Abstract
This paper describes osmotically-driven pressure generation in a membrane-bound compartment while taking into account volume expansion, solute dilution, surface area to volume ratio, membrane hydraulic permeability, and changes in osmotic gradient, bulk modulus, and degree of membrane fouling. The emphasis lies on the dynamics of pressure generation; these dynamics have not previously been described in detail. Experimental results are compared to and supported by numerical simulations, which we make accessible as an open source tool. This approach reveals unintuitive results about the quantitative dependence of the speed of pressure generation on the relevant and interdependent parameters that will be encountered in most osmotically-driven pressure generators. For instance, restricting the volume expansion of a compartment allows it to generate its first 5 kPa of pressure seven times faster than without a restraint. In addition, this dynamics study shows that plants are near-ideal osmotic pressure generators, as they are composed of many small compartments with large surface area to volume ratios and strong cell wall reinforcements. Finally, we demonstrate two applications of an osmosis-based pressure generator: actuation of a soft robot and continuous volume delivery over long periods of time. Both applications do not need an external power source but rather take advantage of the energy released upon watering the pressure generators.
Highlights
The osmotic pressure gradient across a semipermeable membrane separating compartments of differing solute concentrations generates an important driving force in nature
Inspired by plants that use osmotic gradients to accomplish mechanical work, we propose that an osmosis-driven device with a high-osmolarity working fluid could serve as a means of liberating usable potential energy from the chemical potential of the fluid via pressure generation upon watering the device (Figure 1D)
To characterize the dynamics of the system, we modeled the pressure generation by using two differential equations for the change in mass within the cassette over time
Summary
The osmotic pressure gradient across a semipermeable membrane separating compartments of differing solute concentrations generates an important driving force in nature. This gradient, represented as DP, quantifies the entropically-driven tendency of the solvent in such systems to flow into the region of higher solute concentration; its value is equal to the pressure gradient that must be applied across the membrane to counteract this flow. [1] In addition, certain specialized plants have evolved the ability to change the turgor inside different cells in response to external stimuli. The hydrostatic pressure generated by osmosis can be used to generate electric power using only fresh and salt water; this is a very clean alternative energy source.[3,4,5] A 10 kW power plant has recently been built in Tofte, Norway using PRO technology; a megawatt-scale plant is in the works.[6]
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