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The dissolution and microfluidic mass transfer of carbon dioxide in water at high-pressure conditions are crucial for a myriad of technological applications, including microreactors, extractions, and carbon capture, utilization, and sequestration (CCUS) processes. In this experimental work, we use a high-pressure microfluidic method to elucidate the mass transfer process of CO2 in water at high pressure. An intriguing multiphase CO2 flow and dispersions are observed when operating at the pressure-temperature ($P$-$T$) condition close to the CO2 gas-liquid phase boundary ($P=6.5$ MPa and $T=23.5\pm 0.5~^{\circ}$C). We propose a series of strategies to unravel this complex multi-phase dynamics by calculating each phase's volume and mass change in a gas-liquid coexistent CO2 dispersion, estimating the possible CO2 concentration change in water, and comparing with the CO2 solubility data. Finally, we quantify the CO2 mass transfer by directly calculating the CO2 dissolution rate in water and estimating the volumetric mass transfer coefficient ($k_{L}a$). The results show that the mass transfer may be influenced by the specific area ($a$), CO2 concentration gradient in the water slug, and the traveling speed of a dispersion., Comment: 12 pages, 7 figures

Membrane technology is an energy-efficient separation process that is expected to dominate the water treatment industry. However, the high vulnerability of membranes to fouling has limited the expansion of sustainable and energy-efficient membrane processes. Surface patterning has been proven to be an effective method to improve the performance of membranes by increasing the permeate flux and lowering the attachment of fouling materials on membrane surfaces. In this study, a new generation of patterned membranes was prepared using a novel hydrogel facilitated phase separation (HFPS) method. Hydrogel contains a high water content which initiates phase separation as a nonsolvent upon contact with the polymer solution. Structuring a hydrogel mold provides the ability to control the location of the skin layer on the patterned side of the membrane. The performance of the patterned and the unpatterned membranes made with HFPS was examined in a cross-flow filtration system; a significant increase in the pure water flux (~100%) was observed for the patterned membrane. Moreover, a fouling experiment with bovine serum albumin (BSA) solution showed that the patterned membranes maintained 76% higher flux after 90 minutes of operation.

We present a new method—fused pellets printing—to print any thermoplastic materials by converting a screw extruder into a direct source for feeding material into fused deposition modeling (FDM) style 3D printer. We achieve this by feeding thermoplastic pellets into a stand-alone single screw system that melts and pushes the material through a flexible heated hose to the print head. The material is finally deposited through the print head onto the print bed to construct the 3D object. This heated hose decouples the large mass extruder from an FDM print head which can then move with high speed and precision. The result is a very simple 3D printing tool that can take raw input pellets or even recycled thermoplastic scrap, and directly print parts without the need to produce an intermediate, high-quality filament. This technique has been successfully used for pellets of both soft and hard thermoplastics. Using pellets of styrene–ethylene–butylene–styrene, airtight pneumatic soft robotic actuators have been printed in a single process. In theory, this technique should be suitable for any thermoplastic material regardless of their flexibility, stretchability, and hardness which is not possible with currently available commercial FDM systems.

The Experimental Albertan satellite #1 (Ex-Alta 1) was a three-unit cube satellite (CubeSat) developed at the University of Alberta (UAlberta). As the first ‘made in Alberta’ spacecraft, the Ex-Alta 1 CubeSat was the Canadian contribution to the international QB50 CubeSat constellation mission ( www.qb50.eu ). Ex-Alta 1’s mission incorporated four science objectives, two technical objectives, and one educational outreach objective. These are reviewed in the context of the Ex-Alta 1 mission design, as well as future CubeSat constellation missions that may follow demonstration missions such as Ex-Alta 1 and the experiences of the QB50 project. In support of its objectives, Ex-Alta 1 flew the multi-needle Langmuir probe (m-NLP) QB50 standardized payload, a UAlberta developed miniaturized digital fluxgate magnetometer (DFGM) mounted on a two-element articulated deployable boom, a radiation dosimeter from Teledyne, and an open source onboard computer (OBC) developed at UAlberta. Ex-Alta 1 was released by a NanoRacks deployer from the International Space Station on May 26, 2017, and commanded and controlled from a ground-station developed and operated at UAlberta. Ex-Alta 1 operated continuously throughout its mission until it burned up in the atmosphere on November 14, 2018. The last two-way contact occurred on that day at 07:49:29 UTC, when the spacecraft reported an onboard temperature of 47 °C in eclipse at an altitude of 152 km and which marked the effective end of the mission. This paper provides an overview of the Ex-Alta 1 mission, its development, the spacecraft and mission design, and reports preliminary results from the payload focusing on the DFGM instrument. We also report briefly on related education and outreach activities, including the development and delivery of curriculum-targeted and CubeSat focused classroom sessions inspired by the Ex-Alta 1 mission to school pupils in Alberta.

