Active coating of a water drop by an oil film using a MHz-frequency surface acoustic wave

We employ a millimeter-scale piezoelectric acoustic actuator, which generates MHz-frequency surface acoustic waves (SAWs) in a solid substrate, to actively coat a drop of water by a macroscopic film of silicon oil as a paradigm for a small scale and low power coating system. We build upon previous studies on SAW induced dynamic wetting of a solid substrate, also known as the acoustowetting phenomena, to actively drive a model low surface-energy liquid -- silicon oil -- coat a model liquid object -- a sessile drop of water. The oil film spreads along the path of the propagating SAW and comes in contact with the drop, which is placed in its path. The intensity of the SAW determines the rate and the extent to which a macroscopically thick film of oil will climb over the drop to partially or fully cover its surface. The dynamic wetting of the drop by the oil film is governed by a balance between acoustic, capillary, and gravitational contributions. Introducing a water drop as an object to be coated indicates the opportunity to coat liquid phase objects by employing SAWs and demonstrates that oil films which are actuated by SAWs may traverse curved objects and liquid surfaces

Connecting monotonic and oscillatory motions of the meniscus of a volatile polymer solution to the transport of polymer coils and deposit morphology

We study the connection between the polymer deposition patterns to appear following the evaporation of a solution of poly-methyl-methacrylate (PMMA) in toluene, the transport of the polymer coils in the solution, and the motion of the meniscus of the solution. Different deposition patterns are observed when varying the molecular mass and initial concentration of the solute and temperature and are systematically presented in the form of morphological phase diagrams. The modi of deposition and meniscus motion are correlated. They vary with the ratio between the evaporation-driven convective flux and diffusive flux of the polymer coils in the solution. In the case of a diffusion-dominated solute transport, the solution monotonically dewets the solid substrate by evaporation, supporting continuous contact line motion and continuous polymer deposition. However, a convection-dominated transport results in an oscillatory ratcheting dewetting-wetting motion of the contact line with more pronounced dewetting phases. The deposition process is then periodic and produces a stripe pattern. The oscillatory motion of the meniscus differs from the well documented stick-slip motion of the meniscus, observed as well, and is attributed to the opposing influences of evaporation and Marangoni stresses, which alternately dominate the deposition process

Acoustic Resonance Effects and Cavitation in SAW Aerosol Generation

The interaction of surface acoustic waves (SAWs) with liquids enables the production of aerosols with adjustable droplet sizes in the micrometer range expelled from a very compact source. Understanding the nonlinear acousto-hydrodynamics of SAWs with a regulated micro-scale liquid film is essential for acousto-microfluidics platforms, particularly aerosol generators. In this study, we demonstrate the presence of micro-cavitation in an MHz-frequency SAW aerosol generation platform, which is touted as a leap in aerosol technology with versatile application fields including biomolecule inhalation therapy, micro-chromatography and spectroscopy, olfactory displays, and material deposition. Using analysis methods with high temporal and spatial resolution, we demonstrate that SAWs stabilize spatially arranged liquid micro-domes atop the generator's surface. Our experiments show that these liquid domes become acoustic resonators with highly fluctuating pressure amplitudes that can even nucleate cavitation bubbles, as supported by analytical modeling. The observed fragmentation of liquid domes indicates the participation of three droplet generation mechanisms, including cavitation and capillary-wave instabilities. During aerosol generation, the cavitation bubbles contribute to the ejection of droplets from the liquid domes and also explain observed microstructural damage patterns on the chip surface eventually caused by cavitation-based erosion

