Dielectrowetting Control of Capillary Force (Cheerios Effect) between Floating Objects and Wall for Dielectric Fluid

A capillary interaction between floating objects and adjacent walls, which is known as “Cheerios effect”, is a common phenomenon that generates capillary attraction or repulsion forces between them depending on their wettabilities, densities, geometries, and so on. This paper deals with controlling the capillary forces, specifically, acting on objects floating on a dielectric (non-conductive) fluid. A key control input parameter is the wettability (contact angle) of the sidewall adjacent to the floating object. By introducing dielectrowetting to the sidewall and actively changing the contact angle on the sidewall, the capillary force is controlled and easily reversed between attraction and repulsion. In this reversing process, the tilting angle of the sidewall is another critical parameter. A theoretical relation taking the titling angle into account is compared and in good agreement with experimental results obtained from the trajectory of the floating object. Finally, a continuous motion of the floating object is demonstrated using this control where an array of dielectrowetting electrode pads is sequentially activated.

Single-particle tracking of the formation of a pseudoequilibrium state prior to charged microgel cluster formation at interfaces

The correlation between micron-sized particles and their self-assembly at fluid interfaces is important in several applications, including the stabilization of Pickering emulsions and creation of colloidosomes. In this study, through real-time visualization of the diffusion of microgel particles at the air–water interface of an aqueous pendant drop, the formation of a pseudoequilibrium state is observed prior to cluster formation. It is shown here that at the microscopic level, a pendant drop surface has nonuniform principal curvatures and exhibits positive deviatoric curvature (+∆c) gradients. The +∆c gradients confer superdiffusive motion to single ionic microgel particles and are responsible for bringing particles that are initially far apart to common sites on the interface with high curvatures. Prior to two-particle cluster formation, the balance between pairwise repulsion, capillary attraction and +∆c-induced energy that pushes the pair of particles to a high curvature creates a pseudoequilibrium state where the interparticle distance remains relatively invariant for a long period of time. This observation is also noted during higher-order cluster formation. Thereafter, a sufficiently strong long-range attraction potential is activated to facilitate cluster formation. Real-time tracking of the evolution of cluster formation provides useful insights into the interplay between various interactions experienced by ionic microgels.

Attachment-based mechanisms underlying capture and release of pollen grains

Successful insect pollination can be achieved by a sequence of numerous attachment and detachment events at various biological surfaces. However, the quantitative measurements of pollen adhesion on biological surfaces have been poorly studied so far. We performed atomic force microscopy adhesion measurements of pollen on two most important floral parts for Asteraceae in a course of pollination: the stigma and style of Hypochaeris radicata plant . The results indicated distinct adhesive properties of them—the pollen adhesion on stigmatic surfaces drastically increased over prolonged contact time, while the pollen adhesion increase on stylar surfaces was rather restrained. Based on the observation with cryo-scanning electron microscopy, we explained the experimental results by the presence of morphological features in form of flexible stigmatic papillae that may play a crucial role in enhancing both capillary attraction and van der Waals forces. The distinct adhesive properties seemingly originate from the unique adhesive tasks that each of the floral parts requires to achieve successful pollination. The insights into the adhesive interaction between pollen and the floral parts, obtained in the present study, may lead to better understanding of pollination mechanisms, which are strongly related to our food production. Additionally, the novel pollen adhesive mechanisms learned from the stigma of the studied Asteraceae plant can inspire biomimetic designs of spontaneous gripping systems.

Negative normal stress differences N1–N2 in a low concentration capillary suspension

Negative normal stress differences are reported in capillary suspensions, i.e. particle suspensions in a two-fluid system that creates strong capillary attraction, at a solid concentration of 25%. This volume fraction has heretofore been too low to show such normal stress differences.

Filtration of Floating Particles by Collectors: Influence of the System Geometry on the Efficiency

Important industrial processes including oil extraction, mineral processing and wastewater treatment, rely on the separation of buoyant particles from a liquid phase. The capillary attraction between floating particles and fixed collectors can be leveraged to improve the efficiency of the separation process. The capture of an advected floating particle by a fixed cylindrical obstacle is due to direct interception and capillary attraction for sub-millimeter particles. The capillary attraction stems from the local deformation of the air/liquid interface. Previous work has established that floating particles placed on the surface of a still liquid bath, spontaneously move toward or away from one another depending on their surface properties. More recently, a numerical study has considered the competition between hydrodynamic and capillary interactions as floating particles are advected past a fixed cylinder. This seminal work revealed that capillary interactions can enhance the capture of particles at low flow velocity. Building on these results, we develop a numerical approach to study the interactions between advected particles and an array of obstacles. The results are obtained with the finite element modeling of the fluid flow in the channel, in presence of obstacles. Assuming that the particles do not alter the fluid flow, we solve the momentum conservation equation for each advected particle using the Basset Boussineq Oseen equation. If contact occurs, we assume that the particle is captured by the obstacle, thus neglecting inertial effects. We demonstrate that an array of obstacles can capture most of the particles traveling down the channel. First, we show that the efficiency of an array of obstacles, i.e. the fraction of particles captured depends on interfacial and hydrodynamic effects. For example, parameters such as the Reynolds number, capillary length, contact angle and collector size influence the trapping efficiency. Second we vary the geometry of the array and seek to minimize the amount of static material needed to get the maximum efficiency. These results provide guidelines for the design of efficient filters.

The Influence of Rainfall Humidity on the Moisture Regime of Envelope Structures with Internal Insulation

While the matters of the moisture of external walls of historical buildings it turned out that apart from classical sources of moisture (such as capillary attraction, condensation etc.) there is another source which is often left behind – rainfall humidity which leaks to perimeter walls from the exterior. Whilst the Czech approach of standard assessment works with condensed moisture only, some foreign authors (especially those from Germany) point out a notable influence of rainfall humidity on the moisture regime of the mentioned constructions. Its amount exceeds the amount of rainfall that leaks into the construction due to diffusion by several times. The issue deserves to be examined in more detail, but the use of nanotechnology could help to solve or improve the problem. In some cases it would be possible to apply the suspension with added nanoparticles into the insulated masonry and improve the properties of masonry, which is facing to rainwater