Purpose This paper aims to target to print functionally gradient materials (FGM) devices made of immiscible polymers in multi-material fused deposition modelling (FDM) systems. The design is intended to improve adhesion of dissimilar thermoplastics without the need for chemical compatibilization so that filaments from many different sources can be used effectively. Therefore, there is a need to invent an alternative solution for printing multiple immiscible polymers in an FDM system with the desired adhesion. Design/methodology/approach In this study, the authors have developed a bi-extruder for FDM systems which can print two thermoplastics through a single nozzle with a static intermixer to enhance bonding between input materials. The system can also change the composition of extrudates continuously. Findings The uniqueness of this extruder is in its easy access to the internal channel so that a static intermixer can be inserted, enabling deposition of mechanically interlocked extrudates composed of two immiscible polymers. Without this intermixer, the bi-extruder extrudes with simple side-by-side co-extrusion having no mechanical interlocking. The bi-extruder was characterized by printing objects using pairs of materials including polylactic acid, acrylonitrile butadiene styrene and high impact polystyrene. Microscope images of the cross-sections of the extrudates confirm the ability of this bi-extruder to control the composition as desired. It was also found that the mechanically interlocked extrudates composed of two immiscible polymers substantially reduces adhesion failures within and between filaments. Originality/value In this study, the first-ever FDM extruder with a mechanical blending feature next to the nozzle has been designed and used to successfully print FGM objects with improved mechanical properties.

The membrane is the heart of a membrane separation process. Therefore, the development of sustainable and energy-efficient membrane separation processes relies primarily on the physicochemical and thermomechanical characteristics of the membrane material. In this chapter, we first present a brief overview of polymeric membranes and their classifications, as well as the most common types of membrane separation processes. Next, the governing equations for gas and liquid transport phenomena in both porous and dense membranes are provided. The common methods to fabricate porous and dense membranes are then explained. We then present an overview of various types and the fabrication methods of nanocomposite membranes. Finally, we discuss the future prospects of membrane technology and nanocomposite membranes.

Hydrogel-facilitated phase separation (HFPS) has recently been applied to make microstructured porous membranes by modified phase separation processes. In HFPS, a soft lithographically patterned hydrogel mold is used as a water content source that initiates the phase separation process in membrane fabrication. However, after each membrane casting, the hydrogel content changes due to the diffusion of organic solvent into the hydrogel from the original membrane solution. The absorption of solvent into the hydrogel mold limits the continuous use of the mold in repeated membrane casts. In this study, we investigated a simple treatment process for hydrogel mold recovery, consisting of warm and cold treatment steps to provide solvent extraction without changing the hydrogel mold integrity. The best recovery result was 96%, which was obtained by placing the hydrogel in a warm water bath (50 &deg, C) for 10 min followed by immersing in a cold bath (23 &deg, C) for 4 min and finally 4 min drying in air. This recovery was attributed to nearly complete solvent extraction without any deformation of the hydrogel structure. The reusability of hydrogel can assist in the development of a continuous membrane fabrication process using HFPS.