Pattern deposition of colloidal particles

Colloidal forces are known to influence the pattern deposition of nanoparticles off a volatile carrier liquid. Several experimental studies suggest a direct connection between colloidal forces and the morphology of the particulate deposit [1,2,3]. Specifically, variations in the zeta potential of the suspended particles and substrate; and variations in the ionic strength in the suspension were found to alter the geometry of the deposit. We use theory and experiment to investigate the connection between colloidal forces and the pattern deposition of colloidal particles off a volatile carrier liquid. We connect between colloidal forces and pattern deposition by considering the adhesion of particles to the solid substrate and the coagulation of particles in the suspension [4,5]. Our initial theoretical approach is based on an asymptotic â long wave type â model for the deposition process. In this model we employ the interactionâ force boundary layer theorem to account for the rate of adhesion of particles to the solid substrate. The theorem allows for manifesting the dynamic process of adhesion in a form that is reminiscent of a first order chemical reaction in a dynamic advection-diffusion equation for the transport of particle mass in the liquid. Similarly, we employ the augmented Smoluchowski theorem to account for particle aggregation. Hence, the rate of aggregation is manifested in a form that is reminiscent of multiple second order chemical reactions. The coefficients of the reaction terms are associated with the energy barriers for adhesion or coagulation. Comparing the predictions of our asymptotic model to experiment, we find that the rate of adhesion, coagulation, and diffusion of particles in the volatile liquid as well as the rate of liquid evaporation govern the deposition of particles. Moreover, fast adhesion of particles to the solid substrate and fast diffusion of particles in the liquid disperse the spatial distribution of the particulate deposit. Fast coagulation and fast evaporation support the deposition of denser patterns of particles of sharper spatial boundaries. A different theoretical approach for modelling the deposition problem, which we currently pursue, is to employ phase change type energy functional to account for particle coagulation and adsorption in the volatile liquid film. Such an approach should naturally incorporate the physics associated with concentrated particulate systems and particle volume effects that are not accounted for in the current asymptotic model. References 1. R. Bhardwaj, X. Fang, et al. Self-Assembly of Colloidal Particles from Evaporating Droplets: Role of DLVO Interactions and Proposition of a Phase Diagram, Langmuir 26 (7833â 7842) 2010 2. M. Anyfantakis, Z. Geng, et al. Modulation of the Coffee-Ring Effect in Particle/Surfactant Mixtures: the Importance of Particleâ Interface Interactions. Langmuir 31 ( 4113â 4120) 2015 3. E. Homede, A. Zigelman, et al. Signatures of van der Waals and electrostatic forces in the deposition of nanoparticle assemblies; J. Phys. Chem. Lett. 9 (5226â 5232) 2018 4. A. Zigelman and O Manor. Simulations of the dynamic deposition of colloidal particles from a volatile sessile drop, J. Colloids Interface Sci. 525 (282-290) 2018 5. A. Zigelman and O. Manor. The deposition of colloidal particles from a sessile drop of a volatile suspension subject to particle adsorption and coagulation, J. Colloids Interface Sci. 509 (195) 2018Non UBCUnreviewedAuthor affiliation: Technion - Israel Institute of Technology HaifaResearche

Bubbles, drops and the physics of their collisions

© 2010 Dr. Ofer ManorThis thesis covers bubbles, drops and collisions of these particles with each other or with solids under the influence of surface-active species, electrical double layer or polymeric brushes. In particular, it discusses a stage in which the particles are at close proximity and the drainage of a thin intervening film determines whether a collision will lead to particle attachment or coalescence. Measurements of particle collisions that took place in atomic force microscopes and a surface force apparatus are studied using numerical analysis in order to unravel the complicated and astonishing physics of these interactions. Further analysis predicts and explains different aspects and mechanisms of such particle collisions. The goal of this thesis is to expand knowledge in emulsion and suspension stability, under various physiochemical constraints, and to improve the measurement capability associated with particle collision efficiency

On the sensitivity of the evaporative pattern deposition of particulate mass to the ionic strength in kinetically stable suspensions

The deposition of particulate mass from a volatile suspension is a common process. Usually, the employed suspensions are designed to be kinetically stable, which is achieved by employing surface forces of molecular origin, e.g., the electrical double layer (EDL) or steric forces, to render high energy barriers to particle attachments. One may expect that a high energy barrier in the original suspension will render the deposit morphology solely connected to particle convection in the volatile liquid and, once most of the carrier liquid has evaporated, to capillary attraction between detached particles to each other and to the solid substrate. However, we show that variations in the magnitude of large energy barriers to particle attachments in our original suspensions are connected to variations in the deposit morphology following the evaporation of the carrier liquid. In our experiments, the different original EDL induced energy barriers are large and traverse tens and hundreds of KBT in magnitude. Nevertheless, the evaporation of the carrier liquid during the deposition process supports the convection of mass toward the three phase contact line between the suspension, substrate, and vapor phases. The convection of particle and ion mass dynamically increases particle concentration and ionic strength in the vicinity of the contact line. The elevated ionic strength reduces the energy barriers to particle attachments in that vicinity, which appears to locally support particle coagulation and adsorption effects and hence to alter the deposit morphology. Thus, the morphology of the deposit may show considerable sensitivity to the specific magnitude of energy barrier to particle attachments in the original, kinetically stable, suspension

Influence of surfactants on the force between two bubbles

The retardation of the interfacial velocity due to the presence of surface-active species is a key feature that determines the magnitude of the dynamic interaction force between colliding bubbles. Here we derive simple measures to quantify the influence of a surface-active species during a head-on collision between bubbles to be used as guidelines in the design and analysis of emulsion stability and related experiments. These measures are derived from a theoretical model that was found to be consistent with experiment and are shown to characterize the interfacial dynamics without the need to use numerical analysis. It is shown that a surface mobility may change with the geometry of the film between the bubbles for a specific amount of a surface-active species. However, small amounts of surface-active species are sufficient to immobilize the interfaces under most physical conditions as found in earlier studies