How rough?



Fig. 1: Gobiesox maeandricus; photo from © Don Loarie, licensed under a Attribution-NonCommercial-ShareAlike License:

The intertidal is fundamentally rough in several senses of the word. I suspect that the crowd might both have useful wisdom here and a willingness to gather data that might answer the question of intertidal rugosity.

For context, I am a fish biologist and at the moment I am fascinated by an intertidal fish that can stick to slimy, rough rocks with nearly unbelievable tenacity. We have published an article showing clingifsh stick equally well to 1000 grit sandpaper and 45 grit paper. I was surprised by this and subsequent tests on molds of field collected rocks confirm that the fish are very sticky. Now I want to know what the environment looks like to this fish. How do we quantify the roughness of the intertidal over say 7 orders of magnitude from 100 nanometers up to 10cm. If this task can be accomplished in a reasonably rigorous way can we then extend the concept to measure the roughness at many sites where these fish occur and where they don't to see if there is a correlation between the substrate and the sticky fish.

Adam Summer's original post plus a linked image of the fish (image link added 1/22/2014) is above.
New contributions are below.

A proposed method

Here is one simple and cheap approach to visualize surface roughness using a glorified laser pointer (one with a cylindrical lens), a protractor, and a point-and-shoot camera (and some tape, a box, and a ruler; approximate setup shown in Fig. 2). The cylindrical lens spreads the laser beam into a sheet of light illuminating a line on the surface of the subject (a piece of coral in Fig. 2; a mussel in Fig. 3). In figure 3, the light sheet illuminates a mussel shell at an angle of about 33° from the line of sight of the camera. The shell has some ridges that are partially covered with sediment. The illuminated band shifts left and right as the surface moves towards or away from the camera.

Fig. 2: Setup with laser, sample (piece of coral), protractor, and camera. Laser sheet is faintly visible.
Fig. 3: Light sheet illuminates a mussel shell at an angle of about 33° from the camera's line of sight.
Fig. 4: After cropping and processing to extract the center position light sheet in the image of the mussel in Fig. 3.

Using the really nice, free program Image J, the image of the mussel was cropped, the red channel was separated out, and the brightness and contrast were adjusted until only the laser line appeared. Then the center of the line of the illuminated pixels was calculated as the weighted average x-value of illuminated pixels along each line of pixels in the x-direction (weighted by intensity, and calculated using Matlab, but this could be done easily as a macro in Image J). Although the ridges on the mussel seem washed out by the laser in the image, they do show up after processing (graph in Fig. 4). The rest of the wiggle in the line may include noise from the laser and jpg compression error, but may also include real variation in topography.

This method won't work below the scale of a few microns, but with some tuning and adjustment it should allow one to get surface roughness information down to the hundred micron scale, if not the ten micron scale.

biology biomechanics inter-tidal marine ocean oceanography

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