The Interaction Bewteen AFM Probe And The Water/Gas Interface Of Nanobubbles

Nanobubble, the nanoscale gas bubble existed at the water/solid interface, is animportant issue in the field of interfacial physics and chemistry, and has been amystery for decades after the first proposal by Parker et al. to explain the astonishinglong attractive force when measuring the force curve between two solid surfacesimmersed in water solution by surface force apparatus (SFA). Later on, nanobubbleswere observed directly by tapping mode atomic force microscopy (AFM) and posed agreat challenge to the classical thermodynamics because of their unexpected longlifetime as well as the large contact angles. Nanobubbles were believed to play animportant role in many important processes such as protein folding, stability ofcolloids and emulsions, and boundary slip in flowing water, etc. Particularly, study onthe interaction between a nanobubble and a solid probe or a nanoparticle wasfundamentally important to understand the mysterious behavior of their water/gasinterface and strongly interesting to a variety of applications, such as froth flotationwhich is widely used for mineral concentration and water treatment.Interaction between a solid surface and a water/gas interface is a classical topicin colloidal and interfacial sciences, and before the nanobubble research, the study ofthe interaction between a macroscopic solid particle and a micron-sized gas bubblehas been carried out for many years. Conventionally, in order to obtain the basic lawof the interaction between a particle and a gas bubble, the direct measurement of theforce versus separation or as so called force curve is necessary. In the earlier years,SFA was the only technique used to measure the force curves between two solidsurfaces immersed in solutions. The emergence of AFM and the development of thecolloidal probe technique made it possible to study the interaction between a particleand a micron-sized gas bubble. During the last20years, a series of researches wereconducted by using this method and most of the studies focused on the forcemeasurements between the hydrophilic or hydrophobic particles and gas bubbles inaqueous solutions. For example, the interaction between a spherical glass particle anda micron-sized gas bubble was measured with the AFM setup by Butt et al. The forcesbetween a carbon sphere and highly ordered pyrolytic graphite (HOPG) surface weremeasured in ethanol aqueous solutions by Zhang et al.However, measuring the interactions between a nanoscopic probe and thewater/gas interface of a nanobubble is more difficult. For example, unlike that in the experiment on micron-sized gas bubbles, nanobubbles are too small to be observed byan optical microscopy, thus it is very hard to locate the AFM probe right on the top ofnanobubbles and then take a direct force curve measurement. By using force volumemode of AFM, Zhang et al. were able to obtain the force curve on nanobubbles tomeasure their interfacial tensions in Tween20solution. However, in pure water theycould not determine the surface tension of nanobubbles because the loaded force inforce volume mode could not be held enough smaller easily and the probe wouldusually penetrate deeply into the water/gas interface. Besides, force volume mode ofAFM is very slow and the thermal drift usually results in a difficulty to make anaccurate measurement. Recently, PeakForce Quantitative Nano-Mechanics(PF-QNM), a new AFM mode based on the force feedback signals, has beendeveloped by which the force-distance curves between the probe and samples can beeasily captured. PF-QNM has been widely used for measuring the mechanicalproperties of various samples including the stiffness of nanobubbles.A phenomenon was frequently found as so called “snap-in” in the force curvemeasurement based on PF-QNM in our previous work: an attractive force uponapproach which increased in gradient suddenly at a large separation (20-30nm). Thislarge attractive “snap-in” is hard to be understood since the AFM probe used in theexperiments was made of Si3N4which was hydrophilic. In water, Si3N4surface isslightly negatively charged. Since the water/gas interface is usually believednegatively charged too, a small repulsion but not a large attraction should be expectedwhen a Si3N4probe approaching the nanobubbles. In our case, the “snap-in” oftenoccurred in most experiments but sometimes disappeared. Interestingly, we noted thatthe “snap-in” phenomenon was also observed in the studies of the force curvebetween a hydrophilic particle and a micron-sized gas bubble, which puzzled theresearchers for many years and caused huge debates. For example, Ducker et al. foundthe “snap-in” frequently occurred when they measured the force curve using thecolloid probe technique. But Fielden et al. studied the interaction between hydrophilicsilica particles and air bubbles in water using the similar colloid probe technique andonly found the monotonically repulsive force during approaching. Later Butt et al.measured the force between colloidal probes and micron-sized gas bubble and foundthe “snap-in” and adhesion force in a series of experiments. Teschkea et al. alsostudied force curves between the probe with~5nm radius of curvature and~1mmradius air bubbles surfaces in water and found both the attractive forces and repulsive forces in their experiments. Also, other groups did the same measurements in purewater or electrolyte solutions and found mainly the occurrence of no “snap-in” inthe force curves. Finally, Englert et al. concluded that under very clean conditions,they only observed a monotonic repulsive force between the bubble and the particle,without any “jump-into-contact” or adhesion.Since the force curve is very fundamental for the study of the interactionbetween a nanoscopic probe and a nanobubble, any confusion on the force curveshould be figured out before the further exploration. For example, the “snap-in” inforce curve would influence the setpoint in PF-QNM imaging and cause theuncertainty of the loaded forces. More seriously, if the attractive force exists, it willmake the feedback unstable in tapping mode of AFM and usually result in a falsecontrast. This would raise arguments on the height measurement of nanobubbleswhich is a key parameter to determine their contact angles. In this paper, we focusedon the investigation of the application of PF-QNM on nanobubble research. The newpreparation method of nanobubbles have been investigated, this method providedsupplementary preparation methods of nanobubbles without the ethonal exchange step.Established the nanobubble exprimental methods of PF-QNM and investigated theorigin of the “snap-in” in the force curve. After a series of experiments with control,we found that the origin of the “snap-in” was mainly attributed to the hydrophobiccontamination on the probes which usually happened during AFM measurements.By using PF-QNM, the origin of “snap-in” in the force curve between atomicforce microscopy (AFM) probe and the water/gas interface of nanobubbles has beeninvestigated. The “snap-in” frequently happened when the probes were preserved fora certain time or after being used for imaging solid surfaces under the atmosphericconditions. In contrast, imaging in liquids rarely induced the “snap-in”. After a seriesof control experiments, it was found that the “snap-in” was attributed to thehydrophobic interaction between the water/gas interface and the AFM probe, whichwas either modified or contaminated by hydrophobic materials. The hydrophobiccontamination could be efficiently removed by conventional plasma cleaningtreatment to prevent the occurring of the “snap-in”. On the other hand, the adsorptionof sodium dodecyl sulfate onto a nanobubble surface changed the water/gas interfaceinto hydrophilic, which also eliminated the “snap-in” phenomenon.In summary, it is the first time that the nanobubble was prepared by0℃waterdroped on HOPG and the PF-QNM was used to investigated the interaction between nanobubbles and AFM probe. By using PF-QNM the origin of “snap-in” wasattributed to the hydrophobic interaction between the water/gas interface and theAFM probe and the “snap-in” can be eliminated by plasma cleaning treatment.