Molecularly Capped Omniphobic Polydimethylsiloxane Brushes with Ultra-Fast Contact Line Dynamics

Maximizing the wetting resistance of fluorine-free omniphobic membranes for hypersaline wastewater desalination

Membrane distillation (MD) equipped with omniphobic (non-wetting) membranes has found a niche in water reclamation from hypersaline industrial wastewater. Here, we examined the efficacy of non-fluorinated materials as surface coating agents for omniphobic MD membrane fabrication, and identified necessary mechanisms to attain a maximized wetting resistance using fluorine-free materials. We first prepared MD membranes with different surface chemistries using a series of linear alkylsilanes and polydimethylsiloxane (PDMS) as representative fluorine-free, low surface energy materials. Membranes modified with a longer chain alkylsilane exhibited a lower surface energy and demonstrated a greater wetting resistance in direct contact MD experiments using feedwaters of various surface tensions. Despite the nearly identical surface energy measured for the longest alkylsilane and PDMS, PDMS-modified membrane exhibited an extended antiwetting performance as compared to the membrane treated with the longest alkylsilane. To elucidate the source of the distinctive wetting resistance, we examined the nucleation and condensation kinetics on the surfaces with the different surface chemistries via environmental scanning electron microscopy. Our analysis suggests that the membranes treated with long chain alkylsilanes contain surface defects (i.e., hydrophilic regions) whereas the high mobility of the PDMS effectively minimizes the defect exposure, slowing down the condensation and subsequent surface wetting.

Liquid-Like Surfaces with Enhanced De-Wettability and Durability: From Structural Designs to Potential Applications

Liquid-like surfaces (LLSs) with dynamic repellency toward various pollutants (e.g., bacteria, oil, and ice), have shown enormous potential in the fields of biology, environment, and energy. However, most of the reported LLSs cannot meet the demands for practical applications, particularly in terms of de-wettability and durability. To solve these problems, considerable progress has been made in enhancing the de-wettability and durability of LLSs in complex environments. Therefore, this review mainly focuses on the recent progress in LLSs, encompassing designed structures and repellent capabilities, as well as their diverse applications, offering greater insights for the targeted design of desired LLSs. First, a detailed overview of the development of LLSs from the perspective of their molecular structural evolution is provided. Then highlight recent approaches for enhancing the dynamic de-wettability and durability of LLSs by optimizing their structural designs, including linear, looped, crosslinked, and hybrid structures. Later, the diverse applications and unique advantages of recently developed LLSs, including repellency (e.g., liquid anti-adhesion/transportation/condensation, anti-icing/scaling/waxing, and biofouling repellency) are summarized. Finally, Perspectives on potential innovative advancements and the promotion of technology selection to advance this exciting field are offered.

Wetting on silicone surfaces

Silicone is frequently used as a model system to investigate and tune wetting on soft materials. Silicone is biocompatible and shows excellent thermal, chemical, and UV stability. Moreover, the mechanical properties of the surface can be easily varied by several orders of magnitude in a controlled manner. Polydimethylsiloxane (PDMS) is a popular choice for coating applications such as lubrication, self-cleaning, and drag reduction, facilitated by low surface energy. Aiming to understand the underlying interactions and forces, motivated numerous and detailed investigations of the static and dynamic wetting behavior of drops on PDMS-based surfaces. Here, we recognize the three most prevalent PDMS surface variants, namely liquid-infused (SLIPS/LIS), elastomeric, and liquid-like (SOCAL) surfaces. To understand, optimize, and tune the wetting properties of these PDMS surfaces, we review and compare their similarities and differences by discussing (i) the chemical and molecular structure, and (ii) the static and dynamic wetting behavior. We also provide (iii) an overview of methods and techniques to characterize PDMS-based surfaces and their wetting behavior. The static and dynamic wetting ridge is given particular attention, as it dominates energy dissipation, adhesion, and friction of sliding drops and influences the durability of the surfaces. We also discuss special features such as cloaking and wetting-induced phase separation. Key challenges and opportunities of these three surface variants are outlined.

Thickness of Nanoscale Poly(Dimethylsiloxane) Layers Determines the Motion of Sliding Water Drops

Layers of nanometer thick polydimethylsiloxane (PDMS) are applied as hydrophobic coatings because of their environmentally friendly and chemically inert properties. In applications such as heat exchangers or fog harvesting, low water drop friction on surfaces is required. While the onset of motion (static friction) has been studied, the knowledge of dynamic friction needs to be improved. To minimize drop friction, it is essential to understand which processes lead to energy dissipation and cause dynamic friction? Here, the dynamic friction of drops on PDMS brushes of different thicknesses is measured, covering the whole available velocity regime. The brush thickness L turns out to be a predictor for drop friction. 4-5 nm thick PDMS brush shows the lowest dynamic friction. A certain minimal thickness is necessary to form homogeneous surfaces and reduce the attractive van der Waals interaction between water and the substrate. The increase in dynamic friction above L = 5 nm is also attributed to the increasing viscoelastic dissipation of the capillary ridge formed at the contact line. The height of the ridge is related to the brush thickness. Fluorescence correlation spectroscopy and atomic force measurements support this interpretation. Sum-frequency generation further indicates a maximum order at the PDMS-water interface at intermediate thickness.

