Session U24: Surface Tension Effects: General

Superhydrophobic surface immobilization by insoluble surfactant

The effect of insoluble surfactants on Stokes flows over transverse-ridge-type superhydrophobic surface with flat menisci is discussed. The focus is on surface immobilization, where surfactant-induced surface tension forces, here modelled using a Langmuir equation of state, can severely retard menisci. At the core of this work is a novel numerical scheme constructed around a complex variable formulation of the problem. This scheme allows exploration of surfactant effects for the full range of surface Péclet numbers, Marangoni numbers, and surfactant loads. Perturbation theory solutions at the small Péclet and Marangoni number limits are used to validate the numerical results. The results reveal a yet unexplored mechanism for surface immobilization where flow retardation can occur at quite low Péclet and Marangoni numbers. This occurs for high values of surfactant load and is purely a consequence of the non-linearity of the equation of state.

    

Hairy fluid mechanics: Dynamic elastocapillarity in deformable beam arrays

Natural surfaces are frequently textured with arrays of slender, filamentous structures that serve vital functions on a wide variety of length scales. Such surfaces often operate at fluid interfaces and are flexible enough to be deformed by surface tension effects. While previous studies have looked at beam bending due to either viscous drag or surface tension independently, the role of a moving contact line through a deformable medium remains relatively unexplored. Interfacial flows past flexible arrays of fibres are highly coupled: when capillary forces of a moving interface are strong enough to deform a surface, that deformation in turn modifies local flow geometry. Here we study this phenomenon by fabricating channels textured with a "hairy" elastoporous media and subsequently displacing an initial layer of fluid with an immiscible invading phase. We first develop a model to describe the deflection of a one-dimensional array of elastic beams by Laplace pressure. Then, we couple the surface tension model with the local fluid dynamics through the porous media. We find that a defending phase can be fully removed from a rigid, undeformed array. However, for flexible surfaces the onset of elastocapillary effects has a profound effect on drainage at quasi-static flow conditions.   

Factors affecting condensate drainage from inclined subcooled rough plates

Surface roughness plays an important role in dictating the mode of condensation on a subcooled surface, as it directly influences the condensate accumulation and drainage. While wettable surfaces mostly facilitate filmwise condensation, non-wettable surfaces render dropwise condensation. For both types of condensation, condensate drainage and detachment from the condenser surface plays a pivotal role in dictating the condensation heat transfer coefficient. We performed experiments on stainless-steel surfaces (hydrophilic) with different degrees of topological roughness. We altered the angle of inclination (<10^°) and other parameters, such as ambient temperature, relative humidity, etc. to investigate their effect on the condensate drainage from the plate. For a smooth surface, there was no formation of film; however, for rough surfaces under particular conditions, filmwise condensation was observed. For both types of condensation, we report the time and location of gravity-induced detachment of condensate droplets from the plate and identify the influence of the above-mentioned parameters in the dripping phenomenon.  Identifying the conditions that encourage dripping from hydrophilic surfaces during condensation has strong relevance in several engineering applications, starting from aerospace engineering to air conditioning and ventilation systems.   

Wetting ridge migration and oil wicking during condensation frosting on oil-impregnated surfaces

Frosting/icing has adverse impacts on various industries including transportation and power generation. Conventional ice removal methods using chemical, thermal, and mechanical techniques are time-consuming, energy-intensive, costly, and environmentally harmful. Recently, nature-inspired micro-nanostructured surfaces infused with lubricant oil, referred to as slippery liquid-infused porous surfaces (SLIPS) or liquid-infused surfaces (LIS), have shown great promise not only in delaying frosting/icing but in reducing ice adhesion strength when compared to traditional lotus-leaf-inspired hierarchical surfaces. However, before we can adopt these semi-liquid semi-solid composite surfaces for anti-icing applications, the problem of oil depletion in the form of wetting ridge migration needs to be resolved. In this work, we experimentally characterize and model the wetting ridge migration, oil wicking, and oil depletion rate when water droplets freeze on textured oil-impregnated surfaces. The insights gained from this work guide the rational design of oil-impregnated surfaces that can reduce and potentially eliminate oil loss; a major bottleneck that inhibited the use of oil-impregnated surfaces in industrial applications.   

Author not Attending

In this study, the effect of viscosity, of a test liquid on static and dynamic surface tension measurements, is scrutinized for three widely used techniques. For every surface tension measurement technique, formation and destruction of the interface are paramount to determine the force per unit quantification. Forming or destructing the interface is a dynamic process, and the time scale to create the equilibrium interface depends on the measurement technique's working principle. In the case of the pendant drop, the appropriate generation of the hanging drop is required to achieve the acceptable shape factor. Without this acceptance, the correct surface tension value cannot be extracted from the images. Similarly, for non-image-based techniques, as in Bubble pressure and Wilhelmy plate, the formation of the desired interface shape is required to obtain correct and precise surface tension quantification. Thus, appropriate interface shape formation is a dynamic process and irrespective of measurement technique it is an imperative step. Most characterizing liquids have their response time governed by the viscous nature of the liquid. Here we attempted to comment on the viscous relaxation time that one has to consider to obtain the correct surface or interfacial tension measurements.   

