Hong Hu

Fabrication of Well-Defined Mushroom-Shaped Structures for Biomimetic Dry Adhesive by Conventional Photolithography and Molding

Biomimetic dry adhesives have many attractive features, such as reversible and repeatable adhesion against various surfaces. This paper presents a method for the simple fabrication of biomimetic dry adhesives composed of a mushroom-shaped structure, which is based on conventional photolithography and molding. Firstly a masked and a maskless exposure are performed on the top and bottom of a photoresist, respectively, that generates microholes with an undercut after development. This structured photoresist is then used for molding, leading to mushroom-shaped structural features after sacrificing the photoresist. Because of the convenience of photolithography, the proposed method has the potential to fabricate various dry adhesives cost-efficiently.

Biomimetic Mushroom-Shaped Microfibers for Dry Adhesives by Electrically Induced Polymer Deformation

The studies on bioinspired dry adhesion have demonstrated the biomimetic importance of a surface arrayed with mushroom-shaped microfibers among other artificially textured surfaces. The generation of a mushroom-shaped microfiber array with a high aspect ratio and a large tip diameter remains to be investigated. In this paper we report a three-step process for producing mushroom-shaped microfibers with a well-controlled aspect ratio and tip diameter. First, a polymer film coated on an electrically conductive substrate is prestructured into a low-aspect-ratio micropillar array by hot embossing. In the second step, an electrical voltage is applied to an electrode pair composed of the substrate and another conductive planar plate, sandwiching an air clearance. The Maxwell force induced on the air-polymer interface by the electric field electrohydrodynamically pulls the preformed micropillars upward to contact the upper electrode. Finally, the micropillars spread transversely on this electrode due to the electrowetting effect, forming the mushroom tip. In this paper we have demonstrated a polymer surface arrayed with mushroom-shaped microfibers with a large tip diameter (3 times the shaft diameter) and a large aspect ratio (above 10) and provided the testing results for dry adhesion.

Fabrication of bifocal microlens arrays based on controlled electrohydrodynamic reflowing of pre-patterned polymer

An easy method based on electrohydrodynamic (EHD) reflowing of pre-patterned polymer is proposed in this study for the fabrication of bifocal microlens arrays (MLAs). The method comprises two sequential steps, i.e. hot embossing for generating a polymer-based micropillar array and EHD reflowing of the micropillars for the formation of a bifocal MLA with controllable surface shape and optical performance. The EHD reflowing process is achieved by applying a voltage across an electrode pair sandwiching an air gap and the pre-patterned polymer, and the EHD force induced on the air–polymer interface reshapes the pillar array into the MLA. The complex bifocal microlens can be achieved only when the electric intensity is stronger than that required to produce a commonly known Taylor cone, which is formed when the EHD force exactly surpasses the surface tension. Finally, the light through MLA is imaged on a moving charge-coupled device (CCD) camera and leads to an observation of two focal planes.

Discretely Supported Dry Adhesive Film Inspired by Biological Bending Behavior for Enhanced Performance on a Rough Surface

Biologically inspired dry adhesion has recently become a research hot topic because of its practical significance in scientific research and instrumental technology. Yet, most of the current studies merely focus on borrowing the concept from some finer biological contact elements but lose sight of the foundation ones that play an equally important role in the adhesion functionality. Inspired by the bending behavior of the flexible foundation element of a gecko (lamellar skin) in attachment motion, in this study, a new type of dry adhesive structure was proposed, wherein a mushroom-shaped micropillar array behaving as a strongly adhesive layer was engineered on a discretely supported thin film. We experimentally observed and analytically modeled the structural deformation and found that the energy penalty could be largely reduced because of the partial shift from pillar bending to film bending. Such behavior is very analogous in functionality to the lamellar skin in a geckos pads and is helpful in effectively limiting the damage of the contact interface, thus generating enhanced adhesion even on a rough surface.

Generation of Hierarchically Ordered Structures on a Polymer Film by Electrohydrodynamic Structure Formation.

The extensive applications of hierarchical structures in optoelectronics, micro/nanofluidics, energy conservation, etc., have led to the development of a variety of approaches for their fabrication, which can be categorized as bottom-up or top-down strategies. Current bottom-up and top-down strategies bear a complementary relationship to each other due to their processing characteristics, i.e., the advantages of one method correspond to the disadvantages of the other, and vice versa. Here we propose a novel method based on electrohydrodynamic structure formation, aimed at combining the main advantages of the two strategies. The method allows the fabrication of a hierarchically ordered structure with well-defined geometry and high mechanical durability on a polymer film, through a simple and low-cost process also suitable for mass-production. In this approach, upon application of an electric field between a template and a substrate sandwiching an air gap and a polymer film, the polymer is pulled toward the template and further flows into the template cavities, resulting in a hierarchical structure with primary and secondary patterns determined by electrohydrodynamic instability and by the template features, respectively. In this work, the fabrication of a hierarchical structure by electrohydrodynamic structure formation is studied using numerical simulations and experimental tests. The proposed method is then employed for the one-step fabrication of a hierarchical structure exhibiting a gradual transition in the periodicity of the primary structure using a slant template and a flat polymer film, which presents an excellent performance on controllable wettability.

Rectangle-capped and tilted micropillar array for enhanced anisotropic anti-shearing in biomimetic adhesion.

