Session F31: Wetting and Adhesion of Soft Materials: Dynamics and Instability II

Multiphase Flow Through Hairy Channels

Surfaces textured with long, flexible fibers are ubiquitous in nature and often serve vital functions in multiphase systems. Cilia in lung epithelia transport mucus along airways, while the fur of semiaquatic mammals entrains air for insulation while swimming. Inspired by their versatility, we fabricate “hairy” elastic surfaces by casting a curing elastomer in laser-cut acrylic molds. Specifically, we apply arrays of deformable posts in model systems for the displacement of immiscible phases in a Hele-Shaw cell patterned with elastic features, wherein the buckling of the obstacles modifies the local geometry and thus has a strong effect on drainage. By displacing the oil with water at Ca<<1, we study the geometry and contact line dynamics of the evolving interface and the impact of confinement on oil removal. Models for depth-averaged fluid flow and the deflection of elastic beams are adapted to our problem to describe the deformation of the host medium due to interfacial and viscous forces. We find the oil phase captured in bundles, in quantities that differ significantly from the undeformed reference case; this has implications towards both enhanced oil recovery and the development of liquid-infused surfaces, which possess properties such as omniphobicity and drag reduction.   

Rate control of blister inflations and the skin patterns

Surface blistering is commonly observed in various biological processes such as cell apoptosis and locomotion. Despite the variety between systems, blister formation and growth are characterized by the delamination of a thin viscoelastic membrane from its substrate, followed by its large deformation (>100 %) as it is inflated by the infiltration of interstitial fluid. The dynamic of this process is controlled by three rates: the inflation rate, the skin’s viscoelasticity and the delamination rate. Through a combination of experiment and theoretical modeling, we find that competition between these time scales may trigger two instabilities, namely the membrane’s thinning instability and the delamination instability. Utilizing the interplay between these two mechanisms, we are able to control the equilibrium shape of a blister by simply mediating its inflation rate. These findings are then used to understand the surface blisters observed on thermo-sensitive hydrogels as they quickly deswell during their volume phase transition. By understanding the mechanism that governs the interplay between blister inflation and solvent transport, one can explain a diversity of skin patterns observed on the gel surface for different temperature and the gel’s crosslink density.
  

Rupture noise of a moving contact line

Many disordered systems exhibit crackling noise when driven by an external force or filed, such as Barkhausen noise in magnetization of ferromagnetic materials, acoustic emission in plastic deformation and seismic activity in earthquakes. Here, we report the avalanche statistics of a moving contact line (CL) pulled by an AFM-based hanging glass fiber through a liquid-air interface. The measured capillary force acting on the circular CL between the liquid-air interface and fiber surface exhibits zig-zag fluctuations, resulting from stick-slip motion of the CL. In the stick state, the measured capillary force increases linearly with CL displacement. Once it reaches a critical value, the CL slips quickly in the form of avalanches, accompanied by a loss of capillary force δf. We found that the measured δf follows a power-law distribution and the power-law exponent agrees with the predicted value by the ABBM model. The experimental results reveal novel features of the stick-slip dynamics and can help to understand other stick-slip phenomena.   

Soft Wetting and Phase Separation on Swollen Polymer Networks

When a water drop is deposited on a soft adhesive substrate, the surface tension of water drives deformation of the substrate to increase contact area; this is commonly known as soft wetting. In recent soft wetting theories, elastic forces and surface stress oppose substrate deformation. However, very soft, crosslinked materials often contain a liquid phase (i.e. solvent) and is can be considered a swollen network. For a covalently crosslinked network, solvent may separate from network near the contact line to minimize the elastic energy. Although phase separation near contact lines have been considered, the phase distribution at the contact zone is not well understood, and the condition required for phase separation is unclear. We implement confocal microscopy to visualize the crosslinked polymer and solvent phase separately during soft wetting. By controlling the degree of crosslinking and swelling ratio, we investigate both the solvent leeching and the deformation of the network. We find that phase separation does not clearly occur when the swelling ratio is small. We expect that examining the microscale contact zone will help in developing a theory for wetting of soft gels.   

