US3885960A

US3885960A - Method of development of liquid electrostatic images using an hydrophobic barrier liquid

Info

Publication number: US3885960A
Application number: US314212A
Authority: US
Inventors: Terry G Anderson
Original assignee: Bell and Howell Co
Current assignee: Bankers Trust Co
Priority date: 1972-12-11
Filing date: 1972-12-11
Publication date: 1975-05-27
Anticipated expiration: 1992-05-27

Abstract

An improved method of developing a latent electrostatic image disposed on a surface comprises depositing a barrier liquid and an aqueous liquid developer on said surface, holding said barrier liquid and developer on said surface until the developer displaces the barrier liquid from the latent electrostatic imagebearing areas of said surface to form a visible image and thereafter fixing the visible image on a receptor sheet, which may be the said surface or one or more separate sheets placed in contact therewith. The barrier liquid is hydrophobic, and the developer carries a coloring agent which can be an organic dye, inorganic pigment, or the like. Fixing can be accomplished merely by drying the coloring agent in place. The surface tension of the developer is lowered sufficiently by the electric field surrounding the latent electrostatic image to effect the desired physical displacement of the barrier liquid.

Description

United States Patent [1 1 Anderson 14 1 May 27, 1975 [75] Inventor: Terry G. Anderson, Pasadena. Calif.

[73] Assignee: Bell & Howell Company, Pasadena,

Calif.

22 Filed: Dec.1l, 1972 211 Appl.No.:314,2l2

[52] US. Cl. 96/1 LY; 117/37 LE; 252/621, 96/76 R [51] Int. Cl G03g 13/10 [58] Field of Search 117/37 LE; 96/1 LY, l R, 96/76 R; 252/621 L [56] References Cited UNITED STATES PATENTS 2,558,856 7/1951 Land 96/76 R 3,001,888 9/1961 Mctcalfe et al. 252/621 L 3,068,115 12/1962 Gundlach 96/1 LY 3,256,197 6/1966 Fanser et 252/621 L 3,262,806 7/1966 Gourge 96/1 R X 3,328,193 6/1967 Oliphant ct al, 117/37 LE 3,498,917 3/1970 Witter 117/37 LE 3,512,965 5/1970 Matkas 117/37 LE X 3,540,885 11/1970 Honjo et al 117/37 LE X 3,560,203 2/1971 Honjo ct al 117/37 LE X 3,625,897 12/1971 Machida ct al. 252/621 L 3,642,471 2/1972 Sato et a1 117/37 LE X 3,650,738 3/1972 Ikeda et al, 117/37 LE X 3,703,399 ll/1972 Tazaka et al. 117/37 LE FOREIGN PATENTS OR APPLICATIONS 729,134 3/1966 Canada 117/37 LE 1,223,984 3/1971 United Kingdom 96/1 LY 873,080 7/1961 United Kingdom 96/1 LY Primary E.\'aminerRoland E. Martin, Jr.

Attorney, Agent, or Ft'rmNilsson, Robbins, Bissell, Dalgarn & Berliner 57] ABSTRACT An improved method of developing a latent electrostatic image disposed on a surface comprises depositing a barrier liquid and an aqueous liquid developer on said surface, holding said barrier liquid and developer on said surface until the developer displaces the barrier liquid from the latent electrostatic imagebearing areas of said surface to form a visible image and thereafter fixing the visible image on a receptor sheet, which may be the-said surface or one or more separate sheets placed in contact therewith. The barrier liquid is hydrophobic, and the developer carries a coloring agent which can be an organic dye, inorganic pigment, or the like. Fixing can be accomplished merely by drying the coloring agent in place. The surface tension of the developer is lowered sufficiently by the electric field surrounding the latent electrostatic image to effect the desired physical displacement of the barrier liquid.

18 Claims, 8 Drawing Figures METHOD OF DEVELOPMENT OF LIQUID ELECTROSTATIC IMAGES USING AN HYDROPI-IOBIC BARRIER LIQUID FIELD OF THE INVENTION The present invention generally relates to image reproduction and more particularly to the conversion of latent electrostatic images to visible fixed images.

