Fumed silica4/16/2023 ![]() ![]() As a control, the micro-scallop showed no significant forward displacement when the opening and closing cycles were symmetric, as is expected. Again, we track the scallop's position when it is closed (minimum opening angle) and plot the trajectory of the hinge's midpoint as a function of time. Now, a fast-opening followed by a slow-closing step give forward propulsion (see Fig. In the shear-thinning fluid the micro-scallop only moves forward when the opening–closing cycle is opposite to that used in the shear-thickening fluid. As we show here, this property can be exploited in the design and operation of a microswimmer that is simpler to operate than most other existing microrobots. At low shear rates, the two rates are comparable and the total number of entanglements is almost constant, thus the apparent viscosity remains almost constant when the shear rate increases, the rate of disruption becomes dominant and results in a shear-thinning effect. An entangled polymer network typically exhibits shear-thinning behavior, as the rheology is controlled by the rate of entanglement formation and disruption. Propulsion of the micro-scallop operated with a reciprocal but asymmetric actuation sequence is also achieved in a non-Newtonian shear-thinning solution of hyaluronic acid (HA), which is found in a number of biological media (see also Section 8.3.1.1), including saliva, blood, vitreous and synovial fluid. 8.4.2.3 Propulsion in shear-thinning fluids The reason is that during the fast-closing half-period, the fluid in between the two shells exhibits a higher shear rate and thus a higher viscosity than the fluid in front of the swimmer while in the slow opening half-period the viscosity there is no such difference and the fluid in front and at the back of the swimmer has a low viscosity, thus the microrobot propels further forward than backward. ![]() The net displacement is only achievable under asymmetric actuation in a non-Newtonian fluid (see Fig. After several periods a considerable net forward displacement is visible. In each period (one opening and closing cycle), the forward displacement is larger than the backward displacement and the swimmer thus exhibits a small net displacement. Hence, the microrobot indeed swims in the regime of low Re. If we take the characteristic length of the swimmer as 1 mm and the fastest linear velocity as 1000 μm/s and 100 μm/s during the fast closing and slow opening strokes, respectively, then we obtain a Reynolds number Re = 0.5 × 10 − 4 for the closing stroke and Re = 1 × 10 − 4 for the opening stroke, which are both much smaller than 1. As a control, the micro-scallop in the same fluid was actuated with a symmetric wave-form, and, as expected, no net displacement was observed. 8.4.2.2 Propulsion in shear-thickening fluidsįorward net displacement of the micro-scallop in a shear-thickening fluid ( fumed silica in PPG) was achieved by the asymmetric actuation induced by an exponential decaying pulse signal. In the shear-thickening fluid the viscosity is dominant over elasticity, which is an important point for discussing the propulsion mechanism. Hyaluronic acid (6 mg/mL) was used as the shear-thinning fluid. Peer Fischer, in Microbiorobotics (Second Edition), 2017 8.4.2 Propulsion in shear-thickening/thinning fluids 8.4.2.1 Non-Newtonian fluids preparation and rheological measurementįumed silica suspensions (8% w/w) in poly(propylene glycol) (PPG, M w = 725, Sigma-Aldrich) were used as the shear-thickening fluid. Micro- and nanorobots in Newtonian and biological viscoelastic fluids ![]()
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