This letter presents a beam-reconfigurable superstrate antenna with a periodic flexible surface composed of an array of liquid-metal-filled microchannels inside a polymer. Two beam-reconfiguration techniques are used here: 1) stretching the surface along one axis where the radiation pattern depends on the elongation percentage; and 2) reshaping the surface to concave/convex where the pattern depends on surface state. Elongation and reshaping can split the broadside beam to two beams up to ±55° and ±58° off-broadside, respectively. This technique can be used as a simple low-cost alternative to electronic tuning that requires integration of an array of switches/varactors into the surface and hinder the fabrication of large-scale tunable surfaces and aperture antennas.

Spatial thermal control of localized fluid volumes is a major challenge in the lab-on-chip (LOC) field. Thin-film heaters are commonly used in LOC devices to carry out many key functions that require high temperature uniformity, including the polymerase chain reaction and melting curve analysis. However, the conventional heater design approach is to iterate from an initial best guess toward an optimal design, but without prior knowledge of the exact power distribution that is required to produce the desired temperature profile. Such an iterative process enormously increases the time and cost of the design process. We report on a novel methodology for rapid-turnaround design of thin-film heaters that generate a pre-determined high-resolution spatially controlled power density field that yields uniform temperature or any other desired temperature profile in a planar structure. High temperature uniformity in reaction chambers of arbitrary shape and size is achieved with a single metal layer heater, while maximizing energy efficiency and minimizing the footprint. We implement the method in a monolithic polymer chip architecture, and clearly demonstrate highly selective and uniform heating within 1 °C in >94% of the chamber volume in four different designs at 95 °C. [2015-0225]

Dry adhesives have attracted much attention because of their repeatable and reversible attachment. Many research groups have made fruitful achievements in fabricating and designing various dry adhesives. However, most of these studies focus on imitating bioinspired geometry to achieve this smart adhesion, neglecting the contact interface control through their peeling motion. Here, we present an alternative design to achieve this switchable adhesion on the basis of controlling contact areas. This unique design includes micropillars array with large overhanging caps and a "step" located at the center line of the cap. When dragging the pillars in the direction of the upper surface of the step, the lower surface is brought into contact, rapidly yielding stronger adhesion (switched-on state). Alternatively, when dragging the pillars in the direction of the lower surface of the step, the contact areas decrease sharply, leading to weak adhesion (switched-off state). Such switchable property under strong adhesion force is exactly what many practical applications need, and the ability to achieve this property by controlling the adhesion area size presented here opens a new way to dry adhesives design.

Oil/water separation is a critical global challenge due to the ever-increasing generation of oily wastewaters from industrial activities and daily life. Membrane separation processes can effectively remove oil from water; however, synthesizing energy-efficient and anti-oil fouling membranes remains a huge enigma among researchers. In this work, we prepared novel polyamide-imide (PAI) microfiltration membranes using our recently developed hydrogel-facilitated phase separation (HFPS) technique and conventional nonsolvent induced phase separation (NIPS) method. The external surface of the hydrogel mold could be patterned as desired. The prepared membranes, including HFPS-patterned, HFPS-unpatterned, and NIPS, showed high porosity, superhydrophilicity, and underwater superoleophobicity. The underwater oil contact angles of n-hexadecane and mineral oils were higher than 150°, while a complete repellency for diesel oil was observed for all membranes. The ultra-high-water flux of patterned HFPS membranes, 440 L m-2 hour-1 (LMH), made them outstanding candidates for separating oil/water emulsions. For all fabricated membranes, gravity-driven filtration experiments of 9 consecutive oil cyclic filtration tests yielded >99.9% oil removal efficiency. Moreover, after 18 filtration experiments, the flux recovery ratio and flux decline were in the range of 90–100% and 3–20%, respectively.

