Synthetic fibrillar adhesives inspired by nature, most commonly by the gecko lizard, have been shown to strongly and repeatedly attach to smooth surfaces. These adhesives, mostly of monolithic construction, perform on par with their natural analogues on smooth surfaces but exhibit far inferior adhesive performance on rough surfaces. In this paper, we report on the adhesive performance of functionally graded microfibrillar adhesives based on a microfibre with a divergent end and a thin soft distal layer on rough surfaces. Monolithic and functionally graded fibre arrays were fabricated from polyurethanes and their adhesive performance on surfaces of varying roughness were quantified from force–distance data obtained using a custom adhesion measurement system. Average pull-off stress declined significantly with increasing roughness for the monolithic fibre array, dropping from 77 kPa on the smoothest (54 nm RMS roughness) to 19 kPa on the roughest (408 nm RMS roughness) testing surface. In comparison, pull-off stresses of 81 kPa and 63 kPa were obtained on the same respective smooth and rough surfaces with a functionally graded fibre array, which represents a more than threefold increase in adhesion to the roughest adhering surface. These results show that functionally graded fibrillar adhesives perform similar on all the testing surfaces unlike monolithic arrays and show potential as repeatable and reusable rough surface adhesives.

Flow separation and vortex shedding are some of the most common phenomena experienced by bluff bodies under relative motion with the surrounding medium. They often result in a recirculation bubble in regions with adverse pressure gradient, which typically reduces efficiency in vehicles and increases loading on structures. Here, the ability of an engineered coating to manipulate the large-scale recirculation region was tested in a separated flow at moderate momentum thickness Reynolds number, Re θ = 1,200. We show that the coating, composed of uniformly distributed cylindrical pillars with diverging tips, successfully reduces the size of, and shifts downstream, the separation bubble. Despite the so-called roughness parameter, k + ≈ 1, falling within the hydrodynamic smooth regime, the coating is able to modulate the large-scale recirculating motion. Remarkably, this modulation does not induce noticeable changes in the near-wall turbulence levels. Supported with experimental data and theoretical arguments based on the averaged equations of motion, we suggest that the inherent mechanism responsible for the bubble modulation is essentially unsteady suction and blowing controlled by the increasing cross-section of the tips. The coating can be easily fabricated and installed and works under dry and wet conditions, increasing its potential impact on a diverse range of applications.flow control | bio-inspired surface | engineered surface | flow separation | adverse pressure gradient D uring the past few decades, considerable effort has been placed on controlling flow separation (1-4). This phenomenon is usually responsible for increased vibration and drag on bluff bodies as well as higher energy consumption in vehicles. The drag experienced by a body under subsonic motion mostly embodies viscous and pressure (form) effects. The former is a result of friction induced by the near-wall fluid, and the latter is a result of pressure imbalance around the surface of the body. The separation phenomenon is well exemplified in the canonic case of flow around a foil at a sufficiently high angle of attack. There, the adverse pressure gradient (APG) in the suction side leads to flow deceleration and eventually flow detachment. The direct consequence of this process is a change of the aerodynamic force components, namely lift and drag.Surface roughness plays a significant role in the turbulence dynamics near the wall and, in particular, in the separation regions (5-8). Evidence suggests that randomly distributed roughness, e.g., sand grain roughness, may move the separation point against the flow direction in the case of foils (9, 10); this shift results in drag increase and lift decrease. Experiments by Song and Eaton (11) showed an upstream shift of the separation point in a channel expansion with rough walls. However, various studies have shown that triggering transition to turbulence may reduce separation (12). These findings have motivated the use of flow control strategies such as vortex generators (13) and synthetic jets (3) t...

