Session A25: Surface Tension Effects: Interfacial Phenomena I

Wave-propelled capillary spinners

When a millimetric body is pinned at the interface of a vibrating liquid bath, the relative motion between the object and interface generates outwardly propagating capillary waves. It has recently been demonstrated that objects with a fore-aft mass asymmetry generate an asymmetric wavefield and consequently self-propel in unidirectional motion.  In this work, we demonstrate that chiral objects resting at a vibrating fluid interface can steadily spin, with their angular speed and direction of rotation controlled by the object size and driving parameters.  Scaling laws and a simplified model of the wavefield reveal the underlying physical mechanism of rotation while collapsing experimental data across parameters.  Guided by this discovery, we further demonstrate that bodies with multiple asymmetries can be remotely steered along 2D trajectories via modulation of the driving frequency.  This highly accessible and tunable macroscopic system may further serve as a novel platform for explorations of chiral active and driven matter.   

Synthetic Schlieren Measurements of Laser-Actuated Surface Waves

When an infrared laser is pulsed on a fluid interface, the absorption of light can lead to sudden changes temperature at the interface leading to a localized change in the surface tension of the fluid. The resulting surface tension gradient leads to the emergence of thermocapillary waves, which propagate across the interface. In this talk, we present preliminary experimental characterization of the surface topography of these waves using the synthetic schlieren technique. We compare our results to our earlier measurements of laser-actuated waves generated when a laser is pulsed on a fluid interface subjected to parametric forcing by vertical oscillation of its container.   

All-in-liquid 3D printed structures: Arresting oil-water interfacial layers with emulsions

All-in-liquid devices have broad potential applicability in processes ranging from energy storage to drug delivery and tissue engineering. Conventionally, they are produced by the jamming of nanoparticle-polymer at the oil-water interface, where one liquid is arrested in a desired non-equilibrium shape in the second liquid phase. Such structures lack the multiscale porosity that exits in equivalent solid hierarchies. Remarkably, we report on printing spongy all-in-liquid materials utilizing the direct ink writing technique. Stable liquid columns of nanoparticle dispersions are produced inside micellar solutions due to the rapid formation of a highly viscoelastic emulsion phase at the interface. The diffusion time of the generated emulsified zone into the micellar solution is longer than the convection time scale. Thus, emulsions remain at the interface during the injection period. The printed aqueous phase becomes an emulsion zone, creating a porous texture in the oil phase. Consequently, a 3D structure with flexible walls consisting of layered emulsions is achieved, which is counterintuitive to the current liquid-based printed structures. We show the applications of emulsion-based 3D printed all-in-liquid materials in liquid lab-on-chip devices and natural gas/hydrogen storage.

    

Capillary skimming of hydrophilic ratchet by surface tension

A meniscus formed on a hydrophilic surface can skim a light object or fluid by use of surface tension force of water. This capillary adhesion, however, is insufficient to skim heavier objects or other immiscible fluids. It is found that secure capillary adhesion by symmetric meniscus can occur more easily when the solid surface and water surface form a more obtuse angle. In the present work, we propose a hydrophilic ratchet that is more advantageous for skimming these materials by employing a periodically changing slope of solid surface of the ratchet in contact with the water surface. The optimal size of the ratchet is comparable to the capillary length, surface tension and inertia both play important roles. This ratchet structure is also found to be beneficial in detaching viscous materials stuck on a ratchet surface by inducing capillary flow via ratchet geometry. These advantages are demonstrated to provide an oil skimmer with anti-oil-fouling characteristics, which is especially required for cleaning oil spills of highly viscous oils.   

Fluidic Shaping of Optical Components

Current methods for fabricating lenses or mirrors rely on mechanical processing - such as grinding, machining, and polishing. The complexity of these fabrication processes prohibits rapid prototyping of optical components, and puts a very high price tag on large lenses and on complex optical elements (known as freeform optics).

We present a simple method, based on free-energy minimization of liquid volumes, which allows to quickly shape curable liquids into a wide range of optical components, without the need for any mechanical processing.

The method is performed by injecting a curable liquid into a bounding frame submerged within an immiscible immersion liquid with equal (or nearly equal) density at which surface tension dominates. After the desired shape is obtained, the liquid can be cured to produce a solid object with nanometric surface quality.

We provide a complete theoretical framework for the minimum energy state of the interface of the liquids, resulting from the balance between hydrostatic, gravitational, and surface tension forces, and subjected to geometrical constraints. We describe the wide range of freeform surfaces that can be produced, and demonstrate the rapid fabrication of such elements, as well as measurements of the components’ shape and surface roughness.   