Dynamics of Droplets Moving on Lubricated Polymer Brushes

Understanding the dynamics of drops on polymer-coated surfaces is crucial for optimizing applications such as self-cleaning materials or microfluidic devices. While the static and dynamic properties of deposited drops have been well characterized, a microscopic understanding of the underlying dynamics is missing. In particular, it is unclear how drop dynamics depends on the amount of uncross-linked chains in the brush, because experimental techniques fail to quantify those. Here we use coarse-grained simulations to study droplets moving on a lubricated polymer brush substrate under the influence of an external body force. The simulation model is based on the many body dissipative particle dynamics (MDPD) method and designed to mimic a system of water droplets on poly(dimethylsiloxane) (PDMS) brushes with chemically identical PDMS lubricant. In agreement with experiments, we find a sublinear power law dependence between the external force F and the droplet velocity v, F ∝ vα with α < 1; however, the exponents differ (α ∼ 0.6-0.7 in simulations versus α ∼ 0.25 in experiments). With increasing velocity, the droplets elongate and the receding contact angle decreases, whereas the advancing contact angle remains roughly constant. Analyzing the flow profiles inside the droplet reveals that the droplets do not slide but roll, with vanishing slip at the substrate surface. Surprisingly, adding lubricant has very little effect on the effective friction force between the droplet and the substrate, even though it has a pronounced effect on the size and structure of the wetting ridge, especially above the cloaking transition.

All-atom molecular dynamics simulations of polymer and polyelectrolyte brushes

Densely grafted polymer and polyelectrolyte (PE) brushes, owing to their significant abilities to functionalize surfaces for a plethora of applications in sensing, diagnostics, current rectification, surface wettability modification, drug delivery, and oil recovery, have attracted significant attention over the past several decades. Unfortunately, most of the attention has primarily focused on understanding the properties of the grafted polymer and the PE chains with little attention devoted to studying the behavior of the brush-supported ions (counterions needed to screen the PE chains) and water molecules. Over the past few years, our group has been at the forefront of addressing this gap: we have employed all-atom molecular dynamics (MD) simulations for studying a wide variety of polymer and PE brush systems with specific attention to unraveling the properties and behavior of the brush-supported water molecules and ions. Our findings have revealed some of the most fascinating properties of such brush-supported ions and water molecules, including the most remarkable control of nanofluidic transport afforded by the specific ion and water responses induced by the PE brushes grafted on the inner walls of the nanochannel. This feature article aims to summarize some of our key contributions associated with such atomistic simulations of polymer and PE brushes and brush-supported water molecules and counterions.

Synthesis of Vinyl-Containing Polydimethylsiloxane in An Active Medium

This research deals with the synthesis of copoly(methylvinyl)(dimethyl)siloxanes by the copolycondensation of dimethyldiethoxy- and methylvinyldimethoxysilane in an active medium, followed by thermal condensation in a vacuum. We achieved a range of copolymers exhibiting finely tuned molecular weights spanning between 1500 and 20,000 with regulated functional methylvinylsiloxane units. Analysis of the microstructure showed that the copolymerization predominantly formed products demonstrating a random distribution of units (R~1). However, an increase in the content of vinyl-containing monomers increases the R parameter, indicating an enhanced tendency towards alternating linkages within the copolymer matrix.

Mechanochemical Activation of Silicone for Large-Scale Fabrication of Anti-Biofouling Liquid-like Surfaces

Large-scale preparation of liquid-like coatings with perfect transparency via solventless and room-temperature processes using low-cost and biocompatible materials is of tremendous interest for a broad range of applications. Here, we present a mechanochemical activation strategy for solventless grafting of poly(dimethylsiloxane) (PDMS) onto glass, silicon wafers, and ceramics. Activation is achieved via ball milling PDMS without using any solvents or additives prior to application. Ball milling results in chain scission and generation of free radicals, allowing room-temperature grafting at durations ≤1 h. The deposition of ball-milled PDMS can be facilitated by brushing or drop-casting, enabling large-scale applications. The resulting surfaces facilitate the sliding of droplets at angles <20° for liquids with surface tension ranging from 22 to 73 mN/m. An important application for public health is generating anti-biofouling coatings on sanitary ware. For example, PDMS-grafted surfaces prepared on a regular-size toilet bowl exhibit a 105-fold decrease in the attachment of bacteria from urine. These findings highlight the significant potential of mechanochemical processes for the practical preparation of liquid-like surfaces.