Research Article  Open Access
ShueeiMuh Lin, ChingYao Chang, "ResonanceSize Parameter Relationship and Dynamics of an AFM Subjected to Multimode Excitation and Based on the Modified Couple Stress Theory", Mathematical Problems in Engineering, vol. 2019, Article ID 3147863, 10 pages, 2019. https://doi.org/10.1155/2019/3147863
ResonanceSize Parameter Relationship and Dynamics of an AFM Subjected to Multimode Excitation and Based on the Modified Couple Stress Theory
Abstract
The mathematical model of AFM probe subjected to multimode excitation based on the modified couple stress theory is presented. The semianalytical solution of the system is proposed. The transient behavior and response spectrum of AFM probe subjected to multimode excitation are investigated. It is very helpful to predict the nanotopography and surface properties based on the response of multimodes excitation. The effects of the root excitation, size parameter, and interacting distance on the response spectrum and frequency shift are investigated. The resonant frequency relation of the two systems with different size parameters is discovered and expressed in a formula. The natural frequencies predicted via the formula and those determined by the semianalytical method are significantly consistent.
1. Introduction
Atomic force microscopy (AFM) is a powerful device for scanning the atomicscale topography and property of a sample’s surface [1].
In convention, the literatures investigated the steady behavior of AFM probe [2–9]. Definitely, the investigation of the amplitude and phase behavior in the transient regime will increase significantly the speed of imaging and accuracy. Santos and Gaderlab [10] investigated the tipsample force in transient behavior. Sahoo et al. [11] increased the speed of imaging and control of AFM by studying the transient motion of cantilever signal. Chang et al. [12] presented a new algorithm for high speed AFM imaging of biopolymers by investigating the transition from transient part to steady state regime. Payam [9] studied the transient behavior of tapping modeAFM in the massspringdamper model.
Some literatures [13–16] investigated the multifrequency excitation of FMAFM systems in fast force spectroscopy and material characterization at the atomic scale as well. The topography and material properties of sample’s surface can be measured by using AFM subjected to the simultaneous multimodes excitations. The amplitude signal of the first mode is used to image the surface topography. The phase shift signal of the second mode is usually used to map changes in material properties of the sample’s surface [17–19].
In convention, the structure’s size effect is not considered. Several literatures [20–23] found that the sizedependent effect on the dynamic behavior of the structures in the order of microns or submicrons is significant. Some nonclassical theories with size dependency are the couple stress theory [16, 24, 25], the strain gradient theory [26], the nonlocal theory [27], and the surface elasticity model [28]. Mindlin and Tiersten [24] and Toupin [25] presented the classical couple stress theory with the gradients of rotation and displacement. Yang et al. [29] modified the classical stress theory (MCST) by considering that the density of strain energy is a function of the strain and curvature tensors.
Several literatures [30–34] investigated the size effect on the behavior of beam. Ansari et al. [35] investigated the sizedependent resonant frequency and flexural sensitivity of AFM based on the modified strain gradient theory by using the force gradient method. However, it is well known that the interpretation of frequency shift by using the force gradient method is unsatisfactory [4].
So far, no literature is devoted to investigate the transient behavior of an AFM probe subjected to multimodes excitations based on the modified couple stress theory. In this study, the mathematical model of AFM probe subjected to twomode excitation and the van der Waals force based on the modified couple stress theory is constructed. The semianalytical method is presented. The effects of several parameters on the transient behavior are investigated.
2. Dynamic System Subjected to MultiMode Excitation
2.1. Governing Equation and Boundary Conditions
Consider the transient response of atomic force microscopy excited at the root measuring the interatomic van der Waals force, as shown in Figure 1. The multimode harmonic excitation is presented, as shown in Figure 1. The governing equation isThe boundary conditions are as follows.
