Application of colloid probe atomic force microscopy to the adhesion of thin films of viscous and viscoelastic silicone fluids

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.

Capillary detachment of a microparticle from a liquid-liquid interface

The attachment and detachment of microparticles at a liquid-liquid interface are common in many material systems, from Pickering emulsions and colloidal assemblies to capillary suspensions. Properties of these systems rely on how the particles interact with the liquid-liquid interface, including the detachment process. In this study, we simultaneously measure the capillary detachment force of a microparticle from a liquid-liquid interface and visualize the shape of the meniscus by combining colloidal probe microscopy and confocal microscopy. The capillary behavior is studied on both untreated (hydrophilic) and fluorinated (hydrophobic) glass microparticles. The measured force data show good agreement with theoretical calculations based on the extracted geometric parameters from confocal images of the capillary bridge. It is also evident that contact line pinning is an important aspect of detachment for both untreated and fluorinated particles.

Hair surface interactions against different chemical functional groups as a function of environment and hair condition

OBJECTIVE

The nature and magnitude of molecular interactions on hair surfaces underpin the design of formulated products, of which the application involves a competitive adsorption process between cationic surfactants, fatty alcohols and surface actives such as silicone. The knowledge of molecular interaction with hair surface will not only provide insight on the surface binding affinity but also offer an effective methodology in characterizing surface deposits.

METHODS

Untreated and chemically treated hair samples were treated with either conditioner chassis alone (gel network) or conditioner chassis plus silicone (chassis/TAS). Hair surface interactions against four different chemical functional groups, namely methyl (-CH3 ), acid (-COOH), amine (-NH2 ) and hydroxyl (-OH), were quantified in both ambient and aqueous environment using Chemical Force Microscopy, a method based on atomic force microscopy (AFM).

RESULTS

Surface adhesion on hair in ambient is dominated by capillary force that is determined by both the wettability of hair fibre (hydrophobic vs. hydrophilic), presence of any deposits and the chemical functionality of the AFM cantilever. Capillary force is diminished and replaced by electrostatic interaction when polar groups are present on both hair and AFM cantilever. A distinctively different force, hydrophobic interaction, plays a major role when virgin hair and hydrophobic functionalized AFM cantilever make contact in water.

CONCLUSION

Results acquired by AFM cantilevers of different functional groups show that hydrophobic interaction is a key driver for deposition on virgin hair, whilst electrostatic interaction is the most important one for bleached hair. Interfacial conformation of chassis components upon deposition is determined by the hair surface properties. Our study highlights the possibility of a range of polar groups, not necessarily negatively charged, on the damaged hair. Unlike conventional surface chemical analysis method, it is possible to quantitatively evaluate the interfacial conformation of deposited surface actives on hair, which identifies the target moieties for conditioning products on different types of hair.

Substrate colonization by an emulsion drop prior to spreading

In classical wetting, the spreading of an emulsion drop on a surface is preceded by the formation of a bridge connecting the drop and the surface across the sandwiched film of the suspending medium. However, this widely accepted mechanism ignores the finite solubility of the drop phase in the medium. We present experimental evidence of a new wetting mechanism, whereby the drop dissolves in the medium, and nucleates on the surface as islands that grow with time. Island growth is predicated upon a reduction in solubility near the contact line due to attractive interactions between the drop and the surface, overcoming Ostwald ripening. Ultimately, wetting is manifested as a coalescence event between the parent drop and one of the islands, which can result in significantly large critical film heights and short hydrodynamic drainage times prior to wetting. This discovery has broad relevance in areas such as froth flotation, liquid-infused surfaces, multiphase flows and microfluidics.

In classical wetting, the spreading of a drop on a surface is preceded by a bridge directly connecting the drop and the surface, yet it ignores the solubility of the drop phase in the medium. Here, the authors show that dissolved drop fluid from the parent drop can nucleate on the surface as islands, one of which coalesces with the parent drop to effect wetting.

Lubrication dynamics of swollen silicones to limit long term fouling and microbial biofilms

Bacterial contamination and biofilm formation on medical devices remain a costly and serious healthcare problem. Silicone (polydimethylsiloxane, PDMS) elastomers are common biomaterials but are susceptible to bacterial surface contamination and biofilm growth. 'Self-lubricated' PDMS elastomers (iPDMS) have the potential to greatly reduce rates of cell attachment, biofilm formation and infection. Cross-linked PDMS elastomers immersed in PDMS oil swell to an equilibrium concentration to form a swollen network, and then form a surface liquid layer through syneresis. Herein we have measured the swelling and syneresis kinetics as a function of time, viscosity (1.5 to 10 cSt), and cross-linking density to optimize the surface lubricant layer formation, and resistance to biofouling. The lubricant layer thickness was measured in situ (optical profilometry and AFM) for flat and micro-textured surfaces, as a function of time and swelling ratio, to be in a range from 0.1 to 1 μm, and continuously increases with time. We show this continuous generation is likely due to a gradual, dynamic re-structuring of the elastomer network. Long term antifouling properties of (10 cSt) iPDMS were tested for Pseudomonas aeruginosa growth in a flow culture bioreactor, and after 30 d showed a 10³ to 10⁴ reduction of bacterial cell density for iPDMS compared to conventional PDMS elastomers. This long term performance and non-specific activity makes them highly suitable for biomedical devices, such as urinary catheters.