Entrainment and alignment of fibers by dip coating

Withdrawing an object from a liquid bath leads to coating a thin film on its surface. When particles are dispersed in the liquid, and under the right operating conditions, the particles can be entrained in the coating film and deposited on the surface of the substrate. Whereas recent studies have considered the conditions for entrainment and the resulting coating film for spherical particles, the situation with anisotropic particles remains more elusive. Here, we experimentally investigate the dip coating process when rigid cylindrical fibers are dispersed in the liquid phase. Fibers differ from spherical particles since they have two lengthscales and may align on the substrate. Our experiments provide the condition for fiber entrainment in the coating film and characterize how the substrate geometry plays an important role in this process. We further characterize the alignment of the deposited fibers on different substrates and under different operating conditions. This deposition mechanism can be leveraged for applications where deposition and alignment of fibers are important, such as smart textiles.   

Capillary Coalescence of Elastic Fibers Withdrawn from a Viscous Liquid

The deformation of slender objects by surface tension, an example of elastocapillarity, occurs at a variety of length scales with wide-ranging consequences in both nature and industry. We experimentally investigate the coalescence of arrays of fibers that are withdrawn from a liquid bath. We vary withdrawal speed, liquid viscosity, and fiber geometry and bending stiffness, then observe the effects on the coalescence process, the rise of liquid between the fibers, and volume captured. We rationalize our observations using theoretical models, relating measurements to characteristic length and time scales of the system and relevant dimensionless parameters such as the capillary number.   

Buckling of a monolayer of plate-like particles trapped at a fluid-fluid interface

We investigate experimentally the deformation mechanics of monolayers of rigid plates trapped by capillary forces at an air-liquid or liquid-liquid interface. This study is motivated by the need to understand the compression of 2D nanoparticles (e.g., graphene) at a fluid-fluid interface, the stability of Pickering emulsions and the fundamental differences between spherical particles and plates in these applications as anisotropic particles exhibit different interactions. To investigate when the particle-laden interface buckles, and to study the wavelength of buckling and the force required for compression, we carry out experiments using a one-dimensional chain of flat rigid plates of length L compressed by a moving barrier with an assigned displacement. The main observations are: i) a critical buckling force and a characteristic post-buckling wavelength of about 2 particle lengths (different from spheres); ii) the buckling wavelength is independent of the particle Bond number and the particle length; iii) the existence of gravity-dominated and surface-tension-dominated regimes in which the buckling force (F_b) scales as L³ and L¹, respectively.  To explain these observation, we develop a mathematical model by considering the capillary forces acting on each particle, the particle weight and buoyancy forces. Good agreement between the theory and the experiments is found. Our observations show that an interfacial monolayer of rigid plates does not have a bending rigidity, unlike a monolayer of spheres. Implications for jamming and wrinkling of 2D monolayers of thin rigid disks trapped at a fluid interface are discussed.

    

Drag force on spherical particles trapped at a liquid interface

The dynamics of particles attached to an interface separating two immiscible fluids are encountered in a wide variety of applications. Here we present a combined asymptotic and numerical investigation of the fluid motion past spherical particles attached to a deformable interface undergoing uniform creeping flows in the limit of small Capillary number and small deviation of the contact angle from 90 degrees. Under the assumption of a constant three-phase contact angle, we calculate the interfacial deformation around an isolated particle and a particle pair. Applying the Lorentz reciprocal theorem to the zeroth-order approximation corresponding to spherical particles at a flat interface and the first correction in Capillary number and correction contact angle allows us to obtain explicit analytical expressions for the hydrodynamic drag in terms of the zeroth-order approximations and the correction deformations. The drag coefficients are computed as a function of the three-phase contact angle, the viscosity ratio of the two fluids, the Bond number, and the separation distance between the particles. In addition, the capillary force acting on the particles due to the interfacial deformation is calculated.   

A single parameter can predict surfactant impairment of superhydrophobic drag reduction

Trace surfactants, unavoidable in applications, can impair the drag reduction achieved by superhydrophobic surfaces (SHS), as Marangoni stresses can immobilize the air-water interface. For realistic SHS textures, it is not known how this impairment depends on surfactant type and concentration, flow velocity, and SHS geometry; as a result, mitigation strategies are still needed. We introduce a model for finite-length, streamwise SHS gratings, and perform simulations and experiments. Our model also enables the estimation, based on velocity measurements, of a priori unknown properties of surfactants inherently present in microfluidic systems. We find that the interface can be mobilized if it is longer than a critical length scale, which is determined by the surfactant properties. This mobilization length is more sensitive to the surfactant chemistry that to its concentration, such that even trace-level contaminants may significantly increase drag if they are highly surface-active. SHS impairment is thereby predicted from a single parameter, namely the ratio of interface length and mobilization scale, providing fundamental insight and practical guidance to achieve superhydrophobic drag reduction.   

Spreading Dynamics of Volatile Droplets Deposited on Immiscible Liquid Layers

Spreading dynamics and interfacial instabilities of a volatile liquid released on an immiscible liquid bath creates a highly complex flow for both solutal and thermal Marangoni stresses appear simultaneously. We investigate the development of a droplet of methanol deposited onto an oil bath. We show that the spreading dynamic of the methanol droplet of radius R(t) obeys a R(t)~t^0.25±0.02 scaling. This exponent is found to be independent of the oil bath depth and the initial drop volume. Neglecting the evaporation effects at the early stage of the spreading, a similarity solution based on lubrication approximation is developed, predicting a R(t)~t^1/4 consistent with the experimental observations. It indicates that solutal Marangoni and viscosity are the main ingredient for the drop spreading. Thermal Marangoni, on the other hand, play a prominent role in destabilizing the rim at the periphery of the methanol droplet, and breaking it into hundreds of small droplets. Measuring different parameters of the instability, the underlying physics behind the bursting phenomenon is explained in detail. Using thermal imaging, effect of evaporation on the recession of methanol droplet is also studied.