At x = 0:At x = L:where and denote the crosssectional area and the area moment of inertia, respectively. is the mass density per unit volume, the tip mass, the flexural displacement, Young's modulus, the coordinate along the beam, the length of the beam, and the material length scale parameter. The van der Waals force [11] isin which is the Hamaker constant, the tipsurface distance, and the tip radius.
In terms of the following dimensionless quantities:where is the characteristic length. A small value of is introduced to avoid the numerical transaction error. The dimensionless governing differential equation of the system isThe associated boundary conditions are as follows.
At = 0:
At = 1:
The initial conditions are
2.2. Solution Method
Because the system composed of (8)(13) is nonlinear and the boundary condition is timedependent, it is difficult to directly solve. The semianalytical method is presented here.
2.2.1. Linearization of System
Due to its complexity, the nonlinear general system is approximated by infinite linearized subsystems. At first, the time variable is divided into infinite sections and the dynamic performance of the system is derived step by step. The methodology is described here. The tip displacement is presented by the Taylor series aswhere is an unknown transaction difference to be determined. Based on this relation and the following methods one can determine the displacement . Moreover, the error of the tip displacement must approach zero:Substituting (14) into the nonlinear nonhomogeneous boundary condition (12), the linearized timedependent boundary condition is obtained:So far, the nonlinear system is linearized in each time domain. The approximated system can be exactly solved by the solution method presented in the following sections. Therefore, the overall displacement can be derived step by step.
So far, the linearized subsystem in the time domain can be expressed as follows:The associated timedependent boundary conditions are as follows.
At = 0:
At = 1:
2.2.2. Solution Method of Linearized Subsystem
(A) Transform of Variable. The linearized subsystem includes two nonhomogeneous timedependent boundary conditions. Solving this subsystem, the following transformation of variable is considered:
Substituting relation (22) into (17)(21), two following subsystems are obtained. The first subsystem is expressed in terms of the transformed variable as follows: the transformed governing equation isThe associated boundary conditions are as follows.
At ξ = 0:
At = 1:The corresponding initial conditions areThe second subsystem is expressed in terms of the shifting function . The second transformed governing equation isAt = 0:At = 1:The third subsystem is expressed in terms of the shifting function . The third transformed governing equation isAt = 0:At = 1:The solutions of the two shifting functions are easily discovered as follows:
(B) Solution of the First Subsystem
(B1) Orthogonality of Eigenfunctions. The frequency equation of the first subsystem composed of (23)(27) iswhere . The n mode shapes are which is the same as that given by Rao [36]. The orthogonality of the eigenfunctions is presented as follows:
(B2) Modes Superposition Method. Based on the orthogonality conditions (44), the mode superposition method is used to derive the solution of the first subsystem composed of (23)(27). The transformed variable is expressed in terms of the eigenfunctionsSubstituting (45) into (23) and multiplying it by and integrating it from 0 to 1, one obtains wherein whichThe solution of (40) isSubstituting (48a) and (48b) back into (22) results in the displacement of the cantilever. It should be noted that the tip displacement must satisfy condition (15).
2.3. Numerical Results of Transient Response and Discussion
The influences of the excitation frequency , the excitation amplitude , and tipsample distance on the tip transient response are investigated and shown in Figures 2(a) and 2(b). Figure 2(a) demonstrates that the interacting distance is nm. The initial displacement and velocity are zero and are listed in (13). One has the natural frequency of the cantilever without the interacting force, f_{1} = 53525.9 Hz. When the frequency of root excitation is far from the natural frequency such as 48000 and 60000 Hz the small vibration of the cantilever is harmonically excited. When the frequency of excitation such as 53000 Hz approaches the natural frequency f_{1}, the vibration response is significantly increased. It is obvious that the frequency shift occurs due to the interacting force. Figure 2(b) demonstrates that the interacting distance is D_{0}=2 nm. When the frequency of excitation is 52000 Hz, the vibration response is significantly increased. It is obvious from Figures 2(a) and 2(b) that the smaller the interacting distance D_{0} is, the greater the frequency shift is.
