Wettability

The need to fend off water is as fundamental as the need to acquire it: water absorption by buildings fosters mold growth and structural breakdown, stagnant surface water breeds disease, and waterlogged clothing interferes with body temperature regulation. 

We traditionally rely on chemical coatings to prevent water absorption and retention, but these wear off over time and can be toxic. In contrast, many organisms use built-in topography: water striders keep their legs dry, mosquitoes defog their eyes, and leaves shed raindrops by limiting water contact to the tips of nanoscale bristles on their surfaces. Air fills the rest of the space under the drop, such that the bristles create a patterned air-solid surface on which macroscopic droplets slide and molecules within each droplet diffuse largely as if the drop were in air. 

We are investigating how patterned features govern motion at these unique interfaces, and have recently optimized liquid-surface dynamics to design ice-preventive materials that deflect impacting droplets at sub-freezing temperatures and nucleate only unstable, low-adhesion ice below that. Since topographic patterns disappear if the bristles lie down, water resistance can be turned on and off simply by bending or tilting, and we use this unique feature to design materials that reversibly switch between hydrophobic and hydrophilic behavior in response to environmental conditions. While liquids other than water are more difficult to resist due to their stronger tendency to spread on a surface, we have recently made the surprising discovery that biofilm – a bacterial commune encased in slime – has a unique multiscale topography that fends off not only water but an unprecedented assortment of other liquids, and we are designing previously elusive resilient, highly nonwetting materials based on our intriguing new role model.

Phillips KR, England GT, Sunny S, Shirman E, Shirman T, Vogel N, Aizenberg J. A colloidoscope of colloid-based porous materials and their uses. Chem. Soc. Rev. [Internet]. 2016;45 (2) :281-322. Full TextAbstract
Nature evolved a variety of hierarchical structures that produce sophisticated functions. Inspired by these natural materials, colloidal self-assembly provides a convenient way to produce structures from simple building blocks with a variety of complex functions beyond those found in nature. In particular, colloid-based porous materials (CBPM) can be made from a wide variety of materials. The internal structure of CBPM also has several key attributes, namely porosity on a sub-micrometer length scale, interconnectivity of these pores, and a controllable degree of order. The combination of structure and composition allow CBPM to attain properties important for modern applications such as photonic inks, colorimetric sensors, self-cleaning surfaces, water purification systems, or batteries. This review summarizes recent developments in the field of CBPM, including principles for their design, fabrication, and applications, with a particular focus on structural features and materials' properties that enable these applications. We begin with a short introduction to the wide variety of patterns that can be generated by colloidal self-assembly and templating processes. We then discuss different applications of such structures, focusing on optics, wetting, sensing, catalysis, and electrodes. Different fields of applications require different properties, yet the modularity of the assembly process of CBPM provides a high degree of tunability and tailorability in composition and structure. We examine the significance of properties such as structure, composition, and degree of order on the materials' functions and use, as well as trends in and future directions for the development of CBPM.
Burgess IB, Loncar M, and Aizenberg J. Structural Colour in Colourimetric Sensors and Indicators. J. Mater. Chem. C [Internet]. 2013;1 (38) :6075-6086. Publisher's VersionAbstract
Colourimetric sensors and indicators are widely used because of their low cost and simplicity. A significant challenge associated with the design of this type of device is that the sensing mechanism must be simultaneously optimised for the sensitivity of the response and a visually perceptible colour change. Structural colour, derived from coherent scattering rather than molecular absorption, is a promising route to colourimetric sensor design because colour shifts are tied to changes in one of many physical properties of a material, rather than a specific chemical process. This Feature Article presents an overview of the development of low-cost sensors and indicators that exploit structural colour. Building upon recent advances in structurally adaptive materials design, structural colour sensors have been developed for a wide variety of previously inaccessible physical (e.g. temperature, strain, electric fields) and chemical stimuli (e.g. small organic molecules, charged species, biomacromolecules and metabolites). These devices, often exceeding the state of the art in performance, simplicity or both, have bright prospects for market impact in areas such as environmental monitoring, workplace hazard identification, threat detection, and point-of-care diagnostics. Finding the ideal balance between performance (e.g. sensitivity, specificity, reproducibility, etc.) and simplicity (e.g. colourimetric vs. spectroscopic readout) will be one of the most critical elements in the further development of structural colour sensors. This balance should be driven largely by the market demands and competing technologies.
Hou X, Hu Y, Grinthal A, Khan M, Aizenberg J. Liquid-based gating mechanism with tunable multiphase selectivity and antifouling behaviour. Nature [Internet]. 2015;519 (7541) :70-73. Full TextAbstract
Living organisms make extensive use of micro- and nanometre-sized pores as gatekeepers for controlling the movement of fluids, vapours and solids between complex environments. The ability of such pores to coordinate multiphase transport, in a highly selective and subtly triggered fashion and without clogging, has inspired interest in synthetic gated pores for applications ranging from fluid processing to 3D printing and lab-on-chip systems. But although specific gating and transport behaviours have been realized by precisely tailoring pore surface chemistries and pore geometries, a single system capable of controlling complex, selective multiphase transport has remained a distant prospect, and fouling is nearly inevitable. Here we introduce a gating mechanism that uses a capillary-stabilized liquid as a reversible, reconfigurable gate that fills and seals pores in the closed state, and creates a non-fouling, liquid-lined pore in the open state. Theoretical modelling and experiments demonstrate that for each transport substance, the gating threshold—the pressure needed to open the pores—can be rationally tuned over a wide pressure range. This enables us to realize in one system differential response profiles for a variety of liquids and gases, even letting liquids flow through the pore while preventing gas from escaping. These capabilities allow us to dynamically modulate gas–liquid sorting in a microfluidic flow and to separate a three-phase air–water–oil mixture, with the liquid lining ensuring sustained antifouling behaviour. Because the liquid gating strategy enables efficient long-term operation and can be applied to a variety of pore structures and membrane materials, and to micro- as well as macroscale fluid systems, we expect it to prove useful in a wide range of applications.
Mishchenko L, Aizenberg J, Hatton BD. Spatial Control of Condensation and Freezing on Superhydrophobic Surfaces with Hydrophilic Patches. Adv. Funct. Mater. [Internet]. 2013;23 (36) :4577-4584. Publisher's VersionAbstract
Certain natural organisms use micro‐patterned surface chemistry, or ice‐nucleating species, to control water condensation and ice nucleation for survival under extreme conditions. As an analogy to these biological approaches, it is shown that functionalized, hydrophilic polymers and particles deposited on the tips of superhydrophobic posts induce precise topographical control over water condensation and freezing at the micrometer scale. A bottom‐up deposition process is used to take advantage of the limited contact area of a non‐wetting aqueous solution on a superhydrophobic surface. Hydrophilic polymer deposition on the tips of these geometrical structures allows spatial control over the nucleation, growth, and coalescence of micrometer‐scale water droplets. The hydrophilic tips nucleate water droplets with extremely uniform nucleation and growth rates, uniform sizes, an increased stability against coalescence, and asymmetric droplet morphologies. Control of freezing behavior is also demonstrated via deposition of ice‐nucleating AgI nanoparticles on the tips of these structures. This combination of the hydrophilic polymer and AgI particles on the tips was used to achieve templating of ice nucleation at the micrometer scale. Preliminary results indicate that control over ice crystal size, spatial symmetry, and position might be possible with this method. This type of approach can serve as a platform for systematically analyzing micrometer‐scale condensation and freezing phenomena, and as a model for natural systems.
Phillips KR, Vogel N, Burgess IB, Perry CC, Aizenberg J. Directional Wetting in Anisotropic Inverse Opals. Langmuir [Internet]. 2014;30 (25) :7615-7620. Publisher's VersionAbstract

