Active materials consume energy from the environment and alter the properties of the surrounding. They constitute a novel class of materials with striking properties and promising applications. Understanding such materials requires development of fundamental theoretical and experimental insights. This research is focused on the design of new material for 3D printing termed “active ink”. Active ink contains a small fraction of functionalized self-propelled rod-like particles. Our previous modeling and experimental studies have demonstrated that the presence of even a small fraction of active self-propelled particles in a fluid results in a drastic reduction of viscosity and a dramatic increase of self-diffusivity in response to applied shear flow. The following novel properties will be harnessed to design new inks for 3D printing:

1. Reduction of the effective viscosity by active swimmers will enhance ink transport through the nozzle and will potentially reduce the voxel size and increase the printing speed.
2. Increase of the effective self-diffusivity will enable faster polymerization resulting in resolution enhancement and more accurate 3D feature design.
3. Active particles have a fundamentally different response to applied shear flow compared to their passive counterparts. This may lead to the design of composite materials with novel distribution of particles not possible for passive particles.
4. Functionalization of active particles also will allow tuning the properties of the hardened polymer, e.g. make it magnetic, optically selective, etc.

Figure 1 | Self-propelled gold-platinum rods used as active component in the ink

 

Figure 2 | Illustration of focusing of active particles in a microfluidic nozzle 

 

By extensive Monte Carlo simulations, we demonstrated that trajectories of active particles are strongly affected by the background fluid flow and geometry of the nozzle leading to wall accumulation and upstream motion (rheotaxis). In particular, we described the non-trivial focusing of active rods depending on physical and geometrical parameters. It is also established that the convergent component of the background flow leads to stability of both downstream and upstream swimming at the centerline. The stability of downstream swimming enhances focusing, and the stability of upstream swimming enables rheotaxis in the bulk.

 

Figure 3 | Nozzle outlet distribution histograms for the position y and orientation j of active particles computed for given inverse Stokes numbers σ (ratio of the flow at the centerline to the self-propulsion velocity)

 

Figure 4 | Active particles in a microfluidic nozzle.

Collaborations:

Ayusman Sen (Chemistry), Leonid Berlyand (Math), Anna Balazs (Chem Engineering, U Pitt).

 

Funding:

NSF DMREF

 

Publications:

M Potomkin, A Kaiser, L Berlyand, I Aranson, Focusing of active particles in a converging flow, New Journal of Physics 19 (11), 115005 (2017)