ENERGY HARVESTING WITH BACTERIA
The disordered swarming movement of dense active matter like bacteria can be used to generate power by rotating micro cylindrical rotors. A team of scientist from Oxford University is working on this research. They used computer simulations to demonstrate this. Researches indicate that harvesting of energy directly from bacteria is a good potential way to generate small amounts of power for micro machines.
Dense bacterial suspensions flow spontaneously, but since they are very much disordered it’s very difficult to attain useful power. Oxford team conducted an experiment in which they submerged a lattice of 64 micro rotors which are symmetric, it was found that the bacteria impulsively aligned in such a way that adjacent rotors began to spin in opposite directions which resembled of a wind farm. But case was different when simulation was done with a single rotor, it was kicked around here and there.
This method has a great significance since even a small amount of mechanical work generated from these organisms doesn’t need any input. Spin is caused due to the drag on rotors as a result of spontaneous flows. There is an alternating pattern through the entire system as each disc rotates in opposite direction to that of the neighbour. If the gap size d is small then there is a strong anti- correlation between the angular velocities. If the spacing is increased then the rotors spin randomly as the adjacent rotors de-correlate.
By studying the dynamics of solitary rotors, it was found how different factors like lattice spacing, rotor size and gap between rotors affect the nature of the array of rotors. The energy barrier locks the spin state of each rotor. Energy barrier is caused due to tiny active nematic wall which is formed on its surface.
In a microfluidic design the size of rotor and choice of configuration are specific.so it can be concluded from the findings that it is a promising idea for harnessing power from active matter.
The equations of active nemato hydrodynamics are used. Active nemato hydrodynamics describes clearly about the spatiotemporal dynamics of matter like microtubule, cellular, bacterial suspensions etc. This is based on the theory of liquid crystals. Equation for Nematic tensor is
represents the orientation order of microscopic particles, here q is the magnitude of the orientation order, n is the director, and I is known as the identity tensor, evolved due to a co-rotation term and relaxation through G( rotational diffusivity) of molecular field, and accounts for the nematic distortion free energy and bulk free energy of Landau–de Gennes assuming a single elastic constant K. The velocity field u and total density of the active matter obey the incompressible Navier – Stokes equations, stress tensor P should account for contributions from viscosity h, active contribution to the stress pact = -zQ and the elastic stress that includes pressure P also.
A flow field through the active stress is generated by variation in Q, with the strength determined by the activity coefficient z. Hybrid lattice Boltzmann technique is used to solve the equations of active nemato hydro dynamics. Thermal fluctuations are not included in Boltzmann technique. The lattice Boltzmann method is used to solve momentum equation. Method of lines is used to determine order parameter, in this method spatial discretization is done by finite difference approach and Euler integration is used for temporal evolution. Time steps and discrete space are chosen as unity. K = 0.01, z = 0.01, h = 2/3 and G = 0.34 are the simulation parameters in lattice units. These parameters are responsible for active turbulence in 2D domains of size 200* 200 were used for simulations.
Micro-rotors are modelled as discs that are discretized on the simulation lattice. Rotors are fixed in space but allowed to spin freely with a moment of inertia I = 103 for all radii. Viscous damping is more dominating than inertia.
Video of bacteria driven rotor
Many researches were done on this topics, the following is another such research. In this a gliding bacteria named mycoplasma mobile also known as M. mobile was proposed to rotate a flower shaped micro- rotor. M. mobile has a unique property, it is capable of moving along a wall due to its body shape. During initial experiments the mobility characteristics was studied and it was observed that M. mobile kept moving at the corner whose inner angle is less than 40 degree. This property was employed for the continuous rotation of the flower shaped motor.
GLIDING BACTERIA DRIVEN FLOWER- SHAPED MICRO-ROTOR
The cell structure of bacteria is similar to the shape of flask and its legs are about 50 nm long around its neck with which it glides. The average gliding speed of M. mobile is 2 to 4 µm/sec and the force of driving is 27pN.
There are two major parts in a micro motor
- Flower shaped micro- rotor
- Petal shape guide wall
The M .mobiles is introduced into the periphery of the micro –rotor by the guide wall. It also sees to it that the bacteria’s does not collide to the rotor.
MOTILITY PROPERTY OF M. MOBILE MOVING ALONG WALLS WITH DIFFERENT CORNER ANGLES
In order to find best shape of micro-rotor, the activities of M. mobiles at different corner with different angles are examined. In order to observe the movements of M. mobile around the wall, a drop of medium containing M. mobile was put over it. Following were the conclusions
- M. mobiles were moving straight at corners of angle was less than 40 degree.
- At corners of angle 50-80 degree M. mobile’s stopped for 3 seconds
- At corner of 90 degree n above they glided without stopping along the wall
From these observations it was concluded that the corner angle must be 40 or less than 40 degree.
The figure shows the M. mobile driven micro-rotary motor. The motor has 3 parts SiO2 rotor that just fits into the track, Si circular track and cells of M. mobile, which circles in a unidirectional pattern inside the track. The asymmetric introduction of M. mobiles into the circular track enables the unidirectional movement of cells but the main reason is BIOTIN-STREPTAVIDIN interaction due to which the rotor is rotates in a particular direction.
BIOTIN STREPTAVIDIN INTERACTION
Streptavidin is a protein obtained by purification process from bacterium Streptomyces avidinii. The bonding between streptavidin and biotin is known to be one of the strongest non-covalent bonding. The complex is resistant to organic solvents, ph, temperature, proteolytic enzymes.
The streptavidin-biotin complex has high affinity. There is shape complementary between them.
Due to these interactions the rotor is pulled by the cells and it rotates the rotor which is coupled to generator to rotate the coil in magnetic field and thus power can be generated.