Abstract:
A method and an apparatus for detecting a normal force component and a friction force component between a probe and a sample substance using an interfacial force microscope is disclosed herein. According to one embodiment, a method of measuring normal and friction forces with an interfacial force microscope includes positioning a sample substance on a piezotube and in proximity to a probe suspended from a cantilever such that a molecular force between the sample substance and the probe causes the cantilever to deflect. The method may include converting the deflection of the cantilever into an electrical signal comprising a normal force and a friction force component, and measuring the normal and friction force components.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Non-Provisional application Ser. No. 12/757,542, titled CANTILEVER-BASED OPTICAL INTERFACE FORCE MICROSCOPE, filed on Apr. 9, 2010, which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to interfacial force microscopy and more specifically to a cantilever-based optical interfacial force microscope. 
       BACKGROUND 
       [0003]    Traditional microscope systems are generally unable to measure intermolecular interactions accurately and cost effectively. One type of microscope system is the atomic force microscope (AFM), which has been used to image and/or measure the topography of various surfaces. AFM&#39;s, however, suffer from a mechanical instability that prevents the accurate measurement of intermolecular interactions. In particular, AFM&#39;s are generally unable to control tip snap-in during tip approach and/or tip snap-off during tip retraction. As a result, AFM&#39;s are generally unable to detect intermediate states of various intermolecular interactions such as, for example, the capillary forces between two silicon surfaces. 
         [0004]    Another type of microscope system is the interfacial force microscope (IFM). Traditional IFM&#39;s use an electrical detection process to measure various surface phenomena. IFM&#39;s, however, have not been widely used due to the low sensitivity and technical complexity of their electrical detection process. Thus, traditional microscope systems have generally been unable to measure intermolecular interactions accurately and cost effectively. 
       SUMMARY 
       [0005]    In accordance with the present disclosure, the disadvantages and problems associated with prior microscope systems have been substantially reduced or eliminated. 
         [0006]    In some embodiments, an apparatus comprises an optical detector configured to detect an optical beam reflected from a cantilever. The apparatus may further comprise an optical fiber probe suspended from the cantilever. The apparatus may further comprise a piezotube configured to move a sample substance in proximity to the optical fiber probe, the cantilever configured to deflect in response to an interfacial force between the sample substance and the optical fiber probe. The apparatus may further comprise a feedback controller communicatively coupled to the optical detector and a semiconductive circuit element abutting at least one surface of the cantilever. In response to detecting a movement of the optical beam reflected from the cantilever, the feedback controller may apply a voltage to the semiconductive circuit element, which may cause the semiconductive circuit element to reduce deflection of the cantilever. The voltage applied by the feedback controller may indicate the strength of the interfacial force between the sample substance and the optical fiber probe. 
         [0007]    In other embodiments, a method comprises positioning a sample substance in proximity to an optical fiber probe suspended from a cantilever. An interfacial force between the sample substance and the optical fiber probe may cause the cantilever to deflect. The method may further comprise detecting an optical beam reflected from the cantilever and, in response to a movement of the optical beam reflected from the cantilever, applying a voltage to a semiconductive circuit element abutting at least one surface of the cantilever. In response to the voltage, the semiconductive circuit element may reduce deflection of the cantilever. The voltage may indicate the strength of the interfacial force between the sample substance and the optical fiber probe. 
         [0008]    In yet other embodiments, an apparatus comprises an optical detector configured to detect an optical beam reflected from a cantilever. The apparatus may further comprise a probe suspended from the cantilever and a tray configured to (i) laterally modulate a sample substance in an x-axis direction in relation to the probe, and (ii) move the sample substance in a z-axis direction in relation to the probe. The apparatus may further comprise a feedback controller communicatively coupled to the optical detector and a semiconductive circuit element abutting at least one surface of the cantilever. The cantilever may be configured to deflect in response to a normal interfacial force and a lateral friction force between the sample substance and the optical fiber probe. In response to detecting a movement of the optical beam reflected from the cantilever, the feedback controller may be configured to apply a voltage to the semiconductive circuit element. In response to the voltage from the feedback controller, the semiconductive circuit element may be configured to reduce deflection of the cantilever. The voltage applied by the feedback controller may indicate the strength of the normal interfacial force and the lateral friction force between the sample substance and the optical fiber probe. 
