A microrobotic platform actuated by thermocapillary flows for manipulation at the air-water interface

Future developments in micromanufacturing will require advances in micromanipulation tools. Several robotic micromanipulation methods have been developed to position micro-objects mostly in air and in liquids. The air-water interface is a third medium where objects can be manipulated, offering a good compromise between the two previously mentioned ones. Objects at the interface are not subjected to stick-slip due to dry friction in air and profit from a reduced drag compared with those in water. Here, we present the ThermoBot, a microrobotic platform dedicated to the manipulation of objects placed at the air-water interface. For actuation, ThermoBot uses a laser-induced thermocapillary flow, which arises from the surface stress caused by the temperature gradient at the fluid interface. The actuated objects can reach velocities up to 10 times their body length per second without any on-board actuator. Moreover, the localized nature of the thermocapillary flow enables the simultaneous and independent control of multiple objects, thus paving the way for microassembly operations at the air-water interface. We demonstrate that our setup can be used to direct capillary-based self-assemblies at this interface. We illustrate the ThermoBot’s capabilities through three examples: simultaneous control of up to four spheres, control of complex objects in both position and orientation, and directed self-assembly of multiple pieces.

Influence of Lateral Restraint on Thermocapillary Migration of Wetting Droplets

The present study aims at studying the characteristics of thermocapillary migration with varying levels of lateral restraints. A temperature gradient is created by heating and cooling either side of the substrate. When a droplet is placed near hot side it spreads as thin film and migrates towards the cold side. The advancing end assumes the shape of a parabolic rim while the receding end stays as a thin film. It is observed that the droplet decelerates to attain a steady state velocity and undergoes slight acceleration near the cold end of the substrate. The observed velocity trend follows the temperature gradient on the substrate. The velocity increases with the droplet volume and substrate temperature gradient. The liquid viscosity is observed to have a diminishing effect on migration velocity. The effect of lateral spread confinement is studied by performing experimental trails on substrates with different widths. It is found that reducing the substrate width increases the migration velocity due to increased footprint resulting in larger thermocapillary force. The results observed in the present study highlights the importance of thermocapillary flows in many academic and industrial applications.

Estudio de la evolución de flujos termocapilares inducidos con un laser IR en películas delgadas de agua para la manipulación de micro-objetos

Estudio de la evolución de flujos termocapilares inducidos con un laser IR en películas delgadas de agua para la manipulación de micro-objetos Evolution study of laser-induced thermocapillary flows for manipulation of micro-objects Johan E. Quispe y Emir Vela Departamento de Ingeniería Mecánica, Universidad de Ingeniería y tecnología - UTEC, Jr. Medrano Silva esquina con Av. Miguel Grau, Barranco DOI: https://doi.org/10.33017/RevECIPeru2015.0002/ Resumen En la actualidad, la micromanipulación ha adquirido un papel importante en el ensamblaje de microcomponentes electromecánicos, ya que hace posible manipular objetos a escala micrométrica de diferentes propiedades y formas geométricas para posteriormente realizar un ensamblado y así crear sistemas cada vez más multifuncionales. Igualmente, el campo de la medicina ha alcanzado grandes avances ya que la micromanipulación nos permite manipular células, moléculas y bio-partículas en general, para realizar estudios más profundos en lo que respecta al comportamiento de éstas. Así, se requiere de un método de micromanipulación que sea capaz de desplazar micro-objetos biológicos como componentes de una manera rápida, precisa y múltiple que permita un estudio o producción a gran escala. Los métodos preferidos a la escala micrométrica son los métodos de manipulación sin contacto, ya que éstos permiten la manipulación sin dañar y contaminar las muestras u objetos. Dentro de éstos, la utilización de microfluidos para desplazar objetos es de gran interés en la comunidad científica ya que los fluidos permiten arrastrar a los objetos según la dirección y velocidad del fluido, lo que produce una fuerza que desplaza a los micro-objetos. En este trabajo de investigación se presenta un estudio, a través de simulaciones, de la generación de flujos termocapilares en películas delgadas de un líquido como método de manipulación sin contacto. Este método se está volviendo una opción viable para desplazar objetos a esta escala debido a su alta dinámica que permite desplazar objetos a gran velocidad, y además utilizando un bajo consumo de energía para su generación. Este método consiste en establecer un pequeño gradiente de temperatura en la interface líquido-aire de una película delgada, el cual genera flujos toroidales, centrados en el foco caliente, que hacen posible el desplazamiento de los objetos en el interior del fluido, a través de una fuerza de arrastre que se establece. Este estudio nos permitió comprender cómo es la evolución temporal del flujo y cómo poder generarlos de manera controlada para lograr estrategias de manipulación eficientes y precisas. Se obtuvieron valores de velocidad del flujo del orden de para un haz laser infrarojo que genera aproximadamente 80 mW sobre la muestra. Además, existe una distancia radial con respecto al foco caliente donde sería más conveniente ubicar los objetos para manipularlos debido al perfil de velocidades que se establecen. También se demostró a través de las simulaciones que es posible establecer un tamaño de espesor de fluido para manipular objetos de un determinado tamaño. Descriptores: micromanipulación, flujo termocapilar, tensión superficial Abstract Nowadays, micromanipulation has an important role for assembly of micro-electromechanical components, because it enables to assemble very small components with different material properties and geometrical shapes in order to create complex multifunctional systems. Also, micromanipulation has contributed significantly in the field of medicine enabling manipulation of cells, molecules and bio-particles to study their behavior. Thus, a micromanipulation method that could move biological objects and microcomponents in an accurate, fast and multiple manner is needed. The preferred methods in the microscale are non-contact methods because they allow manipulating objects without damage or contamination. In this scope, microflows are very good candidates to drag micro-objects according to the flows direction and velocity, producing a drag force over the objects. In this research work, a study, through multiphysics simulations, on the thermocapillary flows generation within thin liquid layers is presented as a non-contact manipulation method. The high flows dynamics can move objects at high speed. This method consists of imaparting a very small temperature gradient at the liquid-air interface of a thin liquid layer, thus generating toroidal-shaped flows centered at the hot spot. So the objects inside the flows are dragged. This study allowed us to understand the temporal evolution of flows and how they could be generated in a controlled manner. Besides, it exists a radial distance with respect to the hot spot where is most suitable to place the objects to manipulate them at high speed due to the velocity profile. Flows speeds in the order of were obtained using an infrared laser of about 80 mW. Simulations results showed that it is possible to establish a liquid depth in order to manipulate objects with specific sizes. As a result, manipulation strategies are being carried out to accurately move objects at high speeds. Keywords: micromanipulation, thermocapillary flows, surface tension.

Thermocapillary flows on heated substrates with sinusoidal topography

Two-dimensional steady thermocapillary flows in a liquid layer over a substrate, which has a uniform temperature and sinusoidal topography, are investigated by asymptotic theory. Here, the buoyancy effect is negligible and the interface is not significantly disturbed under low Marangoni number and low capillary number. A temperature gradient along the gas/liquid interface causes recirculating flows. For a small aspect ratio, which yields a sinusoidal topography with a long wavelength relative to the mean depth of the liquid layer, the second-order solutions are obtained analytically. The basic solutions show vertical diffusion of heat and vorticity from the substrate and interface, respectively. In the second corrections, the horizontal diffusion of heat weakens the overall flow and the convection of heat intensifies it.