The wetting behavior of hierarchically wrinkled surfaces has attracted great interest because of its broad application in flexible electronic, microfluidic chip, and biomedicine. However, theoretical studies concerning the relationship between the apparent contact angle and mechanical strain applied on the soft and flexible surface with a hierarchically wrinkled structure are still limited. We established a theoretical framework to describe and understand how prestrain and applied dynamic strain reversibly tune the wettability of the hierarchically wrinkled surface. More specifically, a direct relationship between the mechanical strain and contact angle was built through reversible tuning of the amplitude and the wavelength of the wrinkled structures caused by mechanical strain, which allowed for more precise adjustment of surface wettability. To verify the accuracy of the theoretical relationship between the contact angle and mechanical strain, a soft surface with a hierarchically wrinkled structure was prepared by combining wrinkled microstructures and strip ones. The results showed that the experimental contact angles were in agreement with the theoretical ones within a limited error range. This will be helpful for further investigation on the wettability of hierarchically wrinkled surfaces.

Wearable sweat sensors are essential for providing insight into human physiological health. The currently developed microfluidic sweat sensors have demonstrated the function of collecting and storing sweat. However, they detect more average concentrations of substances based on time periods, which leads to the fact that in situ real-time measurement for multiple biomarkers remains a grand challenge. Here, we propose a wearable epidermal microfluidic patch with integrated microfluidic pumps and micro-valves for accelerated and continuous collection of the sweat, where the micro-pumps ensure the complete separation of old and new sweat for real-time detection of real concentration of biomarkers in sweat. The biomarker concentration at different time periods is detected by introducing a burst valve, which is used to assist in the analysis of the real-time detection. A quantitative relationship between the minimum burst pressure difference required for sequential collection and the size of the microchannel structure is established to overcome the effects of additional resistance at the gas–liquid interface. Additionally, the sensing modules, including sodium ion, chlorine ion, glucose, and pH level in sweat, are integrated into the patch to realize in situ, real-time detection of multiple biomarkers in the human sweat, decoding the correlation between changes in substance concentrations and physiological conditions. This work provides a unique and simplifying strategy for developing wearable sweat sensors for potential applications in health monitoring and disease diagnostics.

Dynamical regulation to unidirectional wetting has received considerable attention in recent years due to its important role in flexible electronics, microfluidics, drug transportation, and so forth. It is of great significance to prepare a surface which allows the droplet to spread directionally on it with the length of droplet reversibly fluctuated within a broad range under external stimulation. In this paper, a double‐gradient wrinkled structure is prepared by constructing the structure‐gradient pillar arrays on a mechanics‐adjusted wrinkled surface and subsequently making chemical gradient by oxygen plasma treatment. Under the synergistic effect of the structural gradient, chemical gradient, and wrinkled structure, the droplet can undergo unidirectional spreading and realize dynamic regulation of the spreading length on the flexible structure. This will provide a propagable method for the regulation of surface wetting.

A droplet that impacts on a superhydrophobic surface will undergo a process of unfolding, contracting, and finally rebounding from the surface. With regards to the pancake bouncing behavior of a droplet, since the retraction process of the droplet is omitted, the contact time is greatly shortened compared to the normal type of bouncing. However, the quantitative prediction to the range of droplet pancake bouncing and the adjustment of pancake bouncing state have yet to be probed into. In this paper, we reported the controllable pancake bouncing of droplets by adjusting the size of the superhydrophobic surface with microstructures. In addition, we also discovered a dimensional effect with regards to pancake bouncing, namely, the pancake bouncing would be more likely to happen on the surfaces with large post spacing for the droplet with the larger radius. The contact time could be reduced to 2 ms by adjusting the size of the microstructures and the radius of the droplets. Based on the relationship between the droplet bouncing state and the surface microstructure size, we are able to propose reasonable dimensions for the surfaces in order to control pancake bouncing.