BACKGROUND AND SUMMARY OF THE INVENTION Most electrostatic copying methods which utilize liquid toners require the use of light sensitive paper, which tends to gray in background areas, show loss of contrast and produce only a single copy from each exposure. Conventional electrophoretic development utilizing charged particles in an insulating medium results in fixed images with lowered contrast in the background areas due to migration of the toner particles. Other conventional copying methods are also subject to such difficulties as lack of high image density, inability to provide a choice of positive or negative development. In addition, many conventional copying methods are relatively extensive and complicated, frequently requiring the use of skilled personnel to control variables within critical limits. Prior art includes the following US. Pat. Nos. 3,124,446; 3,245,381; 3,285,741; 3,367,791; 3,391,082, 3,425,829; 3,455,240; 3,512,965; 3,551,146; 3,556,784; 3,559,570; and 3,560,204.

Accordingly, it would be highly desirable to provide a simple, inexpensive method of electrostatic copying which could be utilized by unskilled personnel to provide one or a plurality of clear, crisp copies of high contrast on ordinary paper and which would provide a choice of negative or positive development.

The present invention provides a method for satisfying the foregoing needs. A barrier, hydrophobic liquid and an aqueous liquid developer are deposited sequentially on a charged surface, the developer liquid preferentially wetting the charged areas of the surface by displacement of the barrier liquid. The barrier liquid protects the uncharged regions from contact with developer liquid providing the background and image contrast. The developer liquid carries a coloring agent which can be an organic dye, inorganic pigment, or the like. Fixing can be accomplished merely by drying the coloring agent in place. Alternatively, the image can be transferred to one or more separate sheets of'paper. The process can be easily, rapidly desired. effectively carried out by unskilled personnel to produce one or a plurality of clear, sharp copies of any desired color, even on ordinary paper, if desired. Background areas of the copy are of high contrast to the high density images. Positive or negative development can be provided by simply applying an appropriate potential across the imaged sheet while applying developer.

Specifically, an aqueous liquid developer is employed, either simultaneously, e.g., in emulsion form, with a hydrophobic barrier liquid or separately after application of the barrier liquid to a surface bearing the latent image. The coloring agent in the developer need not include charged particles, and may be inorganic pigment or organic dye. The electric field surrounding the electrostatic latentimage decreases the surface tension of the developer sufficiently to cause it to displace the barrier liquid from the image areas at the photoconductive surface. Fixing of the resulting visible image can occur with or without a prior print-off to one or more sheets of copy paper. Thus, the present method is readily adaptable to rapid multiple copying for office or home use.

While the following description of the invention will refer to electrostatic images formed on a photoconductive surface, the invention is also applicable to the development of static charge patterns on dielectric papers and the like.

Further features of the present invention are set forth in the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A shows a system comprising a photoconductive surface bearing a latent electrostatic image, the surface being disposed on a grounded substrate;

FIG. 1B depicts the system of FIG. 1A after application thereto of a barrier liquid;

FIG. 1C depicts the system of FIG. 1B during application of an image developer by a roller;

FIG. 1D depicts wet visible image resulting from the application as shown in FIG. 1C;

FIG. 1E depicts the visible image of FIG. 1D permanently fixed on a receptor sheet, in this instance the photoconductive surface;

FIG. 2 schematically depicts in side elevation (partly in section) the system of FIG. 1 adapted for use in a negative development mode;

FIG. 3 schematically depicts the simultaneous application of an emulsion of developer and barrier liquid to a photoconductive surface from a breakable pod; and

FIG. 4 is a diagrammatic portrayal of the surface tension relationship between developer liquid and barrier liquid.

DETAILED DESCRIPTION FIG. 1A

FIG. 1 schematically depicts a typical system in which the present method is practiced. Thus, as shown in FIG. 1A, a latent electrostatic image 12 is provided on a photoconductive surface 14 which may comprise a conventional zinc oxide coating disposed on a paper sheet 16 or other supporting material coated with zinc sulfide, cadmium sulfide, cadmium selenide, mercuric oxide, or the like. Normally, the selected photoconductive material is disposed in fine particulate form in a resin binder, such as polystyrene, polyvinyl ester or melamine formaldehyde or the like. The supporting material can be, for example, a cellulose material such as paper or the like which has been treated to impart conductivity. A particularly suitable photoconductive surface is zinc oxide in a resin binder coated on a conductive paper support.

The support 16 is disposed on an electrically conductive substrate 18 which may be a metal plate or foil of aluminum, bronze, copper, lead, zinc, tin, or the like, electrically grounded as shown at 20 in FIG. 1A.

The latent electrostatic image 12 can be provided in any conventional manner, as by subjecting the photoconductive surface 14 in the illustrated system to a corona discharge so as to cause a uniform electrostatic charge to be deposited thereon. The thus-charged photoconductive surface 14 can be exposed to light or other electromagnetic radiation to which the photoconductive material is sensitive. The areas impinged upon by such radiation are discharged while the areas not irradiated retain their charge. accordingly, a latent electrostatic image 12 shown in FIG. IA is provided. This image is then developed by the present method.