Surface grafting of polymer brushes has become an area of significant research to deliver desired material properties such as smart wetting. In this research the grafting of Poly(N-isopropylacrylamide)(PNIPAAm) by benzophenone-initiated UV polymerization on micro-pillared surfaces was performed. By increasing polymerization time PNIPAAm first coats the top of the mushroom pillars then bridges the gap between pillars rather than penetrating the inter-pillar spaces due to the intrinsic features of Poly(dimethylsiloxane) (PDMS) pillars. This enables controlled grafting via control of polymerization time. Above the lower critical solution temperature (LCST), the grafted PNIPAAm brushes are immiscible with water and the surfaces displayed a strongly hydrophobic state with a large contact angle of 120°. Below the LCST, the grafted PDMS micropillars surface swells and absorbs water significantly faster than flat grafted surfaces, displaying a hydrophilic state with a small contact angle 33°. Interestingly, grafting on micropillars as opposed to flat surfaces enhanced the magnitude of the contact angle change across PNIPAAm’s hydrophobicity-hydrophilicity switch and enhances the speed of graft hydration. The grafted surface was further found to be durable under strong water jetting. Thus, a significant synergetic effect of the grafted PNIPAAm and micropillars delivered a durable temperature-modulated smart wetting property with a transition from the hydrophobic to hydrophilic state.

Micro-structured surfaces can provide useful material properties, such as repellency to water, oil, or alcohols. Multiple polymers were investigated based on micro molding replication fidelity of repellent structures using both advancing and receding contact angle measurements. Five different polymers (Loctite 3525, ST-1060, TC-854, TC-8740, and Teflon AF) were chosen based on a range of durometers, for each of which, a recommended curing process was presented. These polymers were micro-structured via a one-step replica molding to create mushroom shaped fibers with overhanging caps. Teflon AF, a low surface energy polymer, produced innovative superhydrophobic as well as oleophobic Micro Surface Topographies (MSTs). Advancing contact angles (CAs) of these microstructures were 166 ± 4.2, 151 ± 2.9, and 119 ± 2.2 when in contact with a water, ethylene, and olive oil droplet respectively. The highest reproducibility was achieved by using a curing procedure of 100 °C for 2 h. However, even in these conditions, 35% of MSTs were not fully reproduced. ST-1060 was considered a good alternative to Teflon AF since, even after five uses of the same casting mold, the advancing CAs decreased less than 2% when in contact with any liquid tested. Polymeric MSTs resistance to an external force was also examined using a gyratory shaker suggesting that softer materials, such as ST-1060, were required to survive exposure to environmental conditions.

Crashworthiness, energy absorption capacity, and safety are important factors in the design of lightweight vehicles made of fiber-reinforced polymer composite (FRP) components. The relatively recent emergence of the nanotechnology industry has presented a novel means to augment the mechanical properties of various materials. As a result, recent attempts have contemplated the use of nanoparticles to further improve the resiliency of resins, especially when resins are used for mating FRP components. Therefore, a comprehensive understanding of the response of nanoreinforced polymer composites, subjected to various rates of loading, is of paramount importance for developing reliable structures. In this paper, the effects of nanoreinforcement on the mechanical response of a commonly used epoxy resin subjected to four different strain rates, are systematically investigated. The results are then compared to those of the neat resin. To characterize the mechanical properties of the nanocomposite, a combination of the strain rate-dependent mechanical (SRDM) model of Goldberg and his coworkers and Halpin-Tsai’s micromechanical approach is employed. Subsequently, a parametric study is conducted to ascertain the influences of particle type and their weight percentage. Finally, the numerical results are compared to the experimental data obtained from testing of the neat and the nanoreinforced epoxy resin.

A multi-degree of freedom micro-energy harvester has been designed, fabricated, and tested and sub 100 Hz natural frequencies have been achieved. This design's resonant frequencies at its first three mode shapes are within the low ambient vibration frequency range. The structure is fabricated from a silicon substrate with Aluminum Nitride (AlN) energy harvesting elements on thin silicon beams and uses a chip as a proof mass. The nonlinear stiffness due to stretching strain in the structure provides a wider harvestable frequency bandwidth in each mode. The nonlinear load-deflection equation for the second mode shape of the device, which corresponds to vertical oscillation and maximum harvester deflection, has been modeled using finite element simulation. The first three natural frequencies of 71.8, 84.5, and 188.4 Hz were measured experimentally for the presented harvester. A frequency bandwidth of 10 Hz has been obtained for the second mode shape under a base excitation of 0.2 g. A maximum open circuit voltage of 1 V and power output of 136 nW with a load resistance of 2 MΩ have been measured using this harvester. Using a synchronized switch harvesting on inductor (SSHI) electrical interface and Lead Zirconate Titanate (PZT), simulations estimate that the power output could be improved to ∼3.1 μW.

