On the Forces Between Micro and Nano Objects and a Gripper

Introduction

When a particle’s characteristic size is scaled down below a micrometre the role of its weight becomes negligible. When one tries to release such a neutral particle from whatever handling device surface operating on it in air or under vacuum, the particle will not drop down under the gravity but, instead, will stick to its working surface due to the effect of the omnipresent van der Waals forces. If one charges the particle, forces vibration of the working surface, etc., the released particle might move in an uncontrollable way leaving the observation field of the apparatus that controls the performance of the operation. Having in mind the above, one shall not be surprised that handling, feeding, trapping and fixing of micro parts is still the main bottleneck in micro manufacturing and is far from being solved today Cecil (2005). One of the main problems in the micro- and nano-assembly is the precise and reliable manipulation of a micro- or nano-part that includes moving it from a given point , where it is to be taken from, to a point , when it is to be placed on. One looks for further handling, feeding, trapping and fixing strategies that are still to be developed, taking into account the forces appearing at small distances between the gripper’s plates and the micro object, or between the object and a surface to taken off or to be placed on (Figure 1). In addition, the environmental influences are not mastered yet – the numerous parasitic forces due to the environmental influence often make the manipulation behaviour unpredictable.

Figure 1.

Schematic view of the red blood cell manipulation by a micro-gripper

Let us remind that when a material object is situated in vacuum at a distance L from another one the changes which their presence causes onto the existing between them fluctuations of the quantum electromagnetic field leads to a strong mutual force of attraction. At molecular separations of a few nanometers or less, these interactions are the familiar van der Waals forces (Israelachvili, 1985;

Persegian,2006). However, as recognized by Casimir and Lifshitz (Casimir, 1948; Lifshitz, 1956) at larger distances they reveal retardation effects associated with the finite speed of light. When L is of the order of a micrometer or less this force, known as the Casimir force, is the strongest force acting between two uncharged objects. While when the objects are in vacuum the net force is always a force of mutual attraction, in the case when they are immersed in a fluid this force, as it has been recently demonstrated experimentally by Munday et al. (2009), can be also a force of repulsion. The experimental study reported there, as well as the one by Hertlein et al. (2008) on the critical Casimir effect; demonstrate convincingly the correctness of the main ideas presented in the current article. One of the goals of our work is, however, to encourage further experimental studies in the field of fluctuation induced forces between objects of various geometry – spherical, ellipsoidal, cubical, conical, etc., that do not simply demonstrate the existence of the main effects considered, but that qualitatively provide information for the forces between objects of various geometry, material properties, choice of the fluid in which they are immersed, etc., as a function of the distance between then, temperature, fluid’s pressure, etc.