Bidirectional Droplet Manipulation on Magnetically Actuated Superhydrophobic Ratchet Surfaces.

Droplet manipulation has garnered significant attention in various fields due to its wide range of applications. Among many different methods, magnetic actuation has emerged as a promising approach for remote and instantaneous droplet manipulation. In this study, we present the bidirectional droplet manipulation on a magnetically actuated superhydrophobic ratchet surface. The surface consists of silicon strips anchored on elastomer ridges with superhydrophobic black silicon structures on the top side and magnetic layers on the bottom side. The soft magnetic properties of the strips enable their bidirectional tilting to form a ratchet surface and thus bidirectional droplet manipulation upon varying external magnetic field location and strength. Computational multiphysics models were developed to predict the tilting of the strips, demonstrating the concept of bidirectional tilting along with a tilting angle hysteresis theory. Experimental results confirmed the soft magnetic hysteresis and consequential bidirectional tilting of the strips. The superhydrophobic ratchet surface formed by the tilting strips induced the bidirectional self-propulsion of dispensed droplets through the Laplace pressure gradient, and the horizontal acceleration of the droplets was found to be positively correlated with the tilting angle of the strips. Additionally, a finite element analysis was conducted to identify the critical conditions for dispensed droplet penetration through the gaps between the strips, which hinder the droplet's self-propulsion. The models and findings here provide substantial insights into the design and optimization of magnetically actuated superhydrophobic ratchet surfaces to manipulate droplets in the context of digital microfluidic applications.

Dual Adaptation of a Flexible Shape Memory Polymer Adhesive.

A shape memory polymer (SMP) adhesive forms a conformal and hermetic contact with a target surface at the soft, rubbery state and provides a high-strength dry adhesion at the rigid, glassy state. In particular, recent SMP adhesive studies show SMP's ability to adhere to various rough and even underwater yet rigid surfaces. However, achieving and retaining the strong adhesion on flexible target surfaces such as common fabrics has not been reported since a flexible target surface would easily be peeled off from an SMP adhesive, which is too rigid to accommodate the target surface flexing. Here, we introduce the dual adaptation of an SMP adhesive composed of a thin SMP layer and a backing fabric, which involves the shape adaptation to make a strong adhesive contact and the flexure adaptation to tolerate a target surface flexing. To discover the criteria for optimizing both shape and flexure adaptations, we present the theoretical rationale as well as the computational and experimental studies in this work. Based on the findings here, we design a thin SMP adhesive and demonstrate its dry and underwater adhesive performances on common clothes to highlight its potential applications.

A Magnetically Actuated Superhydrophobic Ratchet Surface for Droplet Manipulation.

A water droplet dispensed on a superhydrophobic ratchet surface is formed into an asymmetric shape, which creates a Laplace pressure gradient due to the contact angle difference between two sides. This work presents a magnetically actuated superhydrophobic ratchet surface composed of nanostructured black silicon strips on elastomer ridges. Uniformly magnetized NdFeB layers sputtered under the black silicon strips enable an external magnetic field to tilt the black silicon strips and form a superhydrophobic ratchet surface. Due to the dynamically controllable Laplace pressure gradient, a water droplet on the reported ratchet surface experiences different forces on two sides, which are explored in this work. Here, the detailed fabrication procedure and the related magnetomechanical model are provided. In addition, the resultant asymmetric spreading of a water droplet is studied. Finally, droplet impact characteristics are investigated in three different behaviors of deposition, rebound, and penetration depending on the impact speed. The findings in this work are exploitable for further droplet manipulation studies based on a dynamically controllable superhydrophobic ratchet surface.