Spreading dynamics of water onto soluble polymer coatings with hydrophobic insoluble patterns

The wetting of many food powders is affected by hydrophobic ingredients present at the surface, such as fat. Instant beverages, soups and infant formulas are typical examples.
The spreading of sessile droplets on homogeneous soluble polymer films is dominated by the hydration resulting from water evaporation and condensation ahead of the moving contact line^1,2,3, resulting in a strong dependence of the contact angle (θ) on the contact line speed (U), film thickness and local water content.
This study introduced surface heterogeneity by inkjet printing hydrophobic fat patterns at controlled area fractions 0-50% onto thin soluble polymer coatings of maltodextrin to assess the impact on the water spreading dynamics, at controlled ambient RH. Increasing the area coverage increased strongly θ, for every U. A surface coverage of 26.3% was found to halve U after 10 s of spreading. Fat patterns were shown to alter both the interface shape and the contact line length. A theory is proposed to describe the impact of these hydrophobic insoluble patterns on the water spreading dynamics.
REFERENCES:
1. Tay A. et al. Eur. Phys. J, 2010, 33, 203–210.
2. Dupas J. et al. Lagmuir, 2013, 29, 12572–12578
3. Dupas J. et al. Phys.Rev.Lett, 2014, 112, 188302   

Tape Loop Adhesion

A common method of adhering two parallel surfaces to each other is to place a tape loop between them. Despite the tape loop’s prevalence there has, to our knowledge, been no full description of the mechanics of this system to date. We attack the problem experimentally using a reduced half-loop geometry, which is moved through a compression-retraction (sticking then peeling) cycle from which force-displacement curves are measured. We also imaged the shape of the loop during the experiment with photography and laser scanning confocal microscopy. By using both polydimethylsiloxane (PDMS) and polycarbonate (PC) as ‘tape’ materials, we explored the mechanics of both ‘tacky’ and dry adhesive systems. Notably, we find that the compression curve is completely insensitive to surface interactions. Adapting the ‘sticky-elastica’ model of Majidi and Vella, we show how the entire cycle can be described using only an elastic modulus and critical energy release rate as inputs. Remarkably, a complete model of the cycle allows us to create an incredibly simple measurement of adhesion, involving only a ruler.   

Elastocapillarity and Rolling Dynamics of Solid Nanoparticles

We use molecular dynamics simulations of rolling dynamics of solid nanoparticles with size R_p in contact with soft elastic substrates to elucidate effect of capillary, elastic and friction forces on rolling motion. Our simulations have shown that a nanoparticle can be in stationary, steady rolling, and accelerating states depending on the nanoparticle-substrate work of adhesion, W, the magnitude of the net applied force, F, and the substrate shear modulus G. In the stationary state, the restoring torque produced in the contact area balances the torque due to the external force. The crossover rolling force F_r is proportional to WR_p. In the steady rolling state, F>F_r, the nanoparticle maintains a constant rolling velocity which is a manifestation of the balance between the rolling friction force and the applied force. The observed scaling relationships between the applied force and nanoparticle velocity reflect a viscoelastic nature of the substrate deformation dynamics. A nanoparticle begins to accelerate when the energy supplied to the nanoparticle exceeds the energy dissipated in the contact area due to viscoelastic substrate deformation. Using these simulation results, we have constructed a diagram of states in terms of dimensionless parameters F/WR_p and W/GR_p.   

Understanding Elastomeric Contact Interfaces in the Presence of Water

The traction of tires in the rain or sticking a bandage on wet skin involves understanding the role of interfacial water in adhesion and friction. In most cases, the contact interfaces are not completely dry or wet and instead show patchy contact. Even for smooth surfaces, the contact can be patchy due to difficulty in draining the water upon contact. In this work, we have studied the contact interface between hydrophobic PDMS and hydrophilic sapphire substrate in dry and wet conditions using infrared-visible sum-frequency generation (SFG) spectroscopy and macroscopic Johnson-Kendall-Roberts (JKR) adhesion measurements. SFG spectroscopy provides information on interfacial water and shows direct spectral peaks associated with the surface -OH groups in contact with either water or PDMS elastomers. This molecular information can be directly correlated with the macroscopic work of adhesion, adhesion hysteresis, and friction. We will discuss how the presence of interfacial water is affected by normal load and velocity.   

