Catalytically Induced Electrokinetics for Motors and Micropumps

Abstract

We have explored the role of electrokinetics in the spontaneous motion of platinum-gold nanorods suspended in hydrogen peroxide (H2O2) solutions that may arise from the bimetallic electrochemical decomposition of H2O2. The electrochemical decomposition pathway was confirmed by measuring the steady-state short-circuit current between platinum and gold interdigitated microelectrodes (IMEs) in the presence of H2O2. The resulting ion flux from platinum to gold implies an electric field in the surrounding solution that can be estimated from Ohm's Law. This catalytically generated electric field could in principle bring about electrokinetic effects that scale with the Helmholtz-Smoluchowski equation. Accordingly, we observed a linear relationship between bimetallic rod speed and the resistivity of the bulk solution. Previous observations relating a decrease in speed to an increase in ethanol concentration can be explained in terms of a decrease in current density caused by the presence of ethanol. Furthermore, we found that the catalytically generated electric field in the solution near a Pt/Au IME in the presence of H2O2 is capable of inducing electroosmotic fluid flow that can be switched on and off externally. We demonstrate that the velocity of the fluid flow in the plane of the IME is a function of the electric field, whether catalytically generated or applied from an external current source. Our findings indicate that the motion of PtAu nanorods in H2O2 is primarily due to a catalytically induced electrokinetic phenomenon and that other mechanisms, such as those related to interfacial tension gradients, play at best a minor role.

Geometry of thresholdless active flow in nematic microfluidics

“…Both free energies have to have the same form as (4), since that form is required by rotation invariance, but the Frank constants K 1,2,3 that appear in them need not be equal.…”

“…We now justify the isotropic viscosity approximation. In the limit of weak order, which in the notation of Kuzuu and Doi [32] is the limit S 2 ,S 4 1, our isotropic viscosity approximation becomes valid because α 4 (our activity parameter α should not be confused with Kuzuu and Doi's anisotropic viscosities), which is just the isotropic piece of the viscosity, is much greater than the anisotropic pieces α 1,5,6 of the viscosity, since the latter all vanish when S 2,4 → 0, with (again in the notation of Kuzuu and Doi) η = α 4 /2 ≈ η * C 3 r 2 . Furthermore, the coefficient γ 1 = α 3 − α 2 = 10η * C 3 r 2 S 2 /λ.…”

“…These active components can be subcellular (such as microtubules powered by molecular motors, and actomyosin networks [2,3]), synthetic (e.g., self-propelled colloids [4] or interacting microrobots), or, alternatively, living organisms [5][6][7][8], such as birds, fish [9], microorganisms [10,11], or insects [12]. Hybrid systems composed of motile rod-shaped bacteria placed in nontoxic liquid crystals have also been realized recently [13].…”

Diffusive behaviors of circle-swimming motors

“…Efforts in this area range from synthetic modifications on existing biomotors [1][2][3][4][5] to purely synthetic catalytic bimetallic nanomotors [5][6][7][8]. Motion of the synthetic motors has been achieved using a number of propulsion mechanisms including autodiffusiophoresis [9][10][11], autoelectrophoresis [6,7,[12][13][14], and bubble generation [15,16]. There are numerous reviews of motors and we point to Ebbens and Howse [17] for a general review of motors and to Paxton, Sen, and Mallouk [7] or Wang [18] for reviews of self-electrophoretic motors.…”

Mini and Micro Propulsion for Medical Swimmers

“…When a magnetic field was applied, the rod could be aligned perpendicular to the magnetic field and moved in the direction of the platinum end. Paxton et al [74] focused on the mechanism of the moving. They suggested that the movement was caused by the electro kinetic phenomenon.…”