Non-stick Droplets Research Articles

Manufacture and properties of composite liquid marbles

HypothesisLiquid marbles are non-stick droplets coated with colloidal usually hydrophobic particles. We suggest that “composite” liquid marbles, i.e. bi-liquid droplets, may be prepared with water droplets coated by a thin silicone oil layer containing hydrophobic, colloidal particles. ExperimentsThe process enabling manufacturing water marbles coated with silicone oil containing fumed fluorosilica particles is reported. The marbles remained stable when placed on solid and liquid supports. Bouncing and coalescence of the composite marbles was explored. FindingsNon-coalescence prolonged (ca. 20 min) jumping of composite marbles above a vibrating water bath was observed. Composite marbles withstand coalescence better than colloidal particle-stabilized liquid marbles. The effective surface tension of the composite marbles is markedly lower than that of water marbles coated with fumed fluorosilica particles. The coefficient of restitution of the composite marbles bouncing on a hydrophobic solid substrate is lower than that established for water marbles. This observation is related to the viscous dissipation occurring within the silicone layer making up the composite marbles.

Motion of the liquid on the surface of Leidenfrost droplets and the hairy ball theorem

Leidenfrost droplets are non-stick droplets, rapidly self-propelling above the hot surface. Leidenfrost effect is a phenomenon in which a liquid, in close contact with a mass significantly hotter than the liquid's boiling point, produces an insulating vapor layer which keeps that liquid from boiling rapidly. The field of the liquid velocities arising on the surface of Leidenfrost droplets is considered. The pattern of surface velocities demonstrates the zero points predicted by the Poincaré–Brouwer theorem for surfaces possessing the Euler characteristic χ = 2. Evaporation of droplets does not change their topology. Zero-velocity points appear when the Leidenfrost droplets are at rest and when they are self-propelled. Topological transformations of bouncing Leidenfrost-like non-stick droplets (called liquid marbles) converted under high values of Weber numbers into tori (χ = 0) results in disappearing of zero-velocity points in their surfaces. Zero-velocity points should also necessarily appear at the surfaces of rolling droplets, placed on superhydrophobic surfaces or liquid marbles.

Liquid Marbles, Elastic Nonstick Droplets: From Minireactors to Self-Propulsion.

Liquid marbles are nonstick droplets wrapped by micro- or nanometrically scaled colloidal particles, representing a platform for a variety of chemical, biological, and microfluidics applications. Liquid marbles demonstrate elastic properties and do not coalesce when bounced or pressed. The effective surface tension and Young modulus of liquid marbles are discussed. Physical sources of the elasticity of liquid marbles are considered. Liquids and powders used for the fabrication of liquid marbles are surveyed. This feature article reviews properties and applications of liquid marbles. Liquid marbles demonstrate potential as microreactors, microcontainers for growing micro-organisms and cells, and microfluidics devices. The Marangoni-flow-driven self-propulsion of marbles supported by liquids is addressed.

Interpretation of elasticity of liquid marbles

Liquid marbles are non-stick droplets covered with micro-scaled particles. Liquid marbles demonstrate quasi-elastic properties when pressed. The interpretation of the phenomenon of elasticity of liquid marbles is proposed. The model considering the growth in the marble surface in the course of deformation under the conservation of marble’s volume explains semi-quantitatively the elastic properties of marbles in satisfactory agreement with the reported experimental data. The estimation of the effective Young modulus of marbles and its dependence on the marble volume are reported.

Elastic properties of liquid marbles

Liquid marbles are non-stick droplets coated with colloidal particles. Liquid marbles do not coalesce when pressed one to another or when colliding. The paper is devoted to the study of the quasi-elastic properties of liquid marbles under collisions. It was established that the contact time under collision is independent of the velocity of the cue marble. The liquid marble was modeled by a droplet coated by an elastic shell, representing the colloidal layer covering the marble. The elasticity of the shell is due to the capillary interaction between colloidal particles. The physical model of collisions is proposed. Pathways of viscous dissipation are discussed. Scaling laws describing the collision are derived. The proposed scaling laws governing the marbles’ collisions were verified experimentally. The contact time of the collision scales as the square root of the marbles’ volume as it occurs under bouncing of droplets. Pathways of viscous dissipation are discussed.

Liquid marbles containing petroleum and their properties

Liquid marbles (non-stick droplets) containing crude petroleum are reported. Liquid marbles were obtained by use of fluorinated decyl polyhedral oligomeric silsequioxane (FD-POSS) powder. Marbles containing crude petroleum remained stable on a broad diversity of solid and liquid supports. The effective surface tension of marbles filled with petroleum was established. The mechanism of friction of the marbles is discussed. Actuation of liquid marbles containing crude petroleum with an electric field is presented.