Dry adhesion observed in the feet of various small creatures has attracted considerable attention owing to the unique advantages such as self-cleaning, adaptability to rough surfaces along with repeatable and reversible adhesiveness. Among these advantages, for practical applications, proper detachability is critical for dry adhesives with artificial microstructures. In this study, we present a microstructured array consisting of both asymmetric rectangle-capped tip and tilted shafts, which produce an orthogonal anisotropy of the shearing strength along the long and short dimensions of the tip, with a maximum anti-shearing in the two directions along the longer dimension. Meanwhile, the tilt feature can enhance anisotropic shearing adhesion by increasing shearing strength in the forward shearing direction and decreasing strength in the reverse shearing direction along the short dimension of the tip, leading to a minimum anti-shearing in only one of the two directions along the shorter dimension of the rectangular tip. Such a microstructured adhesive with only one weak shearing direction, leading to well-controlled attachment and detachment of the adhesive, is created in our experiment by conventional double-sided exposure of a photoresist followed by a moulding process.

Role of space charges inside a dielectric polymer in the electrohydrodynamic structure formation on a prepatterned polymer (ESF-PP)

Electrohydrodynamic structure formation on a prepatterned polymer (ESF-PP) can duplicate structures identical to the initial geometry, but with a higher aspect ratio, under the influence of a spatially modulated electric field. In this process, a voltage is applied between a flat template and a flat substrate, sandwiching a prepatterned polymer and an air gap so as to generate an electrohydrodynamic (EHD) force at the air–polymer interface. Subsequently, the prepatterned polymer can be non-uniformly pulled upwards, causing deformation in its micro/nano-structure. Until now, most of the research into ESF-PP has explored various dielectric polymers, which are all considered to be the perfect dielectrics because of their low electrical conductivity. However, the assumption of a perfect dielectric typically creates discrepancies between theoretical analysis and experimental results in terms of the polymer motion and the final morphology. This phenomenon can be attributed to ignoring the action of the small number of free space charges within dielectric polymer motion (although the electrical conductivity of the dielectric polymer may be even lower than that of deionized water), which emphasizes the importance of the influence of space charges inside the dielectric polymer on deformation. This paper explores the role of free space charges by making a comparison between the perfect dielectric polymer and the leaky dielectric polymer on the progressive development, the surface topography and the aspect ratio from experimental tests and numerical simulations, and a discussion of the effect of the different electrical conductivities. Results show that the free charges inside the dielectric polymer can lead to a larger EHD force because of the additional Coulomb force, even at a low conductivity of 10−7 S m−1, thus demonstrating the ability to duplicate a mushroom-like structure with a high aspect ratio, which has wide applications in superhydrophobicity, dry adhesion, nanogenerators, etc.

Mushroom-Shaped Micropillars for Robust Nonwetting Surface by Electrohydrodynamic Structuring Technique

Mushroom-shaped micropillars (MSMPs) have a unique microscopic shape that is suitable to produce a highly robust nonwetting surface. This commercially important property is possible thanks to the specific overhang shape, which has a larger tip diameter than the shaft below. We fabricated such a textured surface using poly(methyl methacrylate) (PMMA) via electrohydrodynamic structuring technique. The method combines hot embossing, electrically induced growth, and electrowetting. Even though PMMA itself is hydrophilic (the intrinsic contact angle is 82°), its modified surface is hydrophobic with an apparent contact angle of 152°. We also studied the robustness of the nonwetting surface by comparing the difference between the static contact angles before and after the samples had been submerged in water. Our results show that only the surface structured with the 43-μm tip-diameter MSMPs (the largest size) can successfully maintain its highly nonwetting property. Because of the large fabrication potential of the MSMPs with a large tip diameter, the described surface structuring technique is a promising candidate for the mass production of artificial and sufficiently robust nonwetting surfaces.

Effects of contact cap dimension on dry adhesion of bioinspired mushroom-shaped surfaces

Dry adhesion observed in small creatures, such as spiders, insects, and geckos, has many great advantages such as repeatability and strong adhesiveness. In order to mimic these unique performances, fibrillar surface with a mushroom shaped end has drawn lots of attentions because of its advantage in efficiently enhancing adhesion compared with other sphere or simple flat ends. Here, in order to study the effects of contact cap dimension on adhesion strength, patterned surfaces of mushroom-shaped micropillars with differing cap diameters are fabricated based on the conventional photolithography and molding. The normal adhesion strength of these dry adhesives with varying cap diameters is measured with home-built equipment. The strength increases with the rise of cap diameter, and interestingly it becomes strongest when the mushroom caps join together.

Mushroom-shaped microfiber array by electrohydrodynamic structuring process for superhydrophobicity

Bioinspired mushroom-shaped microfiber arrayed surface demonstrates superhydrophobicity. Such a textured surface is fabricated by an electrohydrodynamic (EHD)-based structuring process, which is a combination of hot embossing, electrically induced growing and electrowetting. First, an ordinary micropillar array is prestructured on a conductive substrate through the hot embossing process. Second, another planar plate, functioning as the upper electrode, combines the substrate and an air gap into a parallel capacitor, across which an electrical voltage is applied to generate a modulated electric field. The electrically induced Maxwell force could drive the polymer to grow towards upper electrode and spread transversely on it, forming the mushroom-shaped microfibers. Through an analysis on the early-stage kinetics of the pre-structured micropillars, we can find a suitable electrical potential to ensure the electrically-induced force is sufficient to drive the polymer growing upwards. The measured static contact angle of about 152° and contact angle hysteresis of about 11° demonstrate the superhydrophobicity of this mushroom-shaped microfiber arrayed surface.