Porous materials display interesting transport phenomena due to restricted motion of fluids within the nano- to microscale voids. Here, we investigate how liquid wetting in highly ordered inverse opals is affected by anisotropy in pore geometry. We compare samples with different degrees of pore asphericity and find different wetting patterns depending on the pore shape. Highly anisotropic structures are infiltrated more easily than their isotropic counterparts. Further, the wetting of anisotropic inverse opals is directional, with liquids filling from the side more easily. This effect is supported by percolation simulations as well as direct observations of wetting using time-resolved optical microscopy.

Daniel D, Mankin MN, Belisle RA, Wong T-S, Aizenberg J. Lubricant-infused micro/nano-structured surfaces with tunable dynamic omniphobicity at high temperatures. Appl. Phys. Lett. [Internet]. 2013;102 :231603. Full TextAbstract
Omniphobic surfaces that can repel fluids at temperatures higher than 100 °C are rare. Most state-of-the-art liquid-repellent materials are based on the lotus effect, where a thin air layer is maintained throughout micro/nanotextures leading to high mobility of liquids. However, such behavior eventually fails at elevated temperatures when the surface tension of test liquids decreases significantly. Here, we demonstrate a class of lubricant-infused structuredsurfaces that can maintain a robust omniphobic state even for low-surface-tension liquids at temperatures up to at least 200 °C. We also demonstrate how liquid mobility on such surfaces can be tuned by a factor of 1000.
Yao X, Dunn S, Kim P, Duffy M, Alvarenga J, Aizenberg J. Fluorogel Elastomers with Tunable Transparency, Elasticity, Shape- Memory, and Antifouling Properties. Angew. Chem. Int. Ed [Internet]. 2014;53 (17) :4418-4422. Full TextAbstract

Omniphobic fluorogel elastomers were prepared by photocuring perfluorinated acrylates and a perfluoropolyether crosslinker. By tuning either the chemical composition or the temperature that control the crystallinity of the resulting polymer chains, a broad range of optical and mechanical properties of the fluorogel can be achieved. After infusing with fluorinated lubricants, the fluorogels showed excellent resist- ance to wetting by various liquids and anti-biofouling behavior, while maintaining cytocompatiblity.

Kreder MJ, Alvarenga J, Kim P, Aizenberg J. Design of anti-icing surfaces: smooth, textured or slippery?. Nat. Rev. Mater. 2016;1 (1) :15003.Abstract
Passive anti-icing surfaces, or icephobic surfaces, are an area of great interest because of their significant economic, energy and safety implications in the prevention and easy removal of ice in many facets of society. The complex nature of icephobicity, which requires performance in a broad range of icing scenarios, creates many challenges when designing ice-repellent surfaces. Although superhydrophobic surfaces incorporating micro- or nanoscale roughness have been shown to prevent ice accumulation under certain conditions, the same roughness can be detrimental in other environments. Surfaces that present a smooth liquid interface can eliminate some of the drawbacks of textured superhydrophobic surfaces, but additional study is needed to fully realize their potential. As attention begins to shift towards alternative anti-icing strategies, it is important to consider and to understand the nature of ice repellency in all environments to identify the limitations of current solutions and to design new materials with robust icephobicity.