positioning on the substrate. [ 22 ] As such, this has constrained its practical use in applications where uniform droplet volumes, tunable droplet morphology, and precise positioning or large-scale patterning into ordered arrays are desirable. We show in this work that it is possible to circumvent these limitations through selective nucleation, growth, and confi nement of the droplets on arrays of smooth hydrophobic pre-patterned microdomains on the hydrophilic substrate. We observe the droplet volumes in the array to be highly uniform, their contact angles to be related to the dimension of the lateral confi nement of the microdomains, and that it is possible to tune these by varying the saturation level in the bulk liquid. In addition to providing insight into the fundamental underlying mechanism responsible for control of the droplet morphology, we also demonstrate, as a proof-of-concept example application, a technique for the simple fabrication of low-cost and portable nanolens arrays for optical manipulation that is based on photopolymerization of the surface droplets. Figure 1 a–e, which comprise optical images of the polymerized droplets, show their spatial arrangement into highly ordered arrays on the pre-patterned substrates. The inter-droplet spacing, measured as the distance between the centers of two droplets, is observed to closely match that of the hydrophobic patterns, as shown in Figure S1 (Supporting Information). Figure 2 a, on the other hand, shows the close one-to-one correlation between the average lateral diameter of the droplets and that of the patterns. Specifi cally, the lateral diameters of the polymerized droplets were measured to be 1.03 ± 0.12 μm, 2.93 ± 0.34 μm, 4.98 ± 0.14 μm, 9.86 ± 0.26 μm, and 28.9 ± 2.62 μm for pre-patterned arrays with diameters of 1 ± 0.10 μm, 3 ± 0.10 μm, 5 ± 0.11 μm, 10 ± 0.14 μm and 30 ± 0.20 μm, respectively. These results clearly confi rm the formation of the droplets exclusively on the patterned hydrophobic microdomains as well as confi nement of their lateral growth within the domain. It is therefore possible to not only precisely control the dimension, spacing, and location of the droplets, but also to control the droplet number, density, and spatial arrangement through the design of chemical patterns on the substrate surface. DOI: 10.1002/smll.201501105 Droplet Arrays