         [0009]    In one embodiment, a method of measuring normal and friction forces with an interfacial force microscope includes positioning a sample substance on a piezotube such that the sample substance may be positioned in proximity to a probe suspended from a cantilever. Furthermore, a molecular force between the sample substance and the probe may cause the cantilever to deflect. The method may further include engendering lateral modulation and vertical movement of the piezotube relative to the probe. The method may include detecting cantilever deflection and converting the cantilever deflection into an electrical signal. The method may further include measuring both an AC and a DC component of the electrical signal and converting the AC and DC components into a friction force value and a normal force value, respectively. 
         [0010]    In another embodiment, an interfacial force microscope includes a piezotube, a cantilever comprising a probe and being configured to be in proximity to the piezotube, a detector configured to detect deflection of the cantilever, and a feedback loop coupled between the detector on the one hand, and the cantilever and a lock-in amplifier on the other hand. The piezotube may be configured to move a sample substance vertically and horizontally. The cantilever may be configured to deflect in response to a molecular force acting between a sample substance on the piezotube and the probe. The detector may be configured to convert the detection of the cantilever into an electrical signal, wherein the electrical signal may comprise a normal force component and a friction force component. The lock-in amplifier may be operably connected to the piezotube. 
         [0011]    The present disclosure provides various technical advantages. Various embodiments may have none, some, or all of these advantages. One advantage is that a cantilever-based optical interfacial force microscope (COIFM) may employ an optical detection technique and a feedback loop to self-balance a cantilever configured to sense interfacial forces in a sample substance. The configuration of the feedback loop and cantilever may provide enhanced sensitivity of the COIFM to interfacial forces. 
         [0012]    Another advantage is that the COIFM may comprise a cantilever with an optical fiber probe to measure interfacial forces in a liquid environment. The optical fiber probe may have a sufficient length to allow the free end of the optical fiber probe to penetrate a fluid surrounding a sample substance while the cantilever remains suspended above the fluid. By keeping the cantilever suspended above the fluid, the COIFM may prevent the electrical signals of the feedback loop from affecting the interfacial interactions between the probe and the sample substance. Thus, the COIFM may obtain accurate measurements of intermolecular interactions in a liquid environment. 
         [0013]    Yet another advantage is that the COIFM may be configured to laterally modulate a sample substance (e.g., water) while measuring interfacial forces. In some embodiments, the COIFM may measure the normal forces and/or friction forces caused by interfacial liquid structures in ambient environments. Understanding such forces may permit the design of micro-electro-mechanical system (MEMS) devices that reliably operate in humid and/or wet environments. 
         [0014]    Other advantages of the present disclosure will be readily apparent to one skilled in the art from the description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which: 
           [0016]      FIG. 1  illustrates a cantilever-based optical interfacial force microscope (COIFM), according to certain embodiments; 
           [0017]      FIGS. 2A to 2D  are graphs that illustrate the relationships of example electrical signals in a COIFM, according to certain embodiments; 
           [0018]      FIGS. 3A and 3B  illustrate the formation of an optical fiber probe for a COIFM, according to certain embodiments; 
           [0019]      FIG. 4  illustrates a COIFM configured to analyze interfacial liquid structures by laterally modulating a sample substance, according to certain embodiments; and 
           [0020]      FIG. 5  illustrates the lateral modulation of a tray in a COIFM, according to certain embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]      FIG. 1  illustrates a cantilever-based optical interfacial force microscope (COIFM)  10 , according to certain embodiments. COIFM  10  may be configured to detect and/or measure the interfacial forces between molecules in a sample substance  12 . COIFM  10  may employ an optical detection technique and a feedback loop to self-balance a cantilever  14  that senses interfacial forces in the sample substance  12 . The configuration of the feedback loop and cantilever  14  may provide enhanced sensitivity of COIFM  10  to interfacial forces. In some embodiments, COIFM  10  may unveil structural and mechanical information regarding a sample substance  12  at the molecular level. COIFM  10  may comprise at least one light source  16 , cantilever  14 , optical detector  18 , feedback controller  20 , and piezotube  22 . 
