Jetting Micron-Scale Droplets onto Chemically Heterogeneous Surfaces

We report experiments investigating the behaviour of micron-scale fluid droplets jetted onto surfaces patterned with lyophobic and lyophilic stripes. The final droplet shape depends on the droplet size relative to that of the stripes. In particular when the droplet radius is of the same order as the stripe width, the final shape is determined by the dynamic evolution of the drop and shows a sensitive dependence on the initial droplet position and velocity. Numerical solutions of the dynamical equations of motion of the drop provide a close quantitative match to the experimental results. This proves helpful in interpreting the data and allows for accurate prediction of fluid droplet behaviour for a wide range of surfaces.Comment: 14 pages, accepted for publication in Langmui

Slowdown of surface diffusion during early stages of bacterial colonization

We study the surface diffusion of the model cyanobacterium Synechocystis sp. PCC6803 during the incipient stages of cell contact with a glass surface in the dilute regime. We observe a twitching motility with alternating immobile tumble and mobile run periods, resulting in a normal diffusion described by a continuous-time random walk with a coefficient of diffusion D. Surprisingly, D is found to decrease with time down to a plateau. This is observed only when the cyanobacterial cells are able to produce released extracellular polysaccharides, as shown by a comparative study between the wild-type strain and various polysaccharides-depleted mutants. The analysis of the trajectories taken by the bacterial cells shows that the temporal characteristics of their intermittent motion depend on the instantaneous fraction of visited sites during diffusion. This describes quantitatively the time dependence of D, related to the progressive surface coverage by the polysaccharides. The observed slowdown of the surface diffusion may constitute a basic precursor mechanism for microcolony formation and provides clues for controlling biofilm formation

Drop dynamics on chemically patterned surfaces

We compare numerical and experimental results exploring the behaviour of liquid drops moving across a surface patterned with hydrophobic and hydrophilic stripes. A lattice Boltzmann algorithm is used to solve the hydrodynamic equations of motion of the drops allowing us to investigate their behaviour as the stripe widths and the wettability contrast are altered. We explain how the motion of the drop is determined by the interplay between the driving force and the variation in surface force as the drop moves between regions of different contact angle and we find that the shape of the drops can undergo large periodic deviations from spherical. When compared, the numerical results agree well with experiments on micron-scale drops moving across substrates patterned by microcontact printing. © EDP Sciences

Drop dynamics on chemically patterned surfaces

We compare numerical and experimental results exploring the behaviour of liquid drops moving across a surface patterned with hydrophobic and hydrophilic stripes. A lattice Boltzmann algorithm is used to solve the hydrodynamic equations of motion of the drops allowing us to investigate their behaviour as the stripe widths and the wettability contrast are altered. We explain how the motion of the drop is determined by the interplay between the driving force and the variation in surface force as the drop moves between regions of different contact angle and we find that the shape of the drops can undergo large periodic deviations from spherical. When compared, the numerical results agree well with experiments on micron--scale drops moving across substrates patterned by microcontact printing.Comment: 7 pages, 3 figures, Accepted for publication in Europhys. Let

Behaviour of ink droplet media interactions in model systems

In this work we present a study of the characteristics of UV cure ink droplets on different media (model homogeneous and heterogeneous surfaces, PET, treated PET, etc.). By varying the magnitude of chemical heterogeneities on model surfaces, we are able to highlight the parameters that affect droplet shape. Using Lattice Boltzmann simulations of droplet spreading, we show that the location of the impact point of a droplet on patterned substrate with micron size chemical heterogeneity is an important criterion to consider with respect to the equilibrium shape of a droplet. This allows a complete understanding of the effect of chemical heterogeneity on droplet shape, and therefore on printing quality. We also report an experimental study of the morphology of ink droplets adsorbed on chemically defined substrates. This morphology appears to be related with the surface properties of the media considered

Controlled assembly of micrometer-sized spheres: theory and application

Site-selective assembly of 5 m amine-functionalized glass spheres from aqueous suspensions onto gold surfaces patterned with carboxylic acid and methyl-terminated thiols has been achieved through the introduction of a variable tilt flow cell. In situ microscope imaging has been employed to study the four phases of assembly independently, and the relative roles of electrostatic attraction and capillary emersion have been explored. In contradiction to the commonly recognized electrostatic assembly model, detailed theoretical analysis and experimental evidence are presented to support a mechanism where patterning occurs at the point of meniscus contact. Control of pattern quality is demonstrated through the comparison of results obtained from a variety of experiments, and the best conditions for the assembly of monolayer features are identified. Finally, evidence for the extension of this assembly method to the production of singlet sphere arrays is discussed