Particle Fabrication Using Inkjet Printing onto Hydrophobic Surfaces for Optimization and Calibration of Trace Contraband Detection Sensors

Abstract

A method has been developed to fabricate patterned arrays of micrometer-sized monodisperse solid particles of ammonium nitrate on hydrophobic silicon surfaces using inkjet printing. The method relies on dispensing one or more microdrops of a concentrated aqueous ammonium nitrate solution from a drop-on-demand (DOD) inkjet printer at specific locations on a silicon substrate rendered hydrophobic by a perfluorodecytrichlorosilane monolayer coating. The deposited liquid droplets form into the shape of a spherical shaped cap; during the evaporation process, a deposited liquid droplet maintains this geometry until it forms a solid micrometer sized particle. Arrays of solid particles are obtained by sequential translation of the printer stage. The use of DOD inkjet printing for fabrication of discrete particle arrays allows for precise control of particle characteristics (mass, diameter and height), as well as the particle number and spatial distribution on the substrate. The final mass of an individual particle is precisely determined by using gravimetric measurement of the average mass of solution ejected per microdrop. The primary application of this method is fabrication of test materials for the evaluation of spatially-resolved optical and mass spectrometry based sensors used for detecting particle residues of contraband materials, such as explosives or narcotics.

On the response of ultra microbalances to inkjet dispensing of fluids

“…The upper integration limit is the current time while the accumulation is going on, and the length of the accumulation interval thereafter, T (t) = min(t, T 0 ). A transformation of variables gives an integral that is known analytically (12).…”

“…These results coincide quite well with the estimates gained from the static mass measurements. Furthermore, the above analysis of the response behavior, in particular equation (12), shows that, for a weight changing linearly with time, the balance readings will follow that rate with a constant lag after the response time has expired. This ensures that the linear model applies to the evaporation rates without further response correction as well as to the continuous mode method of droplet mass measurement.…”

“…It has been demonstrated [26] that adding a layer of slowevaporating liquid (a silicone oil in that case) will essentially block the evaporation of the dispensed fluid as it collects in the weighing boat, reducing the related uncertainties in the mass measurement. However, this trick will often not be available because no suitable layer fluid is available (e.g., for isobutanol or acetonitrile [12,27]). Also, it exposes dispensed droplets to a higher evaporation rate in flight (discussed further below near the end of section 4.3) due to the lower saturation of the air with vapor, an effect of increasing importance with decreasing droplet size.…”

“…The data points correspond one-to-one to those in figure 9. [1,2,12,27,36,37,38,39,40,41,42]. The estimates in table 9 apply directly to droplets in dry air (nominally zero partial vapor pressure).…”

“…Inkjet gravimetry has been used in recent years for improving the control of inkjet dispensing amounts in terms of mass or volume [1,2,3,4,5,6,7,8] and carried further into applications like the manufacturing of test samples for detection of explosive or narcotic substances [9,10,11,12,13], study of nanomaterial properties [14] and controlled fluid injection in capillary electrophoresis [15]. The concept is to integrate a microbalance with an industrial inkjet printer and use dispensing of a predetermined number of droplets into a weighing boat on the balance to determine the average droplet mass and/or, if the density is known, the droplet volume.…”