Universal Scaling Behavior of the Tackiness of Polymer Melts

This presentation describes the measurement of stickiness or tackiness, as measured by the peak force to pull off a cylindrical probe from neat polyisoprene melts, the latter of varying materials and surface roughness, over a range of separation speeds. The polymers were either linear or star-branched, with the molecular weights of the former ranging from 84 to 476 kg/mol and of the latter from 609 to 1130 kg/mol. Dynamic mechanic measurements of the polymers showed normal and classic behavior to this well-characterized system. We find that when the pulling speed (V_s) is greater than a critical speed (V_c), the maximum tack force (F_max) can be generally described by the following scaling relationships: F_max ~ V_s^1/3 and F_max ~ t_max^-1/2, where t_max the time when the maximum force is reached in the force-time profile. Remarkably, this scaling behavior of the tackiness appears to be universal as it is independent of the adhered surface, the molecular weight distribution, and the linear or branched chain architecture of the polymer melt. We connect these relations to the physics whereby the tack is controlled by the crack propagation along the interface.   

Spreading dynamics of water droplets on hydrophilic surfaces

In the early stage of droplet spreading, the inertia of the drop resists the capillary driven motion, and for liquids with low viscosity the spreading radius has been observed to grow with time as r(t)~t^1/2, independent of surface wettability. In the final stages, the effect of viscous forces acting in the neighbourhood of the three phase contact line become relevant and the competition between surface tension and viscous forces results in extremely slow spreading dynamics and follows what is called Tanner's law, r(t) ~ t^1/10. In this work, we will present results on the spreading behaviour of water droplets of varying sizes on a completely wetting surface investigated using fully atomistic molecular dynamic simulations. The spreading observed is characterized by a monolayer of molecular dimensions that moves ahead of the main bulk part of the droplet. Interestingly, the bulk part initially spreads over the monolayer with increasing radius until a characteristic time t^*; thereafter, the bulk radius shrinks maintaining a constant contact angle until it disappears altogether. We will show that a first principle model based on hydrodynamic theory describes the spreading data rather well in the regime where the low contact angle approximation holds.   

Multi-scale model of gas transport through soap-film membranes used for Artificial Photosynthesis

This activity is part of the recently funded SoFiA project aiming at a new concept of soap-film photosynthetic membrane for carbon dioxide conversion into fuel [1].
Here, we specifically focus on gas transport through those membranes realized in the form of functionalized soap-films. Diffusion phenomena in the water core and surfactant monolayers are described by Fick’s law [2, 3]. Molecular dynamics is used to compute the surfactant distribution and orientation in the membrane monolayers assuming hexaethylene glycol monododecyl ether (C12E6). The obtained atomistic details are then incorporated in a continuum model describing transport phenomena at a larger scale. Numerical predictions are validated against experimental data from literature and used to estimate the characteristic time for fuel-oxygen mixing when separated by photosynthetic soap film membranes.

[1] http://sofiaproject.eu/
[2] Farajzadeh, R., R. Krastev, and Pacelli LJ Zitha. "Foam film permeability: Theory and experiment." Advances in colloid and interface science 137.1 (2008): 27-44.
[3] Princen, H. M., and S. G. Mason. "The permeability of soap films to gases." Journal of Colloid Science 20.4 (1965): 353-375.   

The role of crystallization, dewetting, and contact angle in the formation of high aspect ratio crystals

High aspect ratio crystals have many applications including improved delivery of important ingredients in consumer products. We present a process to produce crystals with a controllable aspect ratio using just oil and water in an easily scalable process. When an oil in water emulsion crystallizes, generally crystallization occurs so quickly the droplets do not deform. By adding surfactants the remaining liquid fraction can be forced to eject the crystals as they grow resulting in an elongated crystal. The contact angle the oil droplet makes with the crystal does not remain constant during this process. The rate of dewetting can be tuned by varying the surfactant concentration. The rate of crystallization can be controlled by vary how quickly the system cools. We examine the crystallization and dewetting behaviors by using a non-equilibrium Monte Carlo model. We examine the behavior of the contact angle using a semi-analytical geometric model. Our results give insight into how the contact angle changes and the dependence of the final crystal morphology on the dewetting and crystallization rates.