Paddlebots: Translation of Rotating Colloidal Assemblies near an Air/Water Interface.

Microbot propulsion requires unique strategies due to the dominance of viscosity and the reversible nature of microscale flows. To address this, swimmers of specific structure that translate in bulk fluid are commonly used; however, another approach is to take advantage of the inherent asymmetry of liquid/solid surfaces for microbots (µbots) to walk or roll. Using this technique, we have previously demonstrated that superparamagnetic colloidal particles can be assembled into small µbots, which can quickly roll along solid surfaces. In an analogous approach, here we show that symmetry can be similarly broken near air/liquid interfaces and µbots propelled at rates comparable to those demonstrated for liquid/solid interfaces.

Multimodal microwheel swarms for targeting in three-dimensional networks.

Microscale bots intended for targeted drug delivery must move through three-dimensional (3D) environments that include bifurcations, inclined surfaces, and curvature. In previous studies, we have shown that magnetically actuated colloidal microwheels (µwheels) reversibly assembled from superparamagnetic beads can translate rapidly and be readily directed. Here we show that, at high concentrations, µwheels assemble into swarms that, depending on applied magnetic field actuation patterns, can be designed to transport cargo, climb steep inclines, spread over large areas, or provide mechanical action. We test the ability of these multimodal swarms to navigate through complex, inclined microenvironments by characterizing the translation and dispersion of individual µwheels and swarms of µwheels on steeply inclined and flat surfaces. Swarms are then studied within branching 3D vascular models with multiple turns where good targeting efficiencies are achieved over centimeter length scales. With this approach, we present a readily reconfigurable swarm platform capable of navigating through 3D microenvironments.

An experimental design for the control and assembly of magnetic microwheels.

Superparamagnetic colloidal particles can be reversibly assembled into wheel-like structures called microwheels (µwheels), which roll on surfaces due to friction and can be driven at user-controlled speeds and directions using rotating magnetic fields. Here, we describe the hardware and software to create and control the magnetic fields that assemble and direct µwheel motion and the optics to visualize them. Motivated by portability, adaptability, and low-cost, an extruded aluminum heat-dissipating frame incorporating open optics and audio speaker coils outfitted with high magnetic permeability cores was constructed. Open-source software was developed to define the magnitude, frequency, and orientation of the magnetic field, allowing for real-time joystick control of µwheels through two-dimensional (2D) and three-dimensional (3D) fluidic environments. With this combination of hardware and software, µwheels translate at speeds up to 50 µm/s through sample sizes up to 5 × 5 × 5 cm3 using 0.75 mT-2.5 mT magnetic fields with rotation frequencies of 5 Hz-40 Hz. Heat dissipation by aluminum coil clamps maintained sample temperatures within 3 °C of ambient temperature, a range conducive for biological applications. With this design, µwheels can be manipulated and imaged in 2D and 3D networks at length scales of micrometers to centimeters.

A novel method for measuring the evaporation of contaminant liquids from sands.

A novel experimental system, electrodynamic levitation, is used to measure the evaporation of liquids from microparticles of sand. The levitator is used to measure the evaporation rate of diethylphthalate (DEP) from microparticles of Saudi Arabian sand at 1 atm pressure and 25 degrees C. Evaporation experiments were conducted for both inland- and coastal-sand microparticles, the diameter of which is 50 microm. The DEP-evaporation rate is determined from gravimetric changes in the DEP-sand-mixture particle, the weight of which is directly proportional to the levitating electric-field intensity. From telemicroscopical observations, it is found that, when the sand particle is enclosed in DEP liquid, the sand-DEP-mixture particle evaporates like a pure DEP droplet. However, when sufficient DEP liquid has evaporated and the DEP is adsorbed into the sand microparticle, the DEP evaporation rate is reduced by a factor of 3-5 as compared with a pure DP droplet.