Slime filled pressurized actuators: Reducing leakage by 400,000 times

Pneumatic and hydraulic actuators are vulnerable
to rupture, micro-cracks, piercing, and tears [1], [2]. Self healing
can repair a damaged actuator’s pressure vessel. Different
approaches focus on the chemical properties or composition
of the pressure vessel [1], [2], [3], [4], or its working fluid [5].
However, these actuators rely on restoration of the air-tightness
of their pressure vessel, requiring a safe environment, limiting
suitable vessel materials, and needing careful fabrication. We
propose inflatable actuators be filled with a viscous, shear
thickening fluid (slime) so the pressure vessel does not have
to be air-tight to function as an effective actuator.
Here slimes were made of simple ingredients (PVA glue,
water, baking soda, shaving cream, and contact lens cleaning
solution) [6]. Three variants (Table I) with increasing viscosity
were made by increasing the weight percentage of the contact
lens solution. Their leakage rates through a 0.4 mm thick 100 %
cotton fabric were compared to those of air and water when
pressurized at 550 kPa. We also tested the effect of the slimes’
high viscosity on extending a 5 cm3 syringe when pressurized
via a length of 5.5 mm inside diameter tubing.
Figure 1 shows the leakage rate of Slime-31 was 0.0005 %/s
compared to the 200 %/s of air. The average syringe extension
rate at a pressure of 50 kPa with air and water was 0.005-
0.009 m/s compared to 410􀀀5 m/s for Slime-31 with a pressure
of 150 kPa (Figure 2). Measurements of the syringe plunger
force compared to the force it should produce given its area
and the pressure applied yielded force efficiencies for the fluids.
A force efficiency of 41-46 % for air and water was noted. The
slime’s force efficiency ranged from 5 % for Slime-31 to an
average of 29 % for Slime-21.
Our results confirm that slimes reduce leakage rates by
up to 400,000 times with the compromise of a corresponding
reduction in speed by 225 times and a reduction in force
efficiency of 9 times. Further, there is a trend in decreasing
leakage rate, speed, and force efficiency of the slimes as their
viscosity increases. Hence, application of slimes to the filling
of actuators’ pressure vessels requires a trade-off between the
relative importance of speed, efficiency, and leak resilience.
Further, slimes’ viscous nature raises the challenge of how they
can be quickly and efficiently pumped. This motivates future
research on their use in soft robotics.