Key Takeaways

• The perpendicular force exerted by the fluid per unit area of the contact surface is called hydrostatic pressure. 

• Osmotic pressure is the pressure applied on the solution which stops the fluid movement through the semi-permeable membrane.

• The osmotic pressure depends on properties such as boiling point elevation, freezing point depression, vapor pressure depression, etc.

Ever wondered about the difference between hydrostatic and osmotic pressure?

Imagine a container with a solution into which a semi-permeable membrane tube filled with solvent molecules is inserted. The solvent molecules travel towards the solution side through the semi-permeable membrane until the osmotic pressure is applied to the solution. Osmotic pressure plays a crucial role in determining the solution's inclination toward absorbing the solvent. The solution in the container also experiences hydrostatic pressure at any point.

To better understand this, let’s explore the differences between hydrostatic pressure vs. osmotic pressure in fluids.

Hydrostatic Pressure

Irrespective of the state, matter exerts pressure. In liquids and gases, the pressure exerted is the same on all sides of the container. The perpendicular force exerted by the fluid per unit area of the contact surface is called hydrostatic pressure. Hydrostatic pressure is always associated with a confined space. For example, the fluids in containers exert pressure on their walls. In such containers, the hydrostatic pressure against the wall is more significant at the bottom than at the top.

Factors Affecting Hydrostatic Pressure 

In a fluid equilibrium, hydrostatic pressure is exerted at any point in time due to the force of gravity. The hydrostatic pressure is maximum at the bottom. The factors affecting hydrostatic pressure are the density of the fluid, acceleration due to gravity, and the depth of the fluid column measured from the surface. Hydrostatic pressure is directly proportional to the depth of the fluid. 

Calculating Hydrostatic Pressure

To calculate hydrostatic pressure, use this formula:

P = pgh

⍴ is the density of the fluid 

h is the height of the fluid column

g is the acceleration due to gravity

The denser the liquid, the higher the hydrostatic pressure exerted on the submerged body.

The Effect of Atmospheric Pressure on Hydrostatic Pressure

Typically, hydrostatic pressure is measured in Pascal. When fluids are at rest or stationary, the pressure that exerts within them forms the hydrostatic pressure measurement. The air in the atmosphere plays a key role in establishing the force on fluids. The downward force exerted at the surface of a fluid by the atmosphere is called atmospheric pressure. The atmospheric pressure is exerted on the surface, whereas hydrostatic pressure is present all throughout the depth of the fluid. Therefore, the total pressure exerted on a fluid (especially a body of water or a body immersed in a fluid) can be given as:

   P = pgh + P_atm

The true hydrostatic pressure can be obtained by subtracting the atmospheric pressure from the absolute pressure measured. Similarly, the local gravity, which varies with latitudinal position and height above sea level, also influences hydrostatic pressure measurements. With temperature variations, the density of the fluid is likely to change, which is why a change in temperature also affects hydrostatic pressure.

Osmosis and Osmotic Pressure

Osmosis is a biophysical phenomenon that supports the movement of solvents through a semi-permeable membrane. In osmosis, solvent flows from a solution through a semi-permeable membrane. The fluid moves from a lower-concentration solution through a semi-permeable membrane.

Osmotic pressure is the pressure applied to the solution, which stops the fluid movement through the semi-permeable membrane. Osmotic pressure is the minimum pressure that is required to limit the fluid movement through the semi-permeable membrane. It is also indicated as the “osmosis index”. Potential osmotic pressure is the highest osmotic pressure created by the solution when separated using a semi-permeable membrane from its pure solvent.

Factors Affecting Osmotic Pressure 

Osmotic pressure is closely related to colligative properties. The osmotic pressure depends on properties such as boiling point elevation, freezing point depression, vapor pressure depression, etc. All these properties are observed in fluids when dissolving solutes in a solution.

Calculating Osmotic Pressure

The osmotic pressure of the ideal solution can be calculated using the formula given below: 

π = iCRT

i is the Vant Hoff factor 

C is the molar concentration of the solute in the solution

R is the universal gas constant

T is the temperature

Hydrostatic Pressure vs. Osmotic Pressure Measurements

So, what is the difference between hydrostatic pressure vs. osmotic pressure?

Let’s look at an example to illustrate the differences. In the body, hydrostatic pressure ensures blood circulation, whereas osmotic pressure helps exchange the necessary fluids in the body.

Another notable difference between hydrostatic pressure and osmotic pressure is how they are measured. By measuring osmotic pressure, typically the concentration of the solution is recorded. Osmometers are used for measuring osmotic pressure, whereas manometers are used for hydrostatic pressure measurements. Gauges such as burden pressure gauges, Mcleod gauges, and piston gauges are also used to measure hydrostatic pressure.

Applications of Osmotic Pressure and Hydrostatic Pressure

The differences between hydrostatic pressure vs. osmotic pressure are clearly highlighted in the environment around us; there are a lot of chemicals and biological processes based on these principles. The transport of water in plants and food preservation using salt and sugar are just a few examples of osmotic applications. Hydrostatic pressure is responsible for the flow of water in a water cooler with an inverted container and hydroelectric power generation, among many other things. 

Cadence’s suite of CFD tools can be used to analyze the effects of hydrostatic pressure and osmotic pressure in complex systems. Subscribe to our newsletter for the latest CFD updates or browse Cadence’s suite of CFD software, including Fidelity and Fidelity Pointwise, to learn more about how Cadence has the solution for you.