In accordance with the present method a barrier liquid and an aqueous liquid developer bearing a coloring agent are deposited on the photoconductive surface 14 by any suitable means, either simultaneously or by application of the barrier liquid first and the developer thereafter. Thus, the barrier liquid and developer may be in emulsion form, if desired, for simultaneous application.

As barrier liquid one can use any suitable volatile, insulating, hydrophobic liquid immiscible with the developer and capable of wetting the photoconductive surface 14. In this regard, suitable nonpolar organic liquids, such as hydrocarbons, preferably paraffinic, or pycloparaffinic hydrocarbons, may be employed as the barrier liquid. Selection of the particular barrier liquid to liquid employed will depend on the particular tech- 1lqUC used in the present method, including the particular photoconductive surface, the particular aqueous developer and its ingredients, and the like. Suitable barrier liquids include iso-paraffin, specifically the mixture sold under the trademark Isopar-G, by Humble Oil and Refining Company, as well as paraffin oil, nliexane, n-heptane, cyclohexane, benzene, toluene, xylene, carbon tetrachloride, perchloroethylene, trichlorotrifluoroethane, and the like hydrocarbons, substituted or unsubstituted and aromatic, aliphatic or cyclic aliphatic, mixtures thereof and the like.

The barrier liquid performs the function of blocking the developer from access to the photoconductive sur face 14 except in those areas bearing the electrostatic image 12. In those areas, the electrical field causes the surface tension of the developer to decrease to such an extent that the developer in those areas physically displaces the barrier liquid, preferentially wets the photoconductive surface 14 and, accordingly, forms the desired visible image corresponding to the latent electrostatic image. In the remaining areas, the barrier liquid continues to prevent the developer from wetting the photoconductive surface 14 and laying down a visible pattern. The line of demarcation between the developer-occupied areas and the liquid barrier-occupied areas lS sharp, so that the visible image exactly reproduces the latent image and so that strong contrast exists between the visible image and the image-free background. and,

The developer is aqueous and includes a suitable col ormg agent which can be organic dye, inorganic pigment or a mixture thereof. For example, pigment base, such as black poster paint, designers gouache, acrylic emulsions, carbon black latexes, iron oxide, cadmium yellow, cadmium red and the like can be used. Other examples of inorganic coloring agents include: potassium permanganate, ferric chloride, cobaltous chloride, cadmium sulfide, cadmium selenide, ceric nitrate ind cupric oxide.

Organic pigments and dyes such as Methylene Blue i available from Eastman Kodak Co.), Brilliant Yellow [available from Harleco Chemical Co. Crystal Violet, Malachite Green, Victoria Blue, Carmine Red, Nigroim C, Chloramine Black, Razon Black, Methylene Viplet, Rose Bengal and Quinoline Yellow (available Trom Allied Chemical Co.) can be used. Other examples of suitable organic coloring agents include Eriochrome Black T, Alizarin and Indigo Carmine.

The coloring agent can be used with aqueous developer in any suitable concentration, for example, about 310 percent, by weight of developer. If the coloring agent is insoluble or' has low solubility in water, a mutual solvent such as dimethyl formamide can be used, for example, in a concentration of about 1-1 5 percent, by weight of the developer. Alcohols, such as isopropyl alcohol, methanol, ethanol and the like can be used for similar purposes. Moreover, such additives are surface tensionlowering agents and can be advantageously used to adjust the initial surface tension of the aqueous developer to a value above but close to that required for displacement of the barrier liquid, Other surface tension-lowering agents miscible with water include acetone. methyl ethyl ketone, lauryl sulfonate and dodecyl sulfonate. Such agents can be used in any suitable concentration, for example, about 15 percent by weight of the developer.

Normally, water is present in the developer in a major proportion, for example, about -90 percent by weight of the developer. When inorganic pigments are used, suspension agents may be employed in order to keep the pigment particles (usually of very small size) in suspension. For example, carboxymethyl cellulose or commercial dispersants such as sold under the name of DAXAD (W. R. Grace & Co.), Dispex (Allied Colloids) or TAMOL (Rohm & Haas), can be used, in concentration of, for example, about l to about 10 percent by weight of the developer.

