Dynamic wetting property investigation of AFM tips in micro/nanoscale

Transcription

1 Dynamc wettng property nvestgaton of AFM tps n mcro/nanoscale The wettng propertes of AFM probe tps are of concern n AFM tp related force measurement, fabrcaton, and manpulaton technques, such as dp-pen nanolthography, nano-dspensng, mcro/nanomanpulaton, and nanotrbologcal nvestgaton, and even basc magng. In these applcatons, the menscus force often domnates the adheson force. As a result, the wettng propertes of cantlever tps play crucal roles n these operatons and may ntroduce artfacts nto the measurement results. Due to ther geometrcal complexty and extremely small sze, there s no drect measurement for AFM tp wettng propertes. In ths study, a novel method s proposed to drectly measure AFM tp wettng propertes at the nanoscale based on the mcro- Wlhelmy balance method. The dynamc contact angles of AFM tps are obtaned through capllary force measurement durng extenson and retracton moton of the AFM tps relatve to nanobubbles usng the nano-wlhelmy balance method. To employ the nano-wlhelmy balance method to AFM tps, the capllary force and the three phase contact lnes need to be accurately measured durng tp-nanobubble nteracton. In ths study, the capllary force measurement s acheved through tp-bubble nteracton, as shown n Fg. The whole nteracton s dvded nto two regons: pyramdal nteracton regon and sphercal nteracton regon. Then the cal forces appled on AFM tps can be gven as: F = 4 = γ t h( z)cos( β + θ ) () where γ s the lqud vapor surface tenson of water, θ s the contact angle, m s the length of the three phase contact lne on the th ( =, 2, 3, and 4) sde wall of the tp, τ s constant for th sde wall of the tp, h (z) s ndentaton depth and β s the angle of the sde wall relatve to the cal drecton.

2 Fg. Schematc of AFM tp-nanobubble nteracton durng extenson (a) and retracton (b) of the AFM scanner. The nteracton s dvded nto two regons, sphercal nteracton regon and pyramdal nteracton regon (c). In the pyramdal nteracton regon, the surface tenson length vares lnearly wth ndentaton depth; and the coeffcent τ can be measured wth much hgher accuracy. Gven θ, β, and γ are constant, the cal force appled to AFM tps durng extenson moton s dfferent from that of the retracton moton due to the exstence of contact angle hysteress. We have: and df dh df dh adv 4 = ( z) = rec 4 = ( z) = γ t cos( β + θ ) (2) adv γ t cos( β + θ ). (3) where θ adv and θ rec are the advancng and recedng contact angles, respectvely. From Eq. (2) and Eq. (3), one can expect that durng both extenson and retracton moton of the AFM scanner, the capllary force should lnearly ncrease wth ndentaton depth. Snce θ adv s larger than θ rec, df adv /dh (z) should be smaller than df rec /dh (z). Moreover, the dynamc contact angles θ adv and θ rec can be obtaned by recordng the capllary force durng tp-nanobubble nteracton. rec

3 (b) Illustraton of tp-bubble nteracton at dfferent cal postons Fg 2. Tp-nanobubble nteracton. (a) A typcal force dstance curve measured on a nanobubble. The whole process s dvded nto seven sectons. (b) Illustraton of the seven sectons. The force dstance curve clearly shows the pyramdal nteracton regon. A typcal force dstance curve obtaned on a nanobubble s shown n Fg 2a. The whole tp-nanobubble nteracton can be dvded nto seven sectons (Fg 2b). In the extenson moton of the AFM scanner, the tp approaches the nanobubble. Before the tp gets contact wth the nanobubble, there s no nteracton force between them (ponts A B). At pont B, the AFM tp contacts the nanobubble, and a menscus brdge forms between the tp and the nanobubble. The tp s rapdly drawn nto the nanobubble due to the capllary force (ponts B C), enterng the pyramdal tp-nanobubble nteracton regon. The force lnearly ncreases wth decreasng pezo cal poston (ponts C D). After pont D, the tp s frst attracted to the sample surface due to electrostatc forces and Van der Waals forces. Then, the tp contacts the sold sample surface, resultng n a rapd lnear ncrease of cantlever deflecton (F G).

4 In the retracton moton, the tp reenters the pyramdal tp-nanobubble nteracton regon (H I). Ths s a result of decreasng length of the three phase contact lne and decreasng nteracton force. Durng retracton, the contact angle s an advancng contact angle. The observed slope of cantlever deflecton v.s. pezo cal poston n secton 5 s lower than that n secton 3, whch s consstent wth the derved mathematc model gven n Eq. (2) and (3). At pont I, the AFM tp enters the sphercal contact regon (J K). In ths secton, the three phase contact lne retreats from the boundary between the pyramdal and sphercal regons, enterng the sphercal regon, untl the menscus breaks and the cantlever deflecton rapdly goes to zero (L M). Two other parameters can be extracted from Fg 2. The dstance D hgt, the heght of the nanobubble, s the cal dsplacement the scanner travels from pont B to pont F (or H). The dstance D adh, the cal dstance between ponts B and L, s related to the maxmum adhesve force between the cantlever tp and the nanobubble. Force volume mode scannng s used. In the force volume mode, the AFM cantlevers scan sample surfaces wth fxed step szes along x and y axes. The result s shown n Fg 3. It shows that D hgt ncreases wth nanobubble heght, followng the profle of the nanobubble. D adh nversely changes wth nanobubble heght. Wth the forcedstance curves over nanobubble surfaces, the dynamc contact angles for each AFM probe can be obtaned wth force dstance curves obtaned near the nanobubble apex usng Eqs. (2) and (3). The advancng contact angle θ adv and recedng contact angle θ rec for the slcon cantlever (RFESP) are 6.8±.6 and 44.3±0.7, respectvely. For the slcon ntrde cantlever (ORC8), the calculated values of θ adv and θ rec are 46.3±. and 42.0±0.7, respectvely.

5 Fg 3. Constructed maps of D hgt (a) and D adh (b) wth force volume measurement of force dstance curves. Yulang Wang, assocate professor, school of mechancal engneerng and automaton, Behang Unversty, E-mal: wangyulang@buaa.edu.cn References [] Yulang Wang*, Humn Wang, Shusheng B, and Bn Guo, Nano-Wlhelmy nvestgaton of dynamc wettng propertes of AFM tps through tp-nanobubble nteracton. Scentfc Reports, 206, 6:3002. [2] Dayong L, Yulang Wang*, Yunlu Pan, Xuezeng Zhao Measurements of slp length for flows over graphte surface wth gas domans, Appled Physcs Letters, 206, 09:5602. [3] Yulang Wang*, Xaola L, Shusheng B, Xaofeng Zhu, and Jnhua Lu, 3D mcro-partcle mage modelng and ts applcaton n measurement resoluton nvestgaton for vsual sensng based axal localzaton n an optcal mcroscope, Measurement Scence and Technology, In press.