We demonstrate the use of a novel efficient metallic susceptor for generating controllable and rapid localized heating for low-cost substrate bonding using commercial microwave ovens. A microwave oven is modeled and enhanced microwave susceptors are designed. Theoretical analysis and simulations confirm our proposed concept and match with our measured data. It is demonstrated that by the use of proposed susceptors, a polymethyl methacrylate substrate can be heated up to 160 °C in less than 8 s. We have also demonstrated that the heat generation is localized and selective, making this technique promising for microfluidic or substrate bonding applications.

We report in this work a novel reversible bonding technique for elastomeric microfluidic devices by integrating gecko-inspired dry adhesives with microfluidic channels which greatly enhances the bonding strength of reversibly sealed channels. The concept is applicable to nearly any elastomer and can be used to bond against any smooth surface which allows for van der Waals interactions. It does not require any solvents or glues or sources for plasma activation or thermal-compressive loading to aid the bonding process and is achievable at zero extra cost. We also demonstrate a quick fabrication technique involving soft master thermo-compressive molding of these microfluidic devices with thermoplastic elastomers. The resultant devices can be used for both pressure driven and non-pressure driven flows. We report the maximum contained pressure of these devices manufactured from two grades of styrene ethylene butylene styrene (SEBS) by conducting a burst pressure test with various substrates.

Polymers offer many advantageous physical properties as structural materials for MEMS and microfluidics. Additionally, polymers support rapid and inexpensive fabrication methods. Patterning metals on polymer microstructures is, however, much more challenging than doing so on conventional substrates such as silicon or glass. As a result, metal layers within polymeric devices are typically fabricated on non-polymeric substrates, and the polymer layers are then fabricated on top, which greatly constrains the variety of structures that can be built. A wider range of devices could be fabricated if there was a reliable method for producing and patterning stable metal films on a photo-polymer. Lab-on-chip (LOC) systems in particular would benefit from low-cost, corrosion-resistant, thermally stable metal films embedded in polymer structures. To date, however, there have been no demonstrations that combine all of these characteristics. We present here a method for patterning a film of aluminum on the photopolymer KMPR and treating the film to stabilize its resistivity. We demonstrate the production and stability of the films and demonstrate that they are suitable for use as heaters and/or temperature sensors at temperatures up to 165 °C – well beyond the required temperatures for mainstay LOC functions such as the polymerase chain reaction (PCR).

This chapter covers the major reported types of manufacturing processes and resulting materials used to reproduce gecko-inspired fibrillar adhesives and similar systems that reversibly adhere based on van der Waals forces. A selection of the large number of structural and mold materials for producing these adhesives is covered, along with a comparison of relative strengths and weaknesses of each process/design from a manufacturing perspective. The challenges of scalable production and use of these adhesives for long term application are covered and suggest that silicone rubbers are most appropriate for durable high quality adhesives, and thermoplastic elastomers more suited for inexpensive, less durable or disposable products.

Fused Deposition Modeling (FDM) is a popular style of additive manufacturing (AM) where a 3D object is fabricated by a molten material deposited into successive 2D layers. Multi-material 3D printing is particularly challenging due to poor adhesion between dissimilar plastics. The majority of currently available multi-material printers use separate nozzles for each material. Therefore, they are incapable of producing devices made of functionally gradient materials (FGM). FGM is a special class of engineering material exhibiting spatially inhomogeneous content, which tailors the material for specific functional and performance requirements. In this work, we present the design and characterization of a bi-material co-extruder system. This specialized extruder is capable of printing two thermoplastic materials through a single nozzle and is also capable of altering the composition of the deposited material while printing. It is also exceptional in that the internal structure of the hot end can be easily accessed and is a more adaptable design than previously reported co-extruders. The extruder was installed on a Geeetech PI3 Pro C printer. Then, some simple structures were printed using pairs of materials including polylactic acid (PLA), acrylonitrile butadiene styrene (ABS) and high impact polystyrene (HIPS). Without any internal mixing features, the composition of the output (for instance, different colored PLA) was determined from microscopic images of cross-section of the extrudates.Copyright © 2016 by ASME