It will be understood that the coloring agent of the developer is selected such that it is insoluble or has a low solubility in the particular barrier liquid under the operating conditions, in order to avoid bleeding" of coloring into barrier liquid and staining of the background areas surrounding the visible image pattern. Moreover, it will be understood that the developer is aqueous and volatile, except for the coloring agent and the suspension agent (if used) and does not depend on charged particles to deposit the coloring agent in place on the latent image areas 12 of photoconductive surface 14.

Fixing of the coloring agent to render permanent the visible image produced thereby requires only drying of the developer, which can be accelerated by heat, air circulation and the like. Before such drying, the visible image can be easily printed off on plain ordinary copy material such as bond paper.

In the event that the developer and barrier liquid are to be applied simultaneously as an emulsion, an emulsifying agent can, if desired, be used, for example, calcium or magnesium soaps (which promote dispersion of aqueous liquids), in a concentration of, for example, about 0.01 to about 1 percent by weight of the developer, in order to aid the formation of the emulsion. Normally, in such emulsions, the barrier liquid and developer are in a volume ratio of between about 3:1 and about 1:3, although the particular ratio is not critical.

Operation In FIG. 1B, the system of FIG. 1A is shown, but after a layer 22 of barrier liquid has been applied to the entire photoconductive surface 14. The barrier layer 22 can be applied by any suitable means, as by a steel roller (not shown), after which (according to the embodi ment of the present method shown in FIG. 1), and referring to FIG. 1C, a roller 24 or the like uniformly applies a coating 26 of aqueous developer over the barrier liquid layer 22.

As previously described, the electrical field sur rounding the image 12 reduces the surface tension of coating 26 in the areas of image 12 to such an extent that coating 26 displaces layer 22 in those areas to form a wet visible image, on photoconductive surface M, as shown in FIG. 1D. This image can then be fixed in place by drying or the like, with consequent removal of the remainder of coating 26 and layer 22, to provide the result shown in FIG. 1E, a permanent visible image exactly reproducing the original latent image.

Reversal Should it be desired to provide a visible image which is the negative or reverse of that provided by the procedure depicted in FIG. 1, this can easily be accomplished by utilizing a system such as is shown schematically in FIG. 2. Thus, in FIG. 2 components similar to those shown in FIG. I bear the same numerals but are succeeded by the letter a. In FIG. 2, a latent image is depicted at 12a disposed on a photoconductive surface 140 on a support 16a in turn backed by a conductive substrate 180 electrically grounded at 20a. Substrate 18a is also electrically interconnected with a metal roller 24a and a power source 28.

A layer 22a of barrier liquid is disposed on the photoconductive surface 14a with a coating 26a of a developer overlying layer 22a. Due to the application of a bias potential from power source 28, the electric field in the areas of the image 12a is neutralized and an electric field is provided on the image-free areas. The strength of the charge depends on the charge density of the latent image, e.g., a source of about 400 volts is generally sufficient for images obtained with commercial corona discharge units. This results in deposition of coating 26a only in the image-free areas, due to a decrease of surface tension in the developer in those areas. The system of FIG. 2 thus provides a fixed visible image which is a negative or reversed image of the original latent electrostatic image, all by the mere activation of power source 28. This is a substantial advantage over many conventional copying systems.

Emulsion The embodiments of the present method illustrated in FIGS. 1 and 2 have been described primarily in terms of sequential application of the barrier liquid and developer. However, as previously noted, these liquids can be applied simultaneously, if desired, for example, in an emulsion, through the use of means such as are depicted schematically in FIG. 3. Thus, in FIG. 3, a photoconductive sheet 30 passes between a pair of pinch rollers 32. An absorbent web 34 bearing a pod 36 on the surface thereof facing sheet 30 also passes be tween rollers 32 along with sheet 30. Rollers 32 crush the pod 36 so as to release an emulsion of developer and barrier liquid therefrom, which emulsion is uniformly distributed on sheet 30 by rollers 32 for development of a visible image in accordance with the present method. Such pod could, if desired, contain a mixture of the developer and barrier liquid or either of these liquids by itself. A plurality of such pods 36 can then be spaced along web 34 for sequential release of liquid at rollers 32. In place of the pod 36 or plurality of pods one can use microencapsulated liquids.

Copying and Fixing After the visible image is formed, as described above, the wet image on the photoconductive surface can either be fixed in place, as by drying or the like, or printed off on one or a plurality of receptor sheets, such as plain bond paper or the like, a distinct advantage over many conventional systems. If the visible image is to be fixed on the photoconductive surface, that surface functions as the receptor sheet. Once the coloring agent is depleted during multiple copying, reapplication of the developer (and barrier liquid as needed) to the photoconductive surface results in a new visible wet image from which to reproduce further copies of the original latent image. Thus, reexposure of the photoconductive surface is not required in order to produce a plurality of copies. A saving in time and effort is thus effected, with a reduction in processing cost.

