Non-mechanical nano- and microscale pumps that function without the aid of an external power source and provide precise control over the flow rate in response to specific signals are needed for the development of new autonomous nano- and microscale systems. Here we show that surface-immobilized enzymes that are independent of adenosine triphosphate function as self-powered micropumps in the presence of their respective substrates. In the four cases studied (catalase, lipase, urease and glucose oxidase), the flow is driven by a gradient in fluid density generated by the enzymatic reaction. The pumping velocity increases with increasing substrate concentration and reaction rate. These rechargeable pumps can be triggered by the presence of specific analytes, which enables the design of enzyme-based devices that act both as sensor and pump. Finally, we show proof-of-concept enzyme-powered devices that autonomously deliver small molecules and proteins in response to specific chemical stimuli, including the release of insulin in response to glucose.

Oscillations of a nominally hemispherical, inviscid drop on a solid plate are considered accounting for the contact line dynamics. Hocking boundary conditions hold on the contact line: the velocity of the contact line is proportional to the deviation of the contact angle from its equilibrium value. Natural oscillations of a drop are studied, and both eigenfrequencies and damping ratios are determined for the axisymmetric modes. The linear oscillations caused by normal vibration of the substrate are considered. Well-pronounced resonant phenomena are revealed. The nonlinear oscillations of a drop are studied.

We consider long-wave Marangoni convection in a liquid layer atop a substrate of low thermal conductivity, heated from below. We demonstrate that the critical perturbations are materialized at the wave number K ∼ √ Bi, where Bi is the Biot number which characterizes the weak heat flux from the free surface. In addition to the conventional monotonic mode, a novel oscillatory mode is found. Applying the K ∼ √ Bi scaling, we derive a new set of amplitude equations. Pattern selection on square and hexagonal lattices shows that supercritical branching is possible. A large variety of stable patterns is found for both modes of instability. Finite-amplitude one-dimensional solutions of the set, corresponding to either steady or traveling rolls, are studied numerically; a complicated sequence of bifurcations is found in the former case. The emergence of an oscillatory mode in the case of heating from below and stable patterns with finite-amplitude surface deformation are shown in this system for the first time.

The behaviour of a non-spherical osmotic motor -an axisymmetric catalytic particle self-propelling in a dilute dispersion of reactant particles -is considered. In concrast to a conventional osmotic motor that creates differences in concentration, and hence in osmotic pressure, due to asymmetry in reaction rate along its surface (e.g. a Janus particle with reactive and non-reactive patches), a non-spherical particle is able to move even with uniform chemical activity on its surface. For small departures from a sphere the velocity of self-propulsion is proportional to the square of the non-sphericity or distortion of the particle shape. It is shown that the inclusion of hydrodynamic interactions (HI) may drastically change the self-propulsion. Except for very slow chemical reactions, even the direction of self-propulsion changes with and without HI. Numerical calculations at finite non-sphericity suggest that the maximum velocity of self-propulsion is obtained by a sail-like motor shape, leading to the name 'chemical sailing'. Moreover, no saturation in the speed of propulsion is found; the motor velocity increases as the area of this 'sail' grows and its thickness decreases. The self-propulsion of a non-spherical particle releasing products of a chemical reaction -a constant flux motor -is also considered.