(a)
(b)
Figure 3 demonstrates the influence of the distance between the tip and a sample surface on the tip response spectrum. It is shown that the smaller the interacting distance is, the larger the resonance frequency shift is.
Figures 4(a) and 4(b) demonstrate the influence of the size parameter on the tip response spectrum for D_{0}= 3, 5 nm. It is observed that the larger the size parameter is, the greater the frequency shift is. Moreover, the smaller the interacting distance D_{0} is, the greater the frequency shift is.
(a)
(b)
Further, the influence of two excitation modes on the tip response spectrum is investigated; Figure 5 demonstrates that if the second frequency of excitation = 76500 Hz is far from the natural frequency f_{1} = 53525.9 Hz, the response spectrum is as shown Figures 3 and 4 with onemode excitation. Moreover, the influence of the interacting distance D_{0} on the response spectrum is slight. However, if the second frequency of excitation = 56500 Hz is close to the natural frequency f_{1} = 53525.9 Hz, several resonant responses occur. Moreover, the influence of the interacting distance D_{0} on the response spectrum is significant. It is concluded that the response spectrum of twomode excitations is significantly different to that of onemode excitation.
3. Prediction of Natural Frequencies
3.1. The System with the Size Effect
The dimensionless governing equation and boundary conditions areThe clamped boundary conditions are as follows.
At = 0:
At = 1:where and , and the tipsurface distance is .
(A) Solution Method. AssumeSubstituting (54) into (49), one obtainswhere . Substituting (54) into the boundary conditions (50)(52), one obtains the following.
At = 0:
At = 1:
Substituting (54) into the boundary conditions (53) and multiplying it by and integrating it from 0 to the period T, , (53) becomeswhere . Further, the semianalytical solution can be determined by using the method by Lin [2].
3.2. The Classical System without the Size Effect
The dimensionless governing equation and boundary conditions areThe clamped boundary conditions are as follows.
At = 0:
At = 1:where and , and the tipsurface distance is .
(A) Solution Method. AssumeSubstituting (65) into (60), one obtainsSubstituting (65) into the boundary conditions (61)(63), one obtains the following.
At = 0:
At = 1:Substituting (65) into the boundary conditions (64) and multiplying it by and integrating it from 0 to the period T, , (64) becomesFurther, the semianalytical solution can be determined by using the method by Lin [2].
3.3. Relation between Natural Frequencies and Size Parameter
It is discovered that if the following relations are satisfiedthe system composed of (55)(59) is similar to that composed of (66)(70) and .
In other words, if the parameters of the two systems are the same, the tip radius relation and , the relation of natural frequencies (71) must be satisfied.
Tables 1 and 2 demonstrate the comparison of the first two natural frequencies determined by using the analytical method [2, 4] and formula (71). It is found that these results are significantly consistent. When the interacting distance is large, the results are the same. When the interacting distance D_{0}=5nm, the errors of the first two frequencies are 0.08% and 0.002%, respectively.


4. Conclusion
The transient response of AFM probe subjected to twomode excitations based on the modified couple stress theory is investigated. The semianalytical solution is presented. The similarity of the classical BernoulliEuler beam system and the system based on the modified couple stress theory is discovered. The relation between the natural frequencies of the two systems is . Several trends about the response spectrum due to twomode excitations are obtained as follows.(1)The response spectrum of twomode excitations is significantly different to that of onemode excitation. If the two frequencies of excitation are close to the natural frequency, several resonant responses occur.(2)The larger the size parameter is, the greater the frequency shift is.(3)The smaller the interacting distance is, the greater the frequency shift is.
Data Availability
All the data used to support the findings of this article are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The support of the Ministry of Science and Technology of Taiwan, R. O. C., is gratefully acknowledged (Grant number: MOST 1062221E168005).
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Copyright © 2019 ShueeiMuh Lin and ChingYao Chang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.