It will be understood that the fixing of the visible image to a permanent form can be accomplished by merely volatilizing the volatile constituents of the developer, leaving the non-volatile coloring as the visible image. Heating, air circulation and other ways of controlling the volatilization can be employed, as previously noted. Other suitable means can be used, such as chemical reactions, including complex formation and polymerization, depending on the particular coloring agent and materials deposited therewith.

General Although I do not intend to rely upon any particular theory or principle of operation, the relationship of the developer liquid A to the immiscible barrier liquid B on the photoconductive surface S can be depicted as illustrated in FIG. 4. In order for liquid A to displace liquid B from the surface S, the angle 9, must be acute [Adam, N.K. The Physics and Chemistry of Surfaces, Third Edition, Oxford Chemistry Press, London (1941), page I88]. The angle 0 will be acute only ifz 7,, cos 0,, 2 7 cos 6,,

where 'y,, and y are surface tensions of liquids A and B with respect to air and 6,, and 0 are contact angles of each of the two liquids on the surface S in the absence of the second liquid. In the present case, the barrier liquid wets the surface S (0 0, y z 20 dy nes/cm, as an example). With water as the liquid phase A, 0 80 and y,, z dynes/cm, and the condition of formula (1) is not fulfilled. An electric field acts to decrease 7 and 6,,. At a critical field strength these changes cause the conditions of formula l to be satistied, and liquid A displaces liquid B from the surface in regions experiencing the electric field. Where the electric field is not sufficiently strong, however, the barrier liquid 8 remains, forming a hydrophobic surface and preventing discoloration of the background.

The following examples will further illustrate the invention.

EXAMPLE I A latent electrostatic image can be formed on the surface of Zinc oxide paper (zinc oxide-resin binder coating) disposed on a grounded metallic plate by exposing that surface to a corona discharge and then exposing the resulting charged surface to a light pattern TABLE I lngredicnts Concentration (7( by weight) water 45 Ecco Coat Sec(coloring 45 agent) lsopropyl Alcohol (surface tension-lowering agent) 10 A conductive black latex from Emerson 8L Cumings' The latent electrostatic image develops to a visible image when the roller is moved at about 2 inches per second, after which the image zinc oxide paper is heated to about 90C for about seconds until the visible image dries to a permanent state and the remainder of the developer evaporates, along with the barrier liquid. The finished image paper is then removed. The image is sharp, of high contrast with an unblemished background and of fine quality.

In a first parallel run, before drying the visible image, it can be printed off by contact with a sheet of plain bond paper.

ln a second parallel run the barrier liquid and developer can be applied simultaneously as an emulsion. The emulsion is formed by mixing the developer and barrier liquid together.

EXAMPLE 2 The procedures set forth for the first run and the two parallel runs in Example 1 are followed, except that a barrier liquid comprising lsopar G and a developer with ingredients as set forth in Table 11 below are substituted for those called for in Example 1, to yield comparable results.

TABLE ll Ingredients Concentration (71 by weight) water 85 Capri Blue dye (coloring agent) 5 lsopropyl Alcohol (sur face tension-lowering agent) out in office-type or home use equipment of simple construction and high durability. Other advantages of the present method are as set forth in the foregoing. Various modifications, alterations, changes and additions can be made in the present method, its steps and parameters. All such modifications, alterations, changes and additions as are within the scope of the appended claims form part of the present invention.

I claim:

1. A method of development of electrostatic images, which method comprises:

depositing a layer of hydrophobic barrier liquid on a surface bearing a latent electrostatic image, said barrier liquid wetting said surface;

depositing a layer of aqueous developer liquid, containing a major proportion by weight of water and a minor proportion by weight of a surface tensionlowering agent, on said layer of barrier liquid, said liquid developer being immiscible with said barrier liquid and bearing a coloring agent dissolved or dispersed therein having no or low solubility in said barrier liquid;

holding said barrier liquid and developer liquid on said surface until said developer liquid displaces said barrier liquid from the areas of said surface bearing said image and wets said surface areas so as to form a visible image; and

thereafter fixing said visible image on a receptor sheet.

2. The method of claim 1 wherein said liquid developer has an initial surface tension higher than that of said barrier liquid and wherein said surface tension of said developer is lowered sufficiently by the electric field accompanying said latent electrostatic image to enable said developer to displace said barrier liquid in said areas.

