A novel force spectroscopy and its application to nano-liquid physics

1. Motivation / Overview

The use of the atomic force microscope (AFM) has become standard in the study of surfaces and forces on the nanoscale. While the AFM can provide an impressive level of detail and information in its typical mode of operation, it only indirectly measures the forces applied to it. Here we discuss a novel technique for dynamic/static force spectroscopy by replacing the standard fixed surface in AFM measurements with a force sensing micro-electromechanical device (MEMS) to provide  both direct force measurements and manipulation of liquid columns. We use this system to make measurements of the visco-elastic properties of nanoscale water columns.

MEMS_1_Overview.jpg Fig 1. Schematic of setup, not to scale.

a) Quartz tuning fork
b) Micro-electromechanical sensor
c) Piezoelectric translator

2. MEMS / Experimental Procedure

1) Micro-electromechanical Device (MEMS)

Fabricated using surface micro-machining techniques

Spring constants between 1 – 50 N/m

A differential capacitance measurement technique is used to make sub nano-newton/meter measurements
MEMS_2_MEMS.jpgFig 2. Scanning electron microscope image of MEMS force sensor

2) Measured Quantity

force can be directly measured.

Since mass and acceleration are small use Newton’s 3rd law to get a force balance.

MEMS_4_Equation.jpg MEMS_3_Setup_1.jpg

3) Water Formation

By varying contact time the quantity of water can be adjusted. By altering the retraction velocity after separation the length of the water column can be altered.

MEMS_3_Setup_2.jpg

Fig 3. The approach and retraction cycle performed to form water columns. The plots to the top are the responses of the AFM and MEMS during the cycle. Once water is formed the retraction rate can be varied.

3. Discussion

Here we demonstrate a method of measuring interaction forces (water) between the tip of an AFM cantilever and the surface of a MEMS force sensor. While similar methods of measurement exist, including the interfacial force microscope, these represent the first, to our knowledge, of measurements of water using this simultaneous yet independent measurement approach. The results show that the MEMS force sensor can be used in conjunction with the AFM, while providing independent results.

This works represents a first step in a larger effort to utilize the unique measurement and actuation abilities the MEMS force senor provides, while still gathering important dynamical response data from the AFM.

This work is supported by the Unites States of America National Science Foundation grant# OISE-0853104