Physical mechanisms behind the wet adhesion: From amphibian toe-pad to biomimetics

Highlights

• Some amphibians’ toe-pads possess a firm attachment on wet slippery surfaces.

• We review the main physical mechanisms behind the wet attachment of toe pads.

• Evolved epidermis structures favor high adhesions and frictions under wet cases.

• Bulk softness and sandwiched film dominate the property of wet contacts.

• Strategies of switching the capillary forces achieve reversible wet adhesions.

Abstract

Some amphibians, such as tree frogs, torrent frogs, newts, are able to climb or attach to wet slippery smooth surfaces, even in a vertical or overhanging state, by their reliable reversible adhesions developed on the epidermal of toe pads. It is widely believed that such outstanding function originates from the possible factors of the specialized evolutions of surficial micro/nanostructures, the chemical components of secreted mucus, the solid-liquid behavior of epidermal and the bulk softness of toe pads. In this review, we summarize the main physical mechanisms of these factors behaving underlying the wet adhesion of toe pads from the researches on biological models to artificial counterparts. The discussion of the organism attachments, the interfacial physical forces and the switchable strategies for artificial wet adhesion are also included. The paper gives a deeply, comprehensively understanding of the characters of wet adhesives on amphibians, which performs necessarily for the new strategies of exploring artificial adhesive surfaces.

Introduction

The epidermal of organisms in nature which possess the extraordinary adhesions, such as walking on the ceiling or hang upside down on wet substrates, endowed by their near-surface architectures always impress us deeply. One such example is the fibrillar surface of gecko toe pads [1,2], which impose a strong but switchable role in the adhesion to almost any dry surface [3]. Experimental evidence has suggested that the gecko toe pad is composed of hundreds of thousands of keratinous hairs (called setae), which then branches into hundreds of even finer hairs (called spatula) [4,5]. With these hierarchically organized structures, the toe pads can maximize the van der Waals interaction to substrates by the intimately contacting [6,7]. Those adhesive features provide a great idea for novel surface designing [[8], [9], [10], [11]].

Apart from the fibrillar surface of geckos, nature also provide a number of biological organisms with intelligent structures for facilitating strong adhesion associated with high friction under wet conditions. Amphibians such as tree frogs, torrent frogs, and newts whose toe pads usually patterned with a polygonal topography of epidermal cells separated by mucus-filled channels give great talents of attaching to vertical and overhanging wet surfaces without slipping or falling (Fig. 1) [[12], [13], [14], [15]]. As detailly investigated in previous work, these amphibian toe pad surfaces are super-hydrophilic [16] and, thus, a very thin layer of fluid always comes between the toe pads and the adhering substrates [17], which means the adhesive mechanism is quite different from geckos’ case. It is generally assumed that the adhesive forces arise from the capillary forces and rate-dependent viscous forces, and that the of van der Waals forces has a probable contribution because of the several nanometers of film thickness, but to what extent is unclear [[18], [19], [20]]. The physical properties of toe pads, such as the surficial structures [21,22], elasticity modulus [23], hydrophilicity [16,24], etc., have been demonstrated to play a positive role for the living body wet attachments. For example, the array of specialized polygonal designs of cells is suggested to have a “draining effect” that the surplus water will be removed out of the pad contact area, allowing an intimate contact situation [17,25]. It favors producing a high friction force for the amphibian attachments when climbing on the incline slip surface [16,24,26]. In addition, other celebrated approaches to the wet adhesion such as the suction effect or the chemical interactions found on the marine organisms also gain much attentions, and have been widely reported [[27], [28], [29]].

Motivated by the wide range of technological application, such as climbing robot feet [10,30], novel gripper [31,32], pick-place system [33,34], etc., many researchers devote to fabricating the artificial structured surfaces for copying the super adhesive properties of living organisms, and more than one thousand articles have been published in the field over last two decades, especially for the gecko inspired fibrillary surfaces. Some of them are excellent reviews that have well discussed and systematically summarized the dry adhesion system including the underlying mechanism, the fabricating approaches, and the engineering applications, e.g., refs [9,11,35,36]. However, considering the possible case of artificially executing adhesions or bondings happening under a wet condition, the amphibians’ epidermal adhesive technology, similarly, apply many potential uses for us. Obvious examples of them are improved wet weather tires, nonslip footwear, plaster for surgery able to adhere to tissue, and holding devices for neurosurgery or MEMS device. As a result, it seems necessary to give a comprehensive overview of the physical principles underlying the toe-pad wet adhesions from biological models to artificial counterparts, directing and inspiring us to carry out wide engineering applications.

This article aims to review the current state of toe pad adhesive studying, with the certain perspective of fundamental physical principles from a detailed understanding of microstructures, to tunable strategy of capillary forces. We first provide a short overview of the fascinating ability of wet attachments tested on the amphibian toe-pads (section 2). Section 3 covers the various surficial structures for wet adhesion and friction behaviors from biological epidermis structures to their biomimetics, and discussing the physical mechanisms behind them. In section 4, we reported the bulk softness, the secreted mucus, the sandwiched film in the contacts of tree frog toe-pad, providing further insights into the wedged film stability, wetting case, the bulk viscoelastic dissipation for the interfacial adhesive contacts. Section 5 reports some novel approaches to tune capillary forces between two solids for a controllable wet adhesion. A summary and future perspectives conclude the paper in Section 6.