         [0022]    Light source  16  may emit an optical beam  24  towards cantilever  14 . Optical beam  24  from light source  16  may reflect off at least one surface of cantilever  14 . Optical detector  18  may be positioned to receive optical beam  24  reflected from cantilever  14 . As cantilever  14  is deflected, causing the unsupported end of cantilever  14  to move in the z-axis direction, the angle of reflection of optical beam  24  may change. Based at least in part on the angle of reflection of optical beam  24  from cantilever  14 , COIFM  10  may determine the position of cantilever  14 . 
         [0023]    Light source  16  may comprise any suitable source of electromagnetic radiation. In some embodiments, light source  16  may comprise a laser such as, for example, a semiconductor laser, a solid state laser, a gas laser, a chemical laser, an excimer laser, and/or any suitable type of laser. In other embodiments, light source  16  may comprise a light-emitting diode and/or lamp emitting a low-divergence optical beam  24 . 
         [0024]    As noted above, light source  16  may emit optical beam  24  towards cantilever  14 . Cantilever  14  may comprise a linear member having a fixed end attached to a support  26  and a free end that is not attached to a support. In some embodiments, cantilever  14  may project horizontally from support  26 . The application of a force to the free end of cantilever  14  may cause the free end of cantilever  14  to move in the z-axis direction, resulting in deflection of cantilever  14 . The application of a force to the free end of cantilever  14  may cause a torque and/or stress (e.g., shear stress, compression, and/or tension) in one or more portions of cantilever  14 . In some embodiments, cantilever  14  may comprise a circuit element  28  communicatively coupled to a feedback controller  20  that prevents and/or reduces the deflection of cantilever  14 . 
         [0025]    Cantilever  14  may comprise any suitable type of structural member. In some embodiments, cantilever  14  may comprise a semiconductive material such as, for example, a doped and/or undoped silicon material. In particular embodiments, cantilever  14  may comprise phosphorus doped silicon and/or boron doped silicon. Cantilever  14  may have any suitable dimensions. In some embodiments, cantilever  14  has a length  30  from eighty (80) to one hundred and eighty (180) micrometers (μm). In particular embodiments, cantilever  14  has a length  30  from one hundred and twenty (120) to one hundred and thirty (130) μm. In some embodiments, cantilever  14  has a thickness  32  from two (2) to six (6) μm. In particular embodiments, cantilever  14  has a thickness  32  from three (3) to five (5) μm. In some embodiments, cantilever  14  has a width from forty (40) to seventy (70) μm. In particular embodiments, cantilever  14  has a width from fifty (50) to sixty (60) μm. 
         [0026]    As noted above, cantilever  14  may comprise circuit element  28  that is communicatively coupled to feedback controller  20 . In some embodiments, circuit element  28  comprises a semiconductor stack such as, for example, a zinc oxide stack. Circuit element  28  may be positioned near the fixed end (e.g., base) of cantilever  14 . In conjunction with cantilever  14 , circuit element  28  may act as a bimorph that controls (e.g., prevents and/or reduces) the vertical displacement of the free end of cantilever  14 . Feedback controller  20  may use circuit element  28  to provide voltage activated force feedback of cantilever  14 . In some embodiments, feedback controller  20  may use circuit element  28  for self-sensing of cantilever  14 , for statically deflecting and/or reducing deflection of the free end of cantilever  14 , and/or for oscillating and/or reducing oscillation of cantilever  14 . 
         [0027]    Cantilever  14  in COIFM  10  may be configured to measure intermolecular interactions for various sample substances. In some embodiments, cantilever  14  comprises a probe  36  affixed to the free end of cantilever  14 . A sample substance may be positioned on piezotube  22  in proximity to probe  36 . Intermolecular interactions between probe  36  and the sample substance  12  may exert a force on cantilever  14 , causing a slight deflection of cantilever  14 . Optical detector  18  may detect the deflection of cantilever  14 . In response to the deflection, feedback controller  20  may adjust the voltage  48  applied to circuit element  28  in order to reduce and/or prevent further deflection of cantilever  14 . Based on the voltage  48  required to prevent and/or reduce the deflection of cantilever  14 , COIFM  10  may determine the interfacial forces between probe  36  and the sample substance  12 . This information may be used to analyze characteristics of sample substances  12  such as, for example, interfacial adhesion, interfacial liquid structures, and/or measurements of chemical interactions. 
