Drop generators, piezoelectric

A complete description of droplet generator and of several atomization methods appears in a previous paper [8]. Simple air-stripping or piezoelectric drop generators were employed. The core liquid typically consisted of a polyanion solution, while the receiving bath contained a polycation(s) solution and, in many instances, a divalent cation. 

Abstract This chapter provides information on different types of drop-on-demand drop generators. It starts with thermal or bubble jets, in which a nucleation bubble is used to eject a droplet out of an orifice. This is followed by piezoelectric, pneumatic, microfluidic, electrohydrodynamics (EHD) and aerodynamic droplet generators. For each droplet generator, the principle of operation and major features and characteristics are described. 

Droplet generator Piezoelectric drop on demand Piezoelectric ink jet Piezoelectric micro-/ nanoliter droplet dispenser... 

Narrow-necked polythene bottles (50 or 100-mL), piezoelectric spark generator, sparking plug attached to base plate, gas inlet tubes, 1-L beaker, dropping pipette, measuring cylinder, safety glasses, protective gloves. 

What was said about the state of knowledge on collisions of different miscible liquid drops applies to the case of immiscible liquids also. We find the work by Chen and Chen [49], who investigated the collision of equal-sized droplets of water and Diesel oil. The dynamic viscosities and surface tensions of the two liquids against air at the temperature of the experiments are different by a factor of 3.1 and 2.6, respectively. Drop sizes, produced with the same piezoelectric droplet generators as in Gao et al. [45], ranged between 700 and 800 pm. The result of an experimental survey of the outcome fi om the collisions for varying impact Weber number and non-dimensional impact parameter is a flow chart similar to that in Fig. 7.5a, where the Weber number is defined with the relative velocity of the colliding drops and the liquid properties of Diesel oU. The boundaries between the... 

Ink-jet describes small drops of ink driven to the substrate, usually by an electrostatic field. The drops are formed by a rapid pressure pulse in a small, nearly enclosed chamber. When the pressure increases, a small amount of liquid is ejected from the chamber through a nozzle (the only exit). When the pressure in the chamber decreases again, that liquid is separated from the bulk of the liquid in the chamber by surface tension in a process called pinch-off. The pressure pulse is commonly generated by a piezoelectric material or by heating the liquid via current through resistive material embedded in the wall of the chamber. The ejection process usually leaves the drop with insufficient velocity to drive it to the substrate, so another force, such as an electrostatic field, is used to complete the delivery. The result is a rapid stream of successive drops, controlled electronically. 

A rotating-wheel interface has been devised to couple LC and CE with MS [57]. The LC effluent premixed with the MALDI matrix flows through a fused-silica capillary at the rate of 100 to 400 nLmin . The tip of the separation capillary is in contact with the rotating wheel. The solution deposited is dried rapidly in vacuum of the MALDI source, leaving a narrow trace of cocrystallized analyte-matrix mixture. The wheel is transported to the repeller, where laser desorption-ionization of the analyte-matrix mixture occurs. A similar system that uses a rotating ball coated with matrix for deposition of a single drop is also available for online LC/MALDI-MS [58]. The droplet deposition is controlled by a piezoelectric-actuated droplet generator. 

The design of the print head in a continuous ink-jet system is shown in Fig. 9. In this example of a binary deflection system the drops are generated continuously by a piezoelectric device and pass between a pair of charging electrodes which, depending on the signal applied. 

Sensitivity. The sensitivity of a piezoelectric material is taken to be equal to the generated open-circuit voltage that drop>s across to the contact with the distance t (= thickness) divided by the applied stress or the product g t, where g is the relevant piezoelectric voltage coefficient. The voltage coefficient g is connected with the charge coefficient d via the dielectric permittivity = CrCo according to... 

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