This technology implements trap-door printing on a substrate with combinatorial microdrop arrays to form arbitrary patterns on the substrate as a means to authenticate products and documents. One method of implementing difficult reversibility in the printing and readout relationship utilizes the combination of pigments and phosphors having non-additive color mixing characteristics to make colored microdots. These microdots are produced by microdroppers or inkjet ejectors, each having a certain proportion of pigments and, therefore, would be producing a unique spectral response. Creating and characterizing a microdot having a unique spectral response holds the “cryptographic key.” A re-measurement of the spectral response of a microdot that matches the “key” authenticates the document on which the microdot is placed. Without knowledge of the “key” and because of the non-additive color mixing characteristics of pigments behind a microdot’s spectral response, a counterfeiter, taking the spectral measurement of a microdot from an original document, and then attempting to determine from this spectral information what pigments and what proportions of each were used to make this microdot, will find it very difficult, indeed almost impossible, to replicate an illegitimate copy of the document. Furthermore, utilizing computer controlled microdrop or inkjet technology adds additional layers of security. The first is that the very small amount of material used for each microdot precludes easy, direct chemical analysis of the deposited microdots. The second security factor is that microdrop or inkjet technology can be used to create very dense two-dimensional arrays of up to tens of thousands of microdots. This means that the reverse engineering to identify the “key” to a multi-thousand element array is an intractably difficult problem in nonlinear combinatorial chemistry. Furthermore, printing marks at “secret” locations on a document—that can be a certain “secret” collection of marks on a document or “secret” marks that are integrated into other patterns or letter prints on a document—adds an extra layer of security.

A fluid ejector capable of producing micron sized droplets on demand is constructed of: a quartz tube; a donut shaped piezoelectric element wrapped around one end of the tube and joined with the tube by high-strength epoxy; and a piece of silicon wafer with a micro machined orifice and heat-fused with the same end of the tube. The orifice can be either conical or pyramidal in shape. Its size and shape can be optimized for a particular application, depending on the type of fluid used, and the size of ejected droplets desired. The layer of silicon dioxide which forms naturally on the surface of the silicon wafer allows the wafer to be fusion bonded to the flat bottom rim of the quartz tube when these two components are placed in physical contact and raised to a temperature of 600 deg C. When energized, the piezoelectric element contracts in the mode which squeezes on the quartz tube, thus ejecting micron sized droplets through the orifice. The use of inert and easily sterilized materials like silicon and quartz in the microdrop ejector allows applications with a wide variety of organic and inorganic fluids which may be corrosive or at high temperatures, or may require high levels of sterility. These common materials also make it easy and inexpensive to mass produce ejectors with identical or different orifices for a variety of applications. Fluid pressure can be controlled by a manometer which is filled either with air or an inert gas. Applications of microdrop ejectors designed and fabricated as described include, but are not limited to, the following. They can be used for the generation of aerosols for various studies, weighing macromolecules that are incorporated into such uniform droplets, microfabrication by accretion of material contained in the droplets in arbitrary geometry on a substrate, and ultra-high resolution inkjet printing. A uniform array of ultra-fine droplets may provide the ideal environment for materials analysis using optical excitation as a probe. Also, the droplets can be electrically charged to a uniform level by straightforward means. Time-of-flight analysis of materials, for example, will then be possible.