         [0028]    Probe  36  of cantilever  14  may be any suitable type of probe. In some embodiments, probe  36  may be a semiconductive tip that protrudes vertically from the free end of a horizontally positioned cantilever  14 . In such embodiments, probe  36  may be a pyramid-shaped tip that comprises a silicon material. The pyramid-shaped tip may resemble a spike and/or may have any suitable dimensions. For example, the pyramid-shaped tip may have a height from fifteen (15) to twenty (20) μm. 
         [0029]    In other embodiments, probe  36  may be an optical fiber probe  36 . The use of an optical fiber probe  36  may allow COIFM  10  to measure interfacial interactions in liquid environments. The optical fiber probe  36  may have a sufficient length  40  to allow the free end of the optical fiber probe  36  to penetrate a fluid  42  surrounding a sample substance  12  while cantilever  14  remains suspended above fluid  42 . By keeping cantilever  14  suspended above fluid  42 , COIFM  10  prevents the electrical signals of the force feedback loop from affecting the interfacial interactions between the optical fiber probe  36  and the sample substance  12 . In other words, by keeping cantilever  14  and force feedback loop isolated from fluid  42 , COIFM  10  may obtain accurate measurements of intermolecular interactions associated with the sample substance  12 . 
         [0030]    The optical fiber probe  36  may comprise any suitable type of optical fiber. For example, the optical fiber probe  36  may comprise a glass fiber, a plastic fiber, and/or any suitable type of optical fiber. One end of the optical fiber probe  36  may be affixed to cantilever  14  while the other end (i.e., the free end) of the optical fiber probe  36  is not affixed to any structure. The optical fiber probe  36  may be affixed to cantilever  14  using any suitable technique. For example, an end of the optical fiber probe  36  may be affixed to cantilever  14  with a thermosetting polymer such as, for example, epoxy. The optical fiber probe  36  may have any suitable dimensions. In some embodiments, the optical fiber probe  36  has a trunk diameter  44  from seventy (70) to one hundred and eighty (180) μm. In particular embodiments, the optical fiber probe  36  has a trunk diameter  44  from one hundred and twenty (120) to one hundred and thirty (130) μm. In some embodiments, the optical fiber probe  36  has a length  40  from one to two centimeters (cm). The free end of the optical fiber probe  36  may be sharpened to form a pointed end  46 . In some embodiments, the pointed end  46  of the optical fiber probe  36  has a diameter from fifty (50) to one hundred and fifty (150) nanometers (nm). In particular embodiments, the pointed end  46  of the optical fiber probe  36  has a diameter from eighty (80) to one hundred and twenty (120) nm. 
         [0031]    In some embodiments, probe  36  may comprise a wire having a sharpened tip. The tip of the wire may be sharpened according to any suitable technique such as, for example, chemical etching. Probe  36  may comprise any suitable type of wire. For example, probe  36  may comprise tungsten, titanium, chromium, and/or any suitable material. 
         [0032]    In some embodiments, probe  36  may be coated with one or more layers of material to insulate probe  36  from liquid. A coating may be deposited over probe  36 , cantilever  14 , and/or both probe  36  and cantilever  14 . The coating may prevent the electrical signals of the force feedback loop in COIFM  10  from affecting the interfacial interactions between probe  36  and the sample substance  12 . For example, where probe  36  is a pyramid-shaped silicon tip that extends from cantilever  14 , a coating on probe  36  and/or cantilever  14  may allow COIFM  10  to measure interfacial interactions in a liquid environment. To enhance the resolution and/or sensitivity of COIFM  10 , the coating may not cover the apex of the tip of probe  36 . The coating may comprise any suitable insulating material. For example, the coating may comprise an elastomer (e.g., silicone elastomer, polyisoprene, polyurethane, etc.), a polymer, a polyimide, and/or any suitable material. 
         [0033]    As noted above, interfacial forces between probe  36  and the sample substance  12  may cause some deflection of cantilever  14 , which may cause a change in the reflection of optical beam  24  from cantilever  14 . Optical detector  18  may detect the movement of optical beam  24  reflected from cantilever  14 . In some embodiments, optical detector  18  outputs to feedback controller  20  an electrical signal indicating the amount of deflection of cantilever  14 . Optical detector  18  may be any suitable device that senses the presence and/or movement of optical beam  24 . Optical detector  18  may comprise a transducer that converts an optical signal into an electrical signal. In some embodiments, optical detector  18  may comprise one or more laser detectors, photomultipliers, photodiodes, thermopile detectors, and/or pyroelectric energy detectors. 
         [0034]    Feedback controller  20  may receive from optical detector  18  an electrical signal that indicates the deflection of cantilever  14 . In response to the electrical signal, feedback controller  20  may adjust the voltage  48  applied to circuit element  28  on cantilever  14  in order to prevent and/or reduce the deflection of cantilever  14 . The voltage  48  that is output from feedback controller  20  may be based at least in part on a voltage  50  associated with a set point  51  and a voltage  52  from optical detector  18 . In some embodiments, feedback controller  20  may cause circuit element  28  to create a torque on cantilever  14  in order to achieve a zero error voltage  53 . 
