Experimental investigation of the impact, spreading, and drying of picolitre droplets onto substrates with a broad range of wettabilities

NIP27, 27th International Conference on Digital Printing Technologies, Minneapolis, USA, October 2-6, 2011.The effect of the substrate wettability on the deposition of picolitre droplets by drop-on-demand technology is investigated experimentally. Substrates with a broad range of wettabilities are considered with contact angles ranging from 19∘ for water on glass to 118∘ for water on a super-hydrophobic plasma-treated glass slide. Droplet profiles are recorded using high-speed imaging. The effect of surface wettability on the drying of water droplets is investigated and compared with models and experiments from the literature. Results are presented for picolitre droplets and experimental conditions relevant to inkjet printing. The cases of the pinned contact line and the moving one with a constant contact angle are both addressed. For colloidal fluids, the formation of the deposit is further investigated using an inverted microscope coupled to a particletracking velocimetry system. This experimental technique allows us to track individual solute particles during the various stages of the deposition. Preliminary results show the promise of this technique for characterizing internal flows within the droplet

Drying and Deposition of Picolitre Droplets of Colloidal Suspensions in Binary Solvent Mixtures

This work was supported by EPSRC grant number RG5560

Control of P2X2 Channel Permeability by the Cytosolic Domain

ATP-gated P2X channels are the simplest of the three families of transmitter-gated ion channels. Some P2X channels display a time- and activation-dependent change in permeability as they undergo the transition from the relatively Na+-selective I1 state to the I2 state, which is also permeable to organic cations. We report that the previously reported permeability change of rat P2X2 (rP2X2) channels does not occur at mouse P2X2 (mP2X2) channels expressed in oocytes. Domain swaps, species chimeras, and point mutations were employed to determine that two specific amino acid residues in the cytosolic tail domain govern this difference in behavior between the two orthologous channels. The change in pore diameter was characterized using reversal potential measurements and excluded field theory for several organic ions; both rP2X2 and mP2X2 channels have a pore diameter of ~11 Å in the I1 state, but the transition to the I2 state increases the rP2X2 diameter by at least 3 Å. The I1 to I2 transition occurs with a rate constant of ~0.5 s^-1. The data focus attention on specific residues of P2X2 channel cytoplasmic domains as determinants of permeation in a state-specific manner

Impact of picolitre droplets on superhydrophobic surfaces with ultra-low spreading ratios

EPSR

Drying and Deposition of Picolitre Droplets of Colloidal Suspensions in Binary Solvent Mixtures

Picolitre droplets of colloidal suspensions in a mixture of two solvents are emitted by a drop-on-demand ink jet print head onto coated and uncoated glass substrates. The evaporation rate and internal flows inside the drying droplets are investigated and the deposit pattern formed is related to the drying dynamics. High-speed imaging of the droplet profile from the side al- lows the droplet diameter, height, contact angle and volume to be measured during evaporation. The internal flows throughout dry- ing are visualized by following tracer particles within the fluid with an inverted microscope. The resulting deposits at higher solid content are imaged by scanning electron microscopy in or- der to relate the morphology and fine structure to the internal flows within the droplet. The evaporation of binary solvent mixtures can cause a gra- dient in the surface tension at the liquid-air interface, resulting in a Marangoni flow. The ratio of solvent composition and the surface tension of the more volatile solvent relative to the less volatile solvent is varied, to manipulate the direction and mag- nitude of any introduced Marangoni flow. Pure solvent droplets are compared to mixed binary solvent systems, to determine the importance of Marangoni flows on the morphology of the final deposit

Modal and non-modal stabilities of flow around a stack of plates

Modal and non-modal stabilities of flow around a stack of flat plates are investigated by means of asymptotic stability and transient growth analyses respectively. It is observed that over the parameters considered, both the base flow and the stabilities vary as a function of ReW2/(W−1)2, i.e. the product of the Reynolds number and the square of the expansion ratio of the stack. The most unstable modes are found to be located downstream of the recirculation bubble while the global optimal initial perturbations (resulting in maximum energy growth over the entire domain) and the weighted optimal initial perturbations (resulting in maximum energy growth in the close downstream region of the stack) concentrate around the stack end owing to the Orr mechanism. In direct numerical simulations (DNS) of the base flow initially perturbed by the modes, it is noticed that the weighted optimal initial perturbation induces periodic vortex shedding downstream of the stack much faster than the most unstable mode. This observation suggests that the widely reported vortex shedding in flow around a stack of plates, e.g. in thermoacoustic devices, is associated with perturbations around the stack end

Internal Flows and Particle Transport Inside Picoliter Droplets of Binary Solvent Mixtures

The flows in evaporating droplets of binary mixtures are much more complicated than single solvent systems. Solutal Marangoni flows are generated due to differential evaporation of components. High-speed imaging techniques are used to visualize how internal flows transport particles to build up the end deposit. Circulatory flow along streamlines develops inside droplets at the contact line or central region, depending on the direction of the Marangoni flow. Re-circulation of particles can reduce the build up of a ring stain. Additionally, particles migrate across streamlines to collect at the droplet center independent of where the circulating regions occur. Potential mechanisms for particle migration are discussed, including chemophoresis, thermophoresis and shear-induced migration

Strongly residual coordinates over A[x]

For a domain A of characteristic zero, a polynomial f over A[x] is called a strongly residual coordinate if f becomes a coordinate (over A) upon going modulo x, and f becomes a coordinate upon inverting x. We study the question of when a strongly residual coordinate is a coordinate, a question closely related to the Dolgachev-Weisfeiler conjecture. It is known that all strongly residual coordinates are coordinates for n=2 . We show that a large class of strongly residual coordinates that are generated by elementaries upon inverting x are in fact coordinates for arbitrary n, with a stronger result in the n=3 case. As an application, we show that all Venereau-type polynomials are 1-stable coordinates.Comment: 15 pages. Some minor clarifications and notational improvements from the first versio

Enhanced inverse bremsstrahlung heating rates in a strong laser field

Test particle studies of electron scattering on ions, in an oscillatory electromagnetic field have shown that standard theoretical assumptions of small angle collisions and phase independent orbits are incorrect for electron trajectories with drift velocities smaller than quiver velocity amplitude. This leads to significant enhancement of the electron energy gain and the inverse bremsstrahlung heating rate in strong laser fields. Nonlinear processes such as Coulomb focusing and correlated collisions of electrons being brought back to the same ion by the oscillatory field are responsible for large angle, head-on scattering processes. The statistical importance of these trajectories has been examined for mono-energetic beam-like, Maxwellian and highly anisotropic electron distribution functions. A new scaling of the inverse bremsstrahlung heating rate with drift velocity and laser intensity is discussed.Comment: 12 pages, 12 figure