Many organisms rely on densely packed, tilted and curved fibers of various dimensions to attach to surfaces. While the high elastic modulus of these fibers enables an extremely large number of fibers per unit area, where each fiber stands freely without sticking to its neighbors, the tilt/curvature provides them with the compliance and the directional adhesion properties to attach strongly and efficiently to a surface. Recent studies have revealed that many of such organisms also feature materials with a graded elastic modulus that is tailored towards improving the contact area without sacrificing the fiber density. In particular, for male ladybird beetles, research has shown that the adhesive setae feature a material gradient such that the elastic modulus of the material at the junction between the stalk and the divergent distal end is close to minimum. This soft material acts like a flexible joint, improving the bending compliance of the tip. Here, we mimic this feature using tilted, mushroom-like, stiff fibers comprised of a stiff stalk of elastic modulus 126 MPa, a softer tip of elastic modulus 8.89 MPa, and a joint-like element of elastic modulus 0.45 MPa (very soft), 8.89 MPa (soft), or 126 MPa (stiff) in between. The results from load–drag–pull (LDP) experiments performed along (gripping) and against (releasing) the tilt direction indicate that the soft and the very soft joint fibers performed superior to the stiff joint fibers and maintained directionally dependent performance. The soft joint fibers achieved up to 22 kPa in shear and 110 kPa in pull-off stress in the gripping direction, which are twice and ten times higher than that in the releasing direction, respectively. A model to optimize the elastic modulus of the joint-like elements to enable sliding without peeling of the tips has been proposed.

a b s t r a c tWe demonstrate patterning of metallic glasses using flexible and reusable polymer templates. The elastic deformation of polymer templates is utilized to pattern features of varying dimensions and oblique angles on planar and non-planar surfaces. This is enabled by low thermoplastic processing temperatures of certain metallic glasses and the stability of thermosetting polymers used as the mold making material. The polymer templates are fabricated by standard replica molding of silicon master templates. This provides a scalable method for patterning of metallic glasses which otherwise requires expensive disposable silicon templates.Patterning of materials is an integral component of fabrication process in microelectronics [1], optical devices [2], and surface engineering [3]. There is an overwhelmingly growing demand for patterning smaller features on novel materials along with reduction in cost of scalability. As a result, tremendous progress has been made in advancement of patterning techniques such as lithography [4], nano-imprinting [5,6], embossing [3,7], self-assembly [8,9], LIGA [10,11], and laser processing [12]. Despite these efforts, patterning of metallic materials is far less advanced compared to polymers and semiconductors. This deficit is related to inherent challenges in metal processing such as high surface energy, oxidation, rapid grain growth, and incompatibility with mechanical embossing. Recent work on thermoplastic molding of metallic glasses (MGs) appears to bridge this processing capability gap between metals and polymers [13][14][15][16][17]. Pattern features in the range of sub-100 nm have been demonstrated by thermoplastic embossing of MGs [16,[18][19][20]. After embossing, the MGs can be crystallized by thermal annealing if patterns in crystalline metals are desired [21]. Though thermoplastic shaping of MGs exhibits a great potential for scientific studies [19,[22][23][24][25], the template cost poses a major hurdle for large-scale implementation. Typically, silicon templates prepared by lithography are used because of their precision and rigidity [15,17]. MGs cannot be released from silicon templates without etching because of their thermal expansion mismatch and scalloping roughness of silicon templates. This limits the template usage to a single molding operation even for low-aspect-ratio features without undercuts. Additionally, the employment of rigid templates limits the patterning application to planar surfaces. To overcome these issues, we explore the application of polymer templates for patterning of MGs with low glass transition temperatures. The polymer templates offer several advantages such as: inexpensive fabrication by replica molding of the master template, ease in demolding, reusability, and conformation to non-planar shapes [26][27][28]. Furthermore, elastic deformation (stretching, compression or bending) of polymer templates can be used to vary the size and orientation of features without the need of a new template [29]. The key requirement for using pol...

Understanding how fluid flow interacts with micro-textured surfaces is crucial for a broad range of key biological processes and engineering applications including particle dispersion, pathogenic infections, and drag manipulation by surface topology. We use high-speed digital holographic microscopy (DHM) in combination with a correlation based de-noising algorithm to overcome the optical interference generated by surface roughness and to capture a large number of 3D particle trajectories in a microfluidic channel with one surface patterned with micropillars. It allows us to obtain a 3D ensembled velocity field with an uncertainty of 0.06% and 2D wall shear stress distribution at the resolution of ~65 μPa. Contrary to laminar flow in most microfluidics, we find that the flow is three-dimensional and complex for the textured microchannel. While the micropillars affect the velocity flow field locally, their presence is felt globally in terms of wall shear stresses at the channel walls. These findings imply that micro-scale mixing and wall stress sensing/manipulation can be achieved through hydro-dynamically smooth but topologically rough micropillars.