Shaped composite liquid marbles

Shaped “cubic” non-stick droplets are reported. Shaped composite droplets were manufactured via a two-stage process. In the first stage, cubic foamed-polystyrene particles were hydrophilized with cold radiofrequency plasma. Then particles were wetted with water. In the second stage, they were coated with solid, colloidal particles such as lycopodium, Teflon or carbon black. Thus, “liquid marble”-like non-stick shaped droplets were obtained. The shaped “cubic” droplets remained stable when supported by a NaCl water solution. Shaped Janus droplets coated on one side with dielectric Teflon and with semiconductor carbon black on the other side, were prepared. Janus marbles were actuated with an electric field.

Composite non-stick droplets and their actuation with electric field

Composite non-stick droplets comprised of di-iodomethane and water, coated by a common shell built from hydrophobic particles, are reported. Activation of the composite marbles by an electric field was studied. The water drop climbed onto the di-iodomethane drop when the composite marble was exposed to the electric field. A dimensionless constant describing sensitivity of dielectric droplets to an electric field is introduced. An explanation of the observed phenomena is proposed.

New insights into liquid marbles

Non-stick droplets, or the so-called liquid marbles, have attracted the attention of investigators during the last decade. This paper reviews the recent results in the field of liquid marbles, in particular the behavior of liquid marbles immersed in organic liquids, opening a way to manufacturing Pickering-like emulsions. The behavior of liquid marbles exposed to magnetic and electric fields is discussed. Biological applications of liquid marbles are reviewed.

Non-Stick Droplet Surgery with a Superhydrophobic Scalpel

Cutting of nonstick droplets with a double-faced superhydrophobic blade is reported first. The process of manufacturing the superhydrophobic scalpel is presented. Cutting of water marbles and droplets deposited on a superhydrophobic surface is possible when the velocity of the blade is higher than a certain critical value. An estimation of the critical blade velocity coinciding with experimental findings is presented. Cutting of Janus and glycerol-based marbles is discussed. Coalescence of glycerol nonstick drops and marbles under cutting is treated.

Characteristics of Liquid Marbles Formed with Plasma-Treated Hydrophobic Cellulose Powder

Liquid marbles, non-stick droplets encapsulated by fine particles, were prepared with cellulose powders (CPs). Since CPs were very hydrophilic, they were treated with O2 plasma, followed by the adsorption of tetramethylcyclotetrasiloxane (TMCTS) before liquid-marble formation. Plasma treatment of CPs was carried out in a flask-rotation type reactor, which required 45 min to adsorb adequate amounts of TMCTS. In TMCTS adsorption, a successive reaction with ammonia was utilized for a secure chemisorption of TMCTS. After the adsorption, CPs became very hydrophobic. The prepared liquid marbles, water drops surrounded by TMCTS-adsorbed CP, were so robust that their compressibility exceeded 90%. A liquid marble containing 0.1-M CoCl2 aq solution was applied for an ammonia gas sensor. In the presence of a highly diluted ammonia solution (0.6%) in a desiccator, the color of the liquid marble turned from pink to blue within a few minutes.

On the Nature of the Friction between Nonstick Droplets and Solid Substrates

The lateral rolling of nonstick water and glycerol droplets deposited on solid substrates inclined at the sliding angles was studied. Droplets deposited on lotuslike water-repellent surfaces and liquid marbles (powder-coated droplets) deposited on solid substrates demonstrated similar behavior. The motion of both droplets and marbles featured friction that is far from Amonton type. The sliding angles of nonstick drops are sensitive to the "history" of a drop on the substrate. The friction of glycerol marbles is governed by viscous dissipation, whereas the rolling of water marbles is dictated by adhesion forces acting within the contact area.

How aphids lose their marbles.

Insects provide examples of many cunning stratagems to cope with the challenges of living in a world dominated by surface forces. Despite being the current masters of the land environment, they are at constant risk of being entrapped in liquids, which they prevent by having waxy and hairy surfaces. The problem is particularly acute in an enclosed space, such as a plant gall. Using secreted wax to efficiently parcel and transport their own excrement, aphids were able to solve this problem 200 Myr ago. Here, we report on the physical and physiological significance of this ingenious solution. The secreted powdery wax has three distinct roles: (i) it is hydrophobic, (ii) it creates a microscopically rough inner gall surface made of weakly compacted wax needles making the gall ultra-hydrophobic, and (iii) it coats the honeydew droplets converting them into liquid marbles, that can be rapidly and efficiently moved.