Cai Shaobiao, Bhushan Bharat
Nanotribology Laboratory for Information Storage and MEMS/NEMS (NLIM), 201 West 19th Avenue, The Ohio State University, Columbus, OH 43210-1142, USA.
Philos Trans A Math Phys Eng Sci. 2008 May 13;366(1870):1627-47. doi: 10.1098/rsta.2007.2176.
Adhesive or repulsive forces contributed by both meniscus and viscous forces can be significant and become one of the main reliability issues when the contacting surfaces are ultra smooth, and the normal load is small, as is common for micro/nano devices. In this study, both meniscus and viscous forces during separation for smooth and rough hydrophilic and hydrophobic surfaces are studied. The effects of separation distance, initial meniscus height, separation time, contact angle and roughness are presented. Meniscus force decreases with an increase of separation distance, whereas the viscous force has an opposite trend. Both forces decrease with an increase of initial meniscus height. An increase of separation time, initial meniscus height or a decrease of contact angle leads to an increase of critical meniscus area at which both forces are equivalent. An increase in contact angle leads to a decrease of attractive meniscus force but an increase of repulsive meniscus force (attractive or repulsive dependent on hydrophilic or hydrophobic surface, respectively). Contact angle has a limited effect on the viscous force. For asymmetric contact angles, the magnitude of the meniscus force and the critical meniscus area are in between the values for the two angles. An increase in the number of surface asperities (roughness) leads to an increase of meniscus force; however, its effect on viscous force is trivial. A slightly attractive force is observed for the hydrophobic surface during the end stage of separation though the magnitude is small. The study provides a fundamental understanding of the physics of the separation process and it can be useful for control of the forces in nanotechnology applications.
当接触表面超光滑且法向载荷较小时，弯月面力和粘性力所产生的粘附力或排斥力可能会很显著，并成为主要的可靠性问题之一，这在微纳器件中很常见。在本研究中，对光滑和粗糙的亲水及疏水表面在分离过程中的弯月面力和粘性力进行了研究。给出了分离距离、初始弯月面高度、分离时间、接触角和粗糙度的影响。弯月面力随分离距离的增加而减小，而粘性力则呈现相反的趋势。两种力均随初始弯月面高度的增加而减小。分离时间、初始弯月面高度的增加或接触角的减小会导致两种力相等时的临界弯月面面积增大。接触角的增加会导致弯月面吸引力减小，但弯月面排斥力增加（吸引力或排斥力分别取决于亲水或疏水表面）。接触角对粘性力的影响有限。对于不对称接触角，弯月面力的大小和临界弯月面面积介于两个角度的值之间。表面粗糙度增加会导致弯月面力增大；然而，其对粘性力的影响微不足道。在分离末期，疏水表面会观察到轻微的吸引力，尽管其大小较小。该研究为分离过程的物理原理提供了基本的理解，并且对纳米技术应用中的力的控制可能会有所帮助。