3. The method of claim 2 wherein said coloring agent of said developer is devoid of electrically charged particles.

4. The method of claim 3 wherein said coloring dye is selected from the group consisting of inorganic pigment and organic dye and mixtures thereof.

5. The method of claim 2 wherein said barrier liquid comprises non-polar organic liquid.

6. The method of claim 5 wherein said non-polar organic liquid comprises a hydrocarbon.

7. The method of claim 6 wherein said hydrocarbon comprises a liquid paraffinic hydrocarbon.

8. The method of claim 1 wherein said coloring agent comprises an organic dye and wherein said developer also includes a minor proportion of a solvent for said dye.

9. The method of claim 8 wherein said developer comprises at least about 60 percent by weight of water, about 10-15 percent by weight of surface tensionlowering agent, up to about 15 percent by weight of dyestuff solvent, and about 3-10 percent by weight of dye stock.

10. The method of claim 9 wherein said surface tension-lowering agent comprises isopropyl alcohol, said dyestuff solvent comprises dimethylformamide and said dye comprises chlorophyll.

11. The method of claim 10 wherein said developer is deposited on said surface by a roller.

12. The method of claim 11 wherein said roller is electrically conductive.

13. The method of claim 12 wherein said surface is photoconductive and disposed on an electrically grounded metallic substrate and wherein said roller is electrically connected to a power supply of polarity to induce negative development of said images.

14. The method of claim 1 wherein said visible image is fixed by allowing it to dry on said surface, and wherein said surface is said receptor sheet.

15. The method of claim 1 wherein said visible image is transferred from said surface to a separate receptor sheet by contact therewith and is then fixed on said separate sheet.

(576 UNiTED STATES PATENT OFFICE CERTIFIQATE 0F CGRREfiTlGN Patent No. 3! 5 9 0 Dated May 27, 1975 Q 'yj q fl Terrv G. Anderson It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

a Column 1, line 51, change "desired" to and--.

Column 3, line 2, change "accordingly" to -Accordinqly.

line 50, delete "and,"

Sugncd and Sealed thls twenty-sixth Day Of August 1975 [SEAL] Q Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (ummissimwr oj'Parenrs and Trademarks

Claims

1. A METHOD OF DEVELOPMENT OF ELECTROSTATIC IMAGES, WHICH METHOD COMPRISES: DEPOSITING A LAYER OF HYDROPHOBIC BARRIER LIQUID ON A SURFACE BEARING A LATENT ELECTROSTATIC IMAGE, SAID BARRIER LIQUID WETTING SAID SURFACE; DEPOSITING A LAYER OF AQUEOUS DEVELOPER LIQUID, CONTAINING A MAJOR PROPORTION BY WEIGHT OF WATER AND A MINOR PROPORTION BY WEIGHT OF A SURFACE TENSION-LOWERING AGENT, ON SAID LAYER OF BARRIER LIQUID, SAID LIQUID DEVELOPER BEING IMMISCIBLE WITH SAID BARRIER LIQUID ANB BEARING A COLORING AGENT DISSOLVED OR DISPERSED THEREIN HAVING NO OR LOW SOLUBILITY IN SAID BARRIER LIQUID; HOLDING SAID BARRIER LIQUID AND DEVELOPER LIQUID ON SAID SURFACE UNTIL SAID DEVELOPER LIQUID DISPLACES SAID BARRIER LIQUID FROM THE AREAS OF SAID SURFACE BEARING SAID IMAGE; AND WETS SAID SURFACE AREAS SO AS TO FORM A VISIBLE IMAGE; AND THEREAFTER FIXING SAID VISIBLE IMAGE ON A RECEPTOR SHEET.

9. The method of claim 8 wherein said developer comprises at least about 60 percent by weight of water, about 10-15 percent by weight of surface tension-lowering agent, up to about 15 percent by weight of dyestuff solvent, and about 3-10 percent by weight of dye stock.

12. The method of claim 11 wherein said roller is electrically conductive.

16. The method of claim 15 wherein said visible image is printed off on a plurality of separate receptor sheets without redevelopment and said printed off images are fixed by drying on said receptor sheets.

17. The method of claim 15 wherein said surface is photoconductive zinc oxide paper and said separate receptor sheet is ordinary paper.

18. The method of claim 1 wherein at least one of said developer and barrier liquid is deposited on said surface by release from a pod during crushing thereof by a roller.