Humidity-tolerant rate-dependent capillary viscous adhesion of bee-collected pollen fluids

We report a two-phase adhesive fluid recovered from pollen, which displays remarkable rate tunability and humidity stabilization at microscopic and macroscopic scales. These natural materials provide a previously-unknown model for bioinspired humidity-stable and dynamically-tunable adhesive materials. In particular, two immiscible liquid phases are identified in bioadhesive fluid extracted from dandelion pollen taken from honey bees: a sugary adhesive aqueous phase similar to bee nectar and an oily phase consistent with plant pollenkitt. Here we show that the aqueous phase exhibits a rate-dependent capillary adhesion attributed to hydrodynamic forces above a critical separation rate. However, the performance of this adhesive phase alone is very sensitive to humidity due to water loss or uptake. Interestingly, the oily phase contributes scarcely to the wet adhesion. Rather, it spreads over the aqueous phase and functions as a barrier to water vapor that tempers the effects of humidity changes and stabilizes the capillary adhesion.

Quantifying Liquid Transport and Patterning Using Atomic Force Microscopy

Atomic force microscopy (AFM) provides unique insight into the nanoscale properties of materials. It has been challenging, however, to use AFM to study soft materials such as liquids or gels because of their tendency to flow in response to stress. We propose an AFM-based technique for quantitatively analyzing the transport of soft materials from an AFM probe to a surface. Specifically, we present a method for loading an AFM probe with a single 0.3 to 30 pL droplet of liquid and subsequently measuring the mass of this liquid by observing the change in the vibrational resonance frequency of the cantilever. Using this approach, the mass of this liquid was detected with picogram-scale precision by a commercial AFM system. Additionally, sub-femtoliter droplets of liquid were transferred from the probe to a surface with agreement found between the real-time change in mass of the liquid-loaded probe and the volume of the feature written on the surface. To demonstrate the utility of this approach in studying nanoscale capillary and transport phenomena, we experimentally determine that the quantity of liquid transported from the tip to a surface in a given patterning operation scales as the mass of liquid on the probe to the 1.35 power. In addition to providing new avenues for studying the dynamics of soft materials on the nanoscale, this method can improve nanopatterning of soft materials by providing in situ feedback.

Capillary bridge rupture in dip-pen nanolithography

Here, we explore fluid transfer from a nanoscale tip to a surface and elucidate the role of fluid flows in dip-pen nanolithography (DPN) of liquid inks. We find that while fluid transfer in this context is affected by dwell time and tip retraction speed from the substrate, their specific roles are dictated by the contact angle of the ink on the surface. This is shown by two observations: (1) the power law scaling of transferred fluid with dwell time depends on contact angle, and (2) slower retraction speeds result in more transfer on hydrophilic surfaces, but less transfer on hydrophobic surfaces. These trends, coupled with the observation of a transition from quasi-static to dynamic capillary rupture at a capillary number of 6 × 10(-6), show that the transfer process is a competition between surface energy and viscosity. Based on this, we introduce retraction speed as an important parameter in DPN and show that it is possible to print polymer features as small as 14 nm. Further explorations of this kind may provide a useful platform for studying capillary phenomena at the nanoscale.

A dynamic model of the jump-to phenomenon during AFM analysis

The measurement of the physical properties of surfaces on the nanoscale is a long-standing problem, and the atomic force microscope (AFM) has enabled the investigation of surface energies and mechanical properties over a range of length scales. The ability to measure these properties for softer materials presents a challenge when interpreting data obtained from such measurements, in particular because of the dynamics of the compliant AFM microcantilever. This work attempts to better understand the interaction between an AFM tip and samples of varying elastic modulus, in the presence of attractive van der Waals forces. A theoretical model is presented in which the dynamics of the approach of an atomic force microscope cantilever tip toward a surface, prior to and during the van der Waals-induced jump-to phenomenon, are included. The cantilever mechanics incorporates the motion of the air through which the cantilever moves, the acceleration, inertia, and torque of the cantilever, and the squeezing of the fluid between the cantilever tip and the surface, leading to elastohydrodynamic lubrication and deformation of the substrate. Simulations of the cantilever approach are compared to measurements performed using an atomic force microscope, and the effect of cantilever drive velocity is investigated. Cantilevers presenting (1) spherical colloid probe tips and (2) pyramidal tips are employed, and substrates exhibiting Young's moduli of 3 MPa, 500 MPa, and 75 GPa are measured. The analysis presented could be extended to enhance understanding of dynamic phenomena in micro/nanoelectromechanical systems such as resonators and microrheometers, particularly those which contain soft materials and also where surface interactions are important.