Mechanically compressing a biological cell using solid weight vs water weight Hope to get some fruitful information from people with physics background. I am doing some lab experiments in which i grow cancer cells on a circular surface, before placing solid weight on top of the cell. The total weight placed is 36g and this weight over surface diameter of 24mm would correspond to 5.8mmhg Or 0.7 kPa.
I then looked at cell shape and other biological phenomena From using my compression device.
However, my advisor is asking if i could replicate the result by submerging the cell in a very high water (water with cell nutrients) height. I calculated that to get 5.8 mmhg of pressure in theory by water, i need to submerge cell in 8cm height.
To my amazement i couldnt replicate the same results. Im a biologist by background, and i try to formulate reasons as to why the results differ. An analogy i can think of is that if you slapping or punch someone, it will have different effects. But this would be a lame reasoning. So i did some googling and came across a somewhat similar qn on this forum and i attach the screenshot. and thre link below,
What's the difference between compression by a gas/liquid or by a solid?
It involves some physics which states that compression weight by liquid or air is at equilibrium at all points. Whereas solid weight stacked on something is not in equilibrium?
Hope to get some understanding on this matter. Any opinion is much appreciated!
An update: i realised (guess could be wrong) that liquid pressure acting on the cell in theory would come in all directions - cell is compressed from its sides as well as being compressed from the top. While solid pressure weight is only downward. Is this a concrete evidence?
 A: Good investigation. You're very close to the key difference, which is that force applied from just one side (e.g., a weight from above) produces deviatoric stress. (Indeed, any loading configuration that's not the same in all three directions produces deviatoric stress.) This type of stress involves shear. It contributes to material failure. If you imagine various configurations with right angles inside the material, deviatoric stress will cause some of these angles to change.
In contrast, pressure (or hydrostatic stress, or equitriaxial compressive stress) is a fundamentally different type of stress called dilatational stress, which is calculated as negative one-third the trace of the stress matrix. It is the mediator of volumetric changes. It does not contribute to material failure and does not change right angles in 
continuous objects.
If some cellular mechanism were crucially triggered from deviatoric stress, than I would not expect hydrostatic pressure to produce the same results. Examples include changes in molecular affinity from molecules being sheared, from changes in angular configurations, and from stretching (which arises from Poisson effects with deviatoric loading but cannot arise from hydrostatic pressure). In cell mechanosensitivity, for example, stretching is a key part of the mechanosensory mechanism, sometimes attributed to the behavior of catch bonds.