         [0035]    Feedback controller  20  may comprise any suitable type of controller. For example, feedback controller  20  may be a digital controller, an analog controller, a linear gain controller, and/or a non-linear gain controller. In some embodiments, feedback controller  20  may be a proportional integral derivative (PID) controller. The voltage  48  required from feedback controller  20  to prevent and/or reduce the deflection of cantilever  14  may indicate the strength of the interfacial forces between the sample substance  12  and probe  36 . 
         [0036]    The sample substance  12  may be positioned on piezotube  22  in COIFM  10 . Piezotube  22  may be coupled to a z-axis controller  54  and/or an amplifier  60 , which may cause piezotube  22  to move the sample substance  12  closer to and/or further from probe  36 . Thus, piezotube  22  may move the sample substance  12  in the z-axis direction. The interfacial forces measured by COIFM  10  may depend on the distance between the free end of probe  36  and the sample substance  12 . 
         [0037]    Piezotube  22  may be any suitable type of piezoelectric actuator. Piezotube  22  may comprise a ceramic and/or crystalline material that, in response to an electric field, changes in size. This property may allow piezotube  22  to position the sample substance  12  with accuracy (e.g., better than micrometer precision) in relation to probe  36  in COIFM  10 . Piezotube  22  may be any suitable type of piezoelectric actuator such as, for example, a direct piezo actuator and/or an amplified piezo actuator. 
         [0038]    COIFM  10  may be configured to measure intermolecular interactions associated with any suitable type of sample substance  12 . For example, the sample substance  12  may comprise one or more biological substances such as, for example, proteins, ligands, cellular systems, and/or bacterial systems. As another example, sample substance  12  may comprise a liquid (e.g., water), which may allow COIFM  10  to measure interfacial fluid structures. As yet another example, sample substance  12  may be a solid, gaseous, and/or plasma substance. 
         [0039]    In operation, COIFM  10  may be used to measure intermolecular interactions in a sample substance  12 . The sample substance  12  may be positioned on piezotube  22  in COIFM  10 . Piezotube  22  may be positioned in proximity to probe  36  suspended from the free end of cantilever  14  in COIFM  10 . When COIFM  10  is activated, light source  16  may emit optical beam  24  towards cantilever  14 , which may reflect optical beam  24  towards optical detector  18 . 
         [0040]    COIFM  10  may actuate piezotube  22  in the z-axis direction such that the sample substance  12  on piezotube  22  moves closer to probe  36 . The interfacial forces between the molecules in the sample substance  12  and probe  36  may cause probe  36  to move closer to or further from the sample substance  12 , which may cause a slight deflection of cantilever  14 . The deflection of cantilever  14  may cause optical beam  24  reflected from cantilever  14  to move. The movement of optical beam  24  may be detected by optical sensor, which may, in response, transmit an electrical signal to feedback controller  20 . In response to the electrical signal from optical sensor, feedback controller  20  may apply a voltage  48  to circuit element  28  affixed to cantilever  14 . By applying a voltage  48  to circuit element  28 , feedback controller  20  may prevent and/or reduce the deflection of cantilever  14 . Based at least in part on the amount of voltage  48  required to prevent and/or reduce the deflection of cantilever  14 , COIFM  10  may determine and/or indicate the strength of the interfacial forces in the sample substance  12 . 
         [0041]      FIGS. 2A to 2D  are graphs that illustrate the relationship of example electrical signals in COIFM  10 , according to certain embodiments. The x-axis  202  of each graph represents time and the y-axis  204  of each graph represents a respective voltage in the feedback loop in COIFM  10 . Electrical signals in COIFM  10  may be adjusted to determine the time resolution of COIFM  10 . For example, as illustrated in  FIG. 2A , when the sample substance  12  is not in proximity to probe  36 , COIFM  10  may apply a square wave voltage with a particular amplitude (e.g., 0.2 V) and frequency (e.g., 10 Hz) to the set-point voltage (V set point )  206 . As illustrated in  FIG. 2B , feedback controller  20  may be operable to configure the preamp output (V A-B )  208  to follow the square wave by applying appropriate voltages (V stack )  210  to circuit element  28  affixed to cantilever  14 . The square wave may cause circuit element  28  to create a torque on cantilever  14  in order to achieve a zero error voltage (V error ))  212 , as illustrated in  FIG. 2C . Thus, feedback controller  20  may be configured to optimize the transient response to achieve the appropriate time response for COIFM  10 . As illustrated in  FIG. 2D , COIFM  10  may, in some embodiments, have a practical time resolution that is between one and two milliseconds (ms). 