This paper describes a new fabrication method for flexible electronics made of microfluidic structures filled with liquid metal in a thermoplastic elastomer, styrene ethylene butylene styrene (SEBS). The devices can reversibly be bonded to a larger system with gecko inspired dry adhesives. To demonstrate the feasibility of the method, a mechanically reconfigurable folded dipole has been fabricated and 55% frequency tuning with a 20% to 40% bandwidth for various frequencies has been achieved.

The effectiveness of biomimetic dry adhesives at different ambient pressures was investigated. Biomimetic dry adhesives have great potential for space applications but there have been few studies on how these adhesives perform in low-pressure environments. The best performing geometry for dry adhesives with respect to normal adhesion has previously been determined to be mushroom shaped fibers, but the pressure sensitivity of these designs has been unclear – with some groups reporting pressure dependent adhesion, and others claiming no effect. We have compared the microscale adhesion of mushroom shaped polymer fibers at different ambient pressures and have determined that suction cup effects are negligible for fibers with caps less than 16.4 μm in diameter. Further investigation in this work showed that a simple suction cup model provided estimates for the geometry requirements for a non-negligible suction cup effect and determined that the minimum cap radius for the 10 μm pillar diameter used in this stud...

Dry adhesives containing nonuniform arrays of fibrils were tested for the uniformity of their adhesion strength. These arrays comprised fibrils with nanometer-scale dimensions and lengths tuned from 150 to 1500 nm. The surfaces of the fibrils were rendered hydrophobic through a vapor phase deposition of silane molecules to further tune the adhesion strength of the fibrillar structure. Adhesion force measurements over micrometer-length scales were obtained using a tipless cantilever controlled by a scanning probe microscope. Maps of the adhesion forces depicted diverse variations in adhesion strength with the nonuniform lateral changes in topography. Through an extensive data analysis, differences observed between samples were correlated to changes in processing conditions and surface chemistry modifications. The methods demonstrated in this paper are useful for identifying variations in the adhesion strength of dry adhesives made of nonuniform arrays of fibrils. These advancements are crucial for understa...

Although bioinspired dry adhesives are nearly a decade old, there are to date no available commercial products based on these materials. Although commercialization is a while off, great strides have been made with respect to physical modeling of actual gecko adhesion, synthetic fabrication methods, and introduction of capabilities like self-cleaning, directional behavior, and superhydrophobic behavior into synthetic variations. Despite the large number of fabrication methods available, there are still specific difficulties in manufacturing these materials that limit their use in commercial applications. In this paper, we describe how a simple manufacturing technology can be adapted to create relatively high-strength adhesives at low costs on large areas. Our focus is on determining how larger diameter, easily manufactured fiber shapes can be best designed to adhere to smooth surfaces. This manufacturing method has been used to successfully produce adhesives from a variety of materials, and we demonstrate ...

We present how direction-sensitive biomimetic dry adhesives made from poly(dimethylsiloxane) may be tailored directly with standard lithographic techniques without requirements for angled fibers or caps. Specifically, it is possible to alter both 90° pull-off adhesion strength and directionality solely through the control over precise cap dimensions and offset, and the improved or reduced adhesion strength is determined primarily by the vulnerability of the fiber cap to peeling in different directions. Running experimental trials on two different variations of anisotropic adhesives and two isotropic adhesives reveal that in this method, directionality is due to reducing the adhesion strength in specific orientations rather than improving it in the strong direction, and this has implications for the maximum adhesion strengths that could be reached with direction-sensitive designs. Using a combination of microscale normal adhesion tests, load-drag-pull tests and macroscale peel strength tests, we reveal tha...

