Applying Permanent, Robust Stenciled Patterns of Fine Particles to Elastomeric Surfaces

Our research focuses on understanding large deformations in magnetorheological elastomers under combined mechanical and magnetic field loading. To quantitatively visualize these deformations in our experiments, we use optical tracking techniques like DIC that require permanent markings on the surface of our silicone samples. We use digital image correlation along with a customized universal testing machine capable of applying both mechanical and magnetic loads to our specimens.

The specimen surfaces are marked with patterns to give the software something to track. Without permanent surface markings, deformation-tracking is impossible. Silicones are inherently non-adhesive and highly extensible.

This means that most common paints and marking methods for plastics don't work because they won't stick or they can crack and flake off when the strains are really high. Our protocol addresses the need for a reliable method to mark elastomers undergoing large deformations, enabling accurate optical tracking through repeated mechanical cycles. Unlike most marking methods that fade or flake off with time or high strains, our approach allows specimens to be marked, stored, loaded, and unloaded without degradation.

This ensures consistent tracking and repeatability in experiments, particularly for aging and fatigue studies. To begin, attach the air compressor to the airbrush. Plug the air compressor into an appropriate electrical outlet and turn it on.

Inside a fume hood, while wearing the appropriate personal protective equipment, fill the airbrush with isopropyl alcohol and clean it multiple times. Now, place the stencil on a paper towel inside a fume hood. Keep the spray nozzle approximately 30 centimeters away from the stencil and spray one coat of adhesive onto the stencil in a smooth and continuous motion.

Wait 30 to 60 seconds until the stencil becomes tacky but is no longer wet. Then, place the mold on a paper towel inside a fume hood and use clean, non-latex gloves to carefully pick up the stencil without disturbing the adhesive. Place the adhesive side of the stencil onto the mold and run one finger firmly across the stencil to flatten it, ensuring uniform contact with the mold.

Next, using painter's tape, tape the edges of the stencil to the mold. For digital image correlation, or DIC speckle mold, place the mold on a paper towel inside a fume hood with the mold cavity surface facing up. Keep the spray nozzle approximately 30 centimeters away from the mold and apply one coat of mold release to the mold in a smooth, continuous motion.

Inside a fume hood, use a funnel or a small spoon to transfer approximately 10 milliliters of fine powder into a test tube. Add isopropyl alcohol to the test tube to bring the total volume to 45 milliliters. Seal the test tube and shake it vigorously to mix the suspension.

Now, re-shake the prepared suspension to re-homogenize the contents and fill the airbrush container until it is 75%full with the suspension. Turn on the air compressor and confirm the air pressure is set between 1.38 and 1.72 bar or 20 to 25 pounds per square inch. Inside the fume hood, position the mold with the adhered stencil at a slightly off-vertical angle and aim the airbrush slightly off to the side of the mold to ensure the spray initiates before reaching the mold.

Then, pull the airbrush trigger back gently to begin spraying. Smoothly apply the first coat of the suspension, keeping the nozzle approximately 30 centimeters from the surface and moving in a continuous motion. After the first coat, release the airbrush trigger and observe the surface for uniform distribution of the fine particle suspension.

If needed, apply additional layers across the stencil and mold surface. Wait until the fine particles are visibly dry before removing the stencil and ensure a minimum drying time of 180 to 300 seconds. Then, remove the painter's tape from the top shorter edge of the mold and slowly pull the tape upward to detach the stencil from the mold in one smooth motion.

After turning on the air compressor, position the mold without any stencil at a slightly off-vertical angle for a perpendicular spray application inside the fume hood. To test the airbrush, compress the trigger briefly while spraying onto a spare sheet of paper. Confirm that the resulting speckle pattern is spatially random and of suitable size for the experimental imaging setup.

Now, flick the trigger over the mold to apply the speckle pattern on its surface and wait for at least 180 to 300 seconds until the fine particles are visibly dry. Finally, assemble the mold in preparation for the injection molding process, or store them for a few days prior to injection molding. Following curing of the silicone elastomers, molds can be easily removed.

The patterns or speckles initially sprayed on the molds transfer to the finished elastomer surface with high fidelity. Specimens created for mechanical testing included various geometries, such as plates, cylinders, and cubes with contrasting grid and speckle patterns on opposite surfaces, including white plates with black grids, black plates with white grids, and plates with blue grids. The pattern that adheres to the silicone specimen during curing can be any shape or color.

The pattern does not come off and can be used repeatedly in large deformation experiments. Optical tracking design patterns included grid patterns, sparse DIC speckle patterns, and dense speckle patterns for varying strain measurement needs. Digital image correlation analysis under increasing global strain displayed progressive local displacement in the vertical direction, visualized by a heat map with colors ranging from red to blue.