         [0042]    Although particular voltage levels and time resolutions are described above, it should be understood that COIFM  10  may be configured to operate with any suitable voltage levels and time resolutions. 
         [0043]      FIGS. 3A and 3B  illustrate the formation of an optical fiber probe  36  for COIFM  10 , according to certain embodiments. In some embodiments, a pointed end  46  may be formed on the optical fiber probe  36  by an acid etching technique. 
         [0044]    As noted above, an optical fiber probe  36  may comprise any suitable type of optical fiber. In some embodiments, the optical fiber used to form the optical fiber probe  36  may be uncoated. In other embodiment, a coated optical fiber may be selected, and the coating may then be stripped from at least a portion of the optical fiber. The coating of the optical fiber may be removed by any suitable technique such as, for example, by using a wire stripping device. 
         [0045]    The optical fiber may have any suitable trunk diameter  44 . In some embodiments, the trunk diameter  44  of the uncoated optical fiber may be from seventy (70) to one hundred and eighty (180) μm. In particular embodiments, the trunk diameter  44  may be from one hundred and twenty (120) to one hundred and thirty (130) μm. 
         [0046]    To form a pointed end  46  on the optical fiber probe  36 , an uncoated optical fiber may be positioned vertically in a container  62 . Container  62  may be any suitable type of container such as, for example, an acid resistant beaker. Once the optical fiber probe  36  is positioned in container  62 , an acid  64  may be added to container  62 . A sufficient quantity of acid  64  may be added such that acid  64  immerses the free end of the optical fiber probe  36 . Acid  64  may be any suitable type of acid  64  such as, for example, a monoprotic acid and/or a polyprotic acid. In some embodiments, acid  64  may be a mineral acid, a sulfonic acid, and/or a carboxylic acid. In particular embodiments, acid  64  may be a hydrofluoric acid and/or a hydrochloric acid. 
         [0047]    After acid  64  is added to container  62 , a solvent  66  may be added to container  62 . Solvent  66  may be less dense and/or immiscible in acid  64 . Consequently, solvent  66  may form a separate layer of fluid over acid  64 . The layer of solvent  66  may serve as a protective barrier to the optical fiber probe  36  so that only a controlled portion of the optical fiber probe  36  is dissolved and/or sharpened by acid  64 . 
         [0048]    Solvent  66  may be any suitable type of solvent  66  that is less dense than acid  64  and/or immiscible in acid  64 . For example, solvent  66  may be an aromatic hydrocarbon such as, for example, toluene and/or benzene. As another example, solvent  66  may be hexane and/or cyclohexane. 
         [0049]    In some embodiments, acid  64  in container  62  may form a meniscus  68  on the optical fiber probe  36 . Meniscus  68  may recede as acid  64  dissolves the material (e.g., glass) in the optical fiber. Due to the formation of meniscus  68 , more material (e.g., glass) may be dissolved at the immersed (e.g., distal) end of the optical fiber, which may result in the continuous narrowing of the optical fiber to create a pointed end  46 . The pointed end  46  of the optical fiber may have any suitable diameter  70 . In some embodiments, the diameter  70  of the pointed end  46  may be from fifty (50) to one hundred and fifty (150) nm. 
         [0050]    The optical fiber probe  36  may be left in container  62  for any suitable period of time (e.g., sixty minutes, ninety minutes, etc.) to form the pointed end  46 . Once the pointed end  46  is formed, the optical fiber probe  36  may be removed from container  62  and cleaned. In some embodiments, the pointed end  46  of the optical fiber probe  36  may be polished and/or annealed. Annealing may align the molecules in the pointed end  46  of the optical fiber probe  36  to enhance the accuracy of measurements by COIFM  10 . 
         [0051]    Although an acid etching technique is described above, it should be understood that any suitable technique may be used to form the pointed end  46  on the optical fiber probe  36 . For example, the pointed end  46  on the optical fiber probe  36  may be formed by milling, dry etching, vapor etching, and/or any suitable technique. In some embodiments, the pointed end  46  may be formed on the optical fiber probe  36  by thermal heating of the optical fiber with a laser (e.g., a carbon dioxide laser). In other embodiments, the pointed end  46  may be formed on the optical fiber probe  36  by resistive heating. 
         [0052]    In some embodiments, COIFM  10  may be used to analyze interfacial liquid structures in an ambient environment. To analyze interfacial liquid structures, COIFM  10  may measure the normal force and/or the friction force between probe  36  in COIFM  10  and the sample substance  12 . Measuring the normal force may permit COIFM  10  to monitor the adhesion between probe  36  and the sample substance  12 . Measuring the friction force may allow COIFM  10  to monitor the ordering of molecules in the sample substance  12 . In some embodiments, the friction force may be measured by laterally modulating the sample substance  12  as it is brought into proximity with probe  36 . 