The non-self-adjoint thin film problem: dynamics of fixed-volume pinned films

The deformation of thin liquid films has been investigated for more than a century, owing to their importance in a wide range of natural phenomena and engineering applications. Theoretical studies addressed infinite and periodic domain, or cases that give rise to self-similar solutions. Yet, to the best of our knowledge, the simple case of a fixed-volume liquid film that is pinned on impermeable boundaries — arguably the most relevant for engineering applications such as adaptive optics — has been overlooked.

We here present a theoretical framework and experimental measurements for the dynamics of a thin film within an impermeable domain that is subjected to a normal force distribution at its interface. From a physical perspective, the mass conservation in the systems acts as an additional constraint which drives distinctly different dynamics. Mathematically, this manifests in a non-self-adjoint differential equation that requires more careful treatment in its solution. Under the long wave approximation, we obtain a time-dependent solution of the linearized system. We provide experimental validation, showing very good agreement with the model, by actuating a liquid film using dielectrophoretic forces, and imaging its topography in real time using holographic microscopy.

    

Liquid Transport on Curved Surfaces

The stagnation of liquid on functional surfaces poses unwanted problems like thermal insulation, evaporation, and increased drag. Often, the pathway for achieving liquid transport is via the creation of dropwise surfaces with low adhesion and high droplet mobility. Here, we present a framework for directional liquid transport that utilizes filmwise wetting, instead. We overcome a liquid film's greater resistance to transport by utilizing a variably curved solid interface to effectively mold the liquid film above it for liquid collection. When the film takes on this curvature gradient, liquid motion is initiated by a Laplace pressure gradient that spontaneously forms. Thus, this capillary flow can create an asymmetric distribution of the liquid film, which enables regions for primary liquid collection and primary external transport to be designed exclusively of each other. Importantly, this mechanism, while based on capillary dynamics, is largely resilient to parameters like supersaturation and surface tension because of a greater inherent propensity to form films, therefore overcoming the traditional limits of the dropwise regime. The result of these film dynamics is more efficient liquid collection, either via phase change or particulate flows, in the filmwise regime.   

Confined-vapor driven solutal Marangoni effect controls nucleation mechanisms and crystalline morphologies in an evaporative crystallization

The initiation of crystal nucleation near a contact line in evaporative crystallization is inevitable because the local concentration of suspended particles at the edge is for the most dramatically increased by coffee-ring effect. To control the crystalline pattern, the nucleation location should be handled. In this study, we showed that the nucleation site of the CTAB's crystallization could be controlled from the droplet edge to center by changing the evaporation boundary condition of multi-component droplets. In particular, we installed a confine chamber to capture the evaporated volatile vapors. In this experiment, we also tested different chain lengths of the surfactants (CTAB > TTAB > DTAB) where the gelation induced by micellization became predominant if the chain length decreased. Here, the gelation interrupted the nucleation control. During the talk, we will provide a regime map to distinguish a different crystallization process of cationic surfactants that are determined by three parameters: (1) the ethanol concentration in an ethanol-water mixture, (2) the confinement effect, and (3) the alkane chain length of the surfactants. We believe that the proposed idea has great potential to control the nucleation in the crystallization process and its crystalline morphology.   

Marangoni-based geometrical low-pass-filter

Smooth surfaces are central in a variety of applications, including optics, friction reduction, and anti-fouling surfaces. Current methods for fabrication of smooth surfaces rely almost entirely on mechanical polishing processes, and alternative methods for creating smooth surfaces are highly desirable. Liquid interfaces are a natural candidate, owing to their natural smoothness. We consider a thin liquid film suspended on top of a rigid surface of arbitrary topography and present a novel approach that utilizes the Marangoni effect and capillary forces to replicate the baseline surface. When radiating the liquid with a constant heat flux, Marangoni forces act to shape the interface to match that of the underlying surface, while capillary forces act to smooth high spatial frequencies. In this way, a rough surface created, for example, by 3D printing could be transformed into a smooth surface while preserving the desired shape. We derive and scale the governing evolution equation of the thin film along with the energy equation under constant and uniform heat flux. Analyzing these equations, we identify a non-dimensional parameter that controls the threshold frequency of spatial waves, similar to a low pass filter. This result allows to define conditions for replication and smoothing of the base line substrate using a thin film. We demonstrate and verify this result by multiple experiments.