Bioinspired synthetic dry adhesives offer unique potential in the field of micro assembly and manipulation. These adhesives, which take their basic operating principle from animals like geckos, use van der Waals adhesion forces and mechanical fiber optimization to produce significant forces. For several years, these materials have been of interest for climbing robots and adhesive surfaces due to potential characteristics like self-cleaning or anti-fouling behavior, anisotropic adhesion strengths and non-transferring materials. These same strengths would make these materials an ideal mechanism for handling delicate parts or manipulating structures in the field of micro-electro-mechanical systems (MEMS) packaging. In this paper, I review the specific fabrication processes developed by our group and examine some of the challenges in their integration with MEMS assembly and packaging processes. Improving normal adhesion strength and directionality of adhesives has been completed and work is beginning to focus on reducing polymer transfer and improving the anti-static compatibility of the structural materials used in the adhesives.

When structures fabricated using surface-micromachining contain non-uniform stresses, those structures will deflect vertically during the release step. In this paper, we show how those deflections can be harnessed, and optionally amplified, to assemble out-of-plane components. This method of assembly introduces no new materials or steps into the fabrication process. Structures can be raised to 90° with respect to the substrate, or, with the addition of mechanical interlocks, to any arbitrary angle. In addition to reporting on the assembly of several surface-micromachined optical device using this method, this paper shows a simple free-space optical system. The optical system operates in the plane of the substrate surface, but light is coupled into the system perpendicularly to the substrate surface.

In this paper we present a study into the cleaning properties of synthetic dry adhesives. We have manufactured the adhesive micro-fibres through a low-cost, high yield fabrication method using Sylgard 184 Polydimethylsiloxane (PDMS) as the structural material. We deliberately contaminated the adhesive samples with different sized particles in the micro and macro scales and tested different cleaning methods for their efficacy with respect to each particle size. We investigated different cleaning methods, which included the use of wax moulding, vibration and pressure sensitive adhesives. For adhesion testing we used a custom system with a linear stage and a force sensor indenting a hemispherical probe into the adhesive surface and measuring the pull-off force. To characterize the cleaning efficacy we visually inspected each sample in a microscope and weighed the samples with a microgram-accuracy analytical balance. Results showed that the moulding method induced adhesion recovery in a greater percentage than the other cleaning methods and even helped with the recovery of collapsed posts in some cases. On the other hand pressure sensitive adhesives seem to have the upper hand with regards to certain particle sizes that can potentially pose problems with the moulding method.

Synthetic dry adhesives inspired by the nano- and micro-scale hairs found on the feet of geckos and some spiders have been developed for almost a decade. Elastomeric single level micro-scale mushroom shaped fibres are currently able to function even better than natural dry adhesives on smooth surfaces under normal loading. However, the adhesion of these single level synthetic dry adhesives on rough surfaces is still not optimal because of the reduced contact surface area. In nature, contact area is maximized by hierarchically structuring different scales of fibres capable of conforming surface roughness. In this paper, we adapt the nature's solution and propose a novel dual-level hierarchical adhesive design using Polydimethylsiloxane (PDMS), which is tested under peel loading at different orientations. A negative macro-scale mold is manufactured by using a laser cutter to define holes in a Poly(methyl methacrylate) (PMMA) plate. After casting PDMS macro-scale fibres by using the obtained PMMA mold, a previously prepared micro-fibre adhesive is bonded to the macro-scale fibre substrate. Once the bonding polymer is cured, the micro-fibre adhesive is cut to form macro scale mushroom caps. Each macro-fibre of the resulting hierarchical adhesive is able to conform to loads applied in different directions. The dual-level structure enhances the peel strength on smooth surfaces compared to a single-level dry adhesive, but also weakens the shear strength of the adhesive for a given area in contact. The adhesive appears to be very performance sensitive to the specific size of the fibre tips, and experiments indicate that designing hierarchical structures is not as simple as placing multiple scales of fibres on top of one another, but can require significant design optimization to enhance the contact mechanics and adhesion strength.