         [0053]      FIG. 4  illustrates COIFM  10  configured to analyze interfacial liquid structures by laterally modulating the sample substance  12 , according to certain embodiments. COIFM  10  may comprise light source  16 , cantilever  14 , optical detector  18 , feedback controller  20 , and piezotube  22 , as described above with respect to  FIG. 1 . COIFM  10  may further comprise a lateral modulator  72  and lock-in amplifier  74  communicatively coupled to piezotube  22  and feedback controller  20 . 
         [0054]    Lateral modulator  72  may be operable to modulate piezotube  22  in the x-axis and/or y-axis directions (also called lateral modulation). Lateral modulator  72  may comprise a voltage supply that is configured to actuate the modulation of piezotube  22 . Lateral modulator  72  may be any suitable modulator such as, for example, a piezoelectric actuator. For example, piezotube  22  may comprise a ceramic structure that contracts and/or expands in the x-axis and/or y-axis directions in response to a voltage applied by the voltage supply in lateral modulator  72 . 
         [0055]    Lateral modulator  72  may be communicatively coupled to lock-in amplifier  74 . Lock-in amplifier  74  may be operable to detect and/or measure the lateral modulation of piezotube  22 . Lock-in amplifier  74 , which may act as a homodyne with a low pass filter, may be operable to extract a signal with a known carrier wave from a noisy environment. Lock-in amplifier  74  may be operable to convert the phase (and/or related information such as in-phase and quadrature components) and amplitude of the extracted signal into a time-varying, low-frequency voltage signal. In some embodiments, lock-in amplifier  74  may be configured to measure phase shift associated with the extracted signal. 
         [0056]    In operation, COIFM  10  may laterally modulate the sample substance  12  to gather information regarding interfacial liquid structures in the sample substance  12 . In some embodiments, a sample substance  12  (e.g., a fluid) may be deposited on piezotube  22 . COIFM  10  may then establish a feedback loop between optical detector  18  and circuit element  28  on cantilever  14 . Piezotube  22  may then be actuated in the z-axis direction (i.e., vertically) such that the sample substance  12  is brought near to and/or in contact with the free end of probe  36  in COIFM  10 . As the sample substance  12  is brought into proximity with probe  36 , adhesion forces between the sample substance  12  and probe  36  may cause cantilever  14  to deflect. Optical detector  18  may detect the deflection of cantilever  14 . Based on signals from optical detector  18  and feedback controller  20 , COIFM  10  may measure the adhesions forces between the sample substance  12  and probe  36 . 
         [0057]    For example, and as discussed above, the feedback loop may receive an electrical signal  53  related to the deflection of the cantilever  14 , and the electrical signal  53  may comprise a normal force component and a friction force component. In one embodiment, the normal force component may comprise a DC component of the electrical signal  53 , and the friction force component may comprise an AC component of the electrical signal  53 . The normal force component may be measured at the feedback controller  20 , while the friction force component may be measured at the lock-in amplifier  74 . In some cases, the normal and friction force components may be measured concurrently. For instance, z-axis controller  54  and amplifier  60  may engender movement of piezotube  22  in the z- or vertical axis, while lock-in amplifier  74  and lateral modulator  72  may engender lateral modulation of piezotube  22 . In response to the vertical and lateral modulation of the piezotube  22 , and as a result of molecular force acting between a sample  12  placed on the piezotube  22  and probe  36  of the cantilever  14 , the cantilever  14  may deflect. As cantilever  14  deflects, optical detector  18  may detect the deflection of the cantilever  14  and convert the deflection into an electrical signal  52 . The electrical signal  52  may be compared with an electrical signal  50  from a set point  51  to yield an electrical signal  53 . Feedback controller  20  may be configured to receive the electrical signal  53  and, as described above, induce circuit element  28  to counteract the deflection of cantilever  14 . Also as discussed above, feedback controller  20  may be coupled to lock-in amplifier  74 . 
         [0058]    In one embodiment, the feedback controller  20  may be configured to measure a DC component of the electrical signal  53 . In some cases, the DC component may be converted, using a conversion factor, to lead to a normal force value. Also, the lock-in amplifier  74  may be configured to measure an AC component of the electrical signal  53 . Lock-in amplifier  74  may measure amplitude and/or phase of the AC component or related information (e.g in-phase and quadrature components) at a driving frequency of the lateral modulator  72 . In some cases, the AC component may be converted, using a conversion factor, to lead to a friction force value. The relationship of normal force and friction force may be represented by the equation: 
         [0000]    
       
