TITLE: Mechanical Model
AUTHORS: Wolfram Moebius

[DESCRIPTION]
Mechanical Model

[STEPS]
5. To simulate our device, COMSOL discretises the model spatially and temporally. The spatial discretisation is defined by a mesh of points, created in the Mesh 1 menu. COMSOL will automatically design a mesh, but this should be optimised for different parts of the design manually.

Select the Mesh 1 menu. In the settings, change the Sequence type to User controlled mesh, which will open new submenus. 
Select the Size submenu, and change the predefined element size to Finer - this sets an overall maximum mesh size.
Right click the Mesh 1 menu and select Size. In this new Size 1 submenu settings, change the Geometric entity level to Point. In the Graphics window, select the point where the agar sheet forms a boundary with both the support and the channel.  Change the Element size to Predefined - Extremely fine as this is a stress point.
Click and drag the Size 1 submenu to above the Free Triangular 1 submenu so it takes precedence.
6. To set up the study parameters, we set steps in the Study 1 menu. We then set up visualisations in the Results menu to be viewed whilst the study is running. We can then run a simulation.

Right click the Study 1 menu, selecting Get initial value. This sets up a solution folder the study will be contained in. 
Right click the Results menu and add 2D Plot Group - this will visualise the solution. 
To visualise the agar deformation, add a Surface condition to the new 2D Plot Group. Type v for vertical displacement into the Expression box in Settings 
Right click on the Surface submenu and select Deformation. Change the Scale factor in Settings to 1. 
Select Step 1: Stationary in Study 1 and under Results While Solving, select the plot group just created. Click Compute to run a simulation.
7. To run multiple simulations sequentially, add a Parametric Sweep under Study 1. Any parameters you have defined in Parameters 1 under Global Definitions can be added with the plus icon. Separate values with a space. This will now automatically run until you disable or delete the Sweep.

 


1. To simulate the stresses and deformation due to material properties and fluid flow, we create a 2D model representing agar overhanging a channel, with forces applied depending on the scenario.

Create a new 2D project in COMSOL using the Model Wizard, add the Solid Mechanics (solid) module under Structural Mechanics and select Study
Add the Stationary study step. A new window with the created model should open in COMSOL. 
2. To create the spatial aspect of our model, we use the Geometry 1 menu to add shapes which represent individual PDMS supports or sheets of agar, and set the dimensions and coordinates to arrange the shapes according to the real life geometry.

In the Model Builder on the left of the page, select the Geometry 1 menu within Component 1.
In the Settings, change the Length unit to mm.
Right click Geometry 1 and add a Rectangle to represent the agar sheet. Add further Rectangles for PDMS supports. This will be a Solid type whether it is PDMS or agar.
In the Block Settings, alter the Size and Shape and Position of each to correspond to the real life geometry. The geometry is symmetrical across the length of the device, so we bisect the structure through the middle of the channel and the middle of the PDMS support - the channel and support widths therefore only need to be half the physical value, with the agar sheet the combined width of both (Values used as example below: support w/h = 3/1 mm, agar w/h = 3.5/1.5 mm, agar z = 1 mm). We apply this symmetry later on.
Select Build All Objects to show the geometry in the Graphics window (on the right of the screen). If the whole structure isn't visible, you can zoom using the buttons above the Graphics window.
4. To set the mechanical loads acting on the device, we use the Solid Mechanics menu to specify the channel pressure and weight of the agar sheet.

Select the Solid Mechanics menu and in the Settings, change the Thickness to the length of the channel (in this case 0.07 m).
Right click the Solid Mechanics menu and under Volume Forces, select Body Load. In the Graphics window, select the agar sheet. Set the weight of the agar sheet in the negative y direction of Force, corresponding to F = mg (as we are specifying Force per unit volume, F/v = ρg). Density in COMSOL is mat1.def.rho (in this case, used -9.81[m/s^2]*mat1.def.rho).
Right click the Solid Mechanics menu and under Domain Constraints, select Fixed Constraint. Select the support in the Graphics window to keep this from reacting to forces.
Right click the Solid Mechanics menu and under More Constraints, select Symmetry. In the Graphics window, select the left and right edge of the agar sheet to simulate a continuous sheet of agar.
Right click the Solid Mechanics menu and select Boundary Load. In the Graphics window, select the overhanging part at the bottom of the agar sheet where it is suspended over the channel. In the Settings, change the Load type to Pressure and input the channel pressure (used -200 Pa).
3. To set material properties relating to stress and deformation, we use the Materials menu to specify the Young's modulus, density and Poisson's ratio for PDMS and agar.

Right click the Materials menu and add a Blank Material to represent agar. In Settings, label the material as 'Agar' and set the Young's modulus, Poisson's ratio and density (used 52000 Pa, 0.32 and 1000 kg/m3 respectively). In the Graphics window click on the PDMS support to deselect it (should turn grey).
Add another Blank Material to represent PDMS, and repeat (used 1000000 Pa, 0.35 and 1190 kg/m3 respectively). In the Graphics window click on the PDMS support to select it (should turn blue).