Materials and strategies applicable to the dynamic transport of microdroplets are relevant to surface fluidics, self-cleaning materials, thermal management systems, and analytical devices. Techniques based on electrowetting, topographic micropatterns, and thermal/chemical gradients have advanced considerably, but dynamic microdroplet transport remains a challenge. This manuscript reports the fabrication of mechano-tunable, microtextured chemical gradients on elastomer films and their use in controlled microdroplet transport. Specifically, discreet mechanical deformations of these films enabled dynamic tuning of the microtextures and thus transport along surface-chemical gradients. The interplay between the driving force of the chemical gradient and the microtopography was characterized, facilitating accurate prediction of the conditions (droplet radius and roughness) which supported transport. In this work, the use of microtextured surface chemical gradients in mechano-adaptive materials with microdroplet manipulation functionality was highlighted.

Silicone elastomers are used in a variety of “stretchable” technologies (e.g., wearable electronics and soft robotics) that require the elastomeric components to accommodate varying magnitudes of mechanical stress during operation; however, there is limited understanding of how mechanical stress influences the surface chemistry of these elastomeric components despite the potential importance of this property with regards to overall function. In this study, plasma-oxidized silicone (poly­(dimethylsiloxane)) films were systematically subjected to various amounts of tensile stress and the resulting surface chemical changes were monitored using contact angle measurements, X-ray photoelectron spectroscopy, and gas chromatography–mass spectrometry. Understanding the influence of mechanical stress on these materials made possible the development of a facile method for the rapid, on-demand switching of surface wettability and the generation of surface wettability patterns and gradients. The use of mechanical stress to control surface wettability is broadly applicable to the fields of microfluidics, soft robotics, printing, and to the design of adaptable materials and sensors.

make fabrication difficult. In contrast, mechanically-adaptive superhydrophobic surfaces based on elastomeric materials with mechanically controlled wrinkles [8,9] and microstructure [10,11] offer simple, rapid control in compliant formats useful to, for example, droplet manipulation and water repellency. However, surface instabilities (i.e., microcracks) have limited the operational range of strain for these materials [12] and sophisticated fabrication procedures (e.g., ion etching and direct-write printing) have confined their application space. [13] We sought to develop a versatile process for the fabrication of soft, mechanically compliant, and tunable SH materials that was simple, scalable, and cost-effective. These mechano-adaptive materials would feature mechanically responsive wettability and droplet adhesion with characteristics that surpass the limitations of existing materials. To achieve this goal, we fabricated elastomer-supported, hierarchically structured surfaces comprised of nanoporous films with networks of mechanically tunable microcracks. Specifically, we oxidized silicone rubbers to generate a rigid silica interfacial layer which, through the application of tensile strain, could be fractured along surface instabilities to generate a network of microcracks. We then rendered this surface SH through a simple chemical etching procedure which introduced nanoporosity in the silica layer. Although the chemical oxidation (ultraviolet ozone, UVO) procedure introduced polar functionalities onto the surface, which would typically increase wettability, the hierarchical micro/nano-structuring afforded mechano-tunable superhydrophobic properties that were readily manipulated using small magnitudes of force easily accessible through inexpensive motors/controllers or even biomechanical input from a user.We believe the unique combination of mechano-induced surface microcracks, which have often been viewed as undesirable in adaptive systems owed to the difficulty in predicting surface instabilities, and chemical etching, an extensively exploited process to generate micro/nanotopography in superhydrophobic films, [14][15][16] provides an exciting, unexplored avenue in the field of adaptive SH materials with the potential to nucleate the design of materials with new functionalities and applications. Unlike existing procedures, combining the convenient processing of solution-phase etching and Adaptive materials with tunable superhydrophobic surfaces promise to impact a range of fluid handling technologies; however, adaptive superhydrophobic materials remain difficult to fabricate, control, and switch rapidly. Here, a versatile method for generating hierarchically structured and adaptive superhydrophobic silicone films for the rational control of surface wettability and droplet adhesion is reported. Specifically, mechanical tension is utilized to manipulate networks of microcracks in nanoporous layers supported on elastomeric silicone films, enabling dynamic modulation of superhydrophobicity and droplet adhesion. ...

The effect of anisotropic surface roughness on the spontaneous transport of droplets on chemical wettability gradients has not been investigated. Understanding the details of this process has the potential to unlock new fluid handling functionality critical to the development of next‐generation surfaces with intelligent control capabilities. Herein, the fabrication of chemical gradients with mechanically tunable anisotropic microtopography (microwrinkles with directional roughness) is described and the use of these surfaces in programable microdroplet transport is reported. In particular, the interplay between chemical gradient intensity, microwrinkle orientation, and droplet velocity/trajectory was investigated, enabling the rational synthesis of surface fluidic systems capable of mechanically programmable 2D droplet manipulations, vertical droplet transport, and droplet combination. These findings highlight the sophisticated capabilities of mechanically switchable droplet handling systems and demonstrate new avenues for designing intelligent materials with programable transport properties for potential use in surface/microfluidics, water harvesting, energy generation, bioanalysis, and microreactor design.