         
           
             
               V 
               stack 
             
             = 
             
               
                 
                   3 
                    
                   α 
                 
                 
                   2 
                    
                   β 
                    
                   
                       
                   
                    
                   
                     k 
                     z 
                   
                    
                   
                     L 
                     cant 
                   
                 
               
                
               
                 ( 
                 
                   
                     F 
                     z 
                   
                   + 
                   
                     
                       
                         2 
                          
                         
                           L 
                           tip 
                         
                       
                       
                         L 
                         cant 
                       
                     
                      
                     
                       F 
                       x 
                     
                   
                 
                 ) 
               
             
           
         
       
     
         [0059]    In the foregoing equation, V stack  may represent the applied voltage  48  to circuit element  28 , a may represent α proportional constant, k z  may represent a spring constant, L cant  may represent the length of cantilever  14 , L tip  may represent the probe length, and F z  and F x  may represent the normal and friction forces, respectively. Therefore, the normal force conversion factor may be 2βk z L cant /3α, and the friction force conversion factor may be βk z L cant   2 /3αL tip . The derivation and sample calculations of the above equations can be found in the paper: Byung I. Kim et al.,  Simultaneous Measurement of Normal and Friction Forces Using a Cantilever - Based Optical Interfacial Force Microscope, R   EVIEW OF  S CIENTIFIC  I NSTRUMENTS  82, 05311 (2011), which is hereby incorporated by reference in its entirety. 
         [0060]    By way of example, if the probe is made of 1-10 Ωcm phosphorus doped Si, with a nominal spring constant (k z ) and resonance frequency known to be 3N/m and 50 kHz, respectively, and if the cantilever and probe dimensions are measured to be L cant =485 μm and L tip =20 μm, respectively. Then, measurements may be taken in ambient conditions with relative humidity of 55%. Tip speed may be chosen to be 10 nm/s, and lateral movement achieved by modulating the sample along the long axis direction of the cantilever with a 1 nm amplitude and a frequency of 100 Hz. Based on these numbers, the amplitude of the AC component may be measured at the lock-in amplifier  74  and the DC component at the feedback controller  20 . Based on this information, and using the conversion factors disclosed above, the normal force conversion factor may be calculated to be approximately 5 nN/V and the frictional force conversion factor may be calculated to be approximately 60 nN/V. Of course, one of ordinary skill in the art would recognize that based on any multitude of variables, each respective conversion factor could change significantly. 
         [0061]    Additionally, a memory element (not shown) may be coupled to the feedback controller  20  and/or the lock-in amplifier  74  in order to record values measured by each respective element. The measured values may be recorded as a function of distance, wherein the distance is related to the movement of piezotube  22  by the z-axis controller  54  and/or the amplifier  60 . In some embodiments, the memory element may be coupled internally to the feedback loop. In other cases, the memory element may be external to the microscope and coupled to the feedback loop through any type of wired or wireless connection, as appropriate. 
         [0062]    As illustrated in  FIG. 5 , piezotube  22  may be modulated laterally (e.g., in the x-axis and/or y-axis directions) as piezotube  22  moves the sample substance  12  into contact with probe  36 . As the sample substance  12  approaches and retracts from probe  36 , lock-in amplifier  74  may detect a voltage signal that indicates the effect of friction forces between the sample substance  12  and probe  36 . In some embodiments, COIFM  10  may indicate and/or record information regarding the normal forces, friction forces, and/or the distance between probe  36  and the sample substance  12 . 
         [0063]    An example illustrates certain embodiments of COIFM  10 . In some embodiments, COIFM  10  may measure the effect of interfacial water in micro-electro-mechanical system (MEMS) devices. In such devices, the presence of water may hinder the movement and/or function of micro-electro-mechanical structures. Understanding the effects of interfacial water in MEMS devices may enable designing MEMS devices that effectively operate in humid and/or wet environments. 
         [0064]    In the present example, water may be deposited on a tray  80  (e.g., silicon substrate) on piezotube  22  in an ambient environment, as illustrated in  FIG. 4 . COIFM  10  may be equipped with probe  36  that comprises a silicon tip. COIFM  10  may be placed in an enclosure  76  (e.g., an acryl box) having at least one inlet port  78  for dry nitrogen gas and at least one inlet port  78  for humid water vapor. Appropriate levels of nitrogen gas and water vapor may then be added to enclosure  76  to control the amount of humidity. 
         [0065]    In the present example, COIFM  10  may establish a feedback loop between optical detector  18  and circuit element  28  on cantilever  14 . Lateral modulator  72  may modulate piezotube  22  in the x-axis and/or y-axis directions as piezotube  22  moves in the z-axis direction to bring the water into contact with probe  36 . COIFM  10  may measure both the normal forces and the friction forces between the water and probe  36 . COIFM  10  may collect and/or record data as piezotube  22 , while modulating, approaches and retracts from probe  36 . 
         [0066]    In the present example, chains of water molecules may form between probe  36  and tray  80  on piezotube  22 . When tray  80  on piezotube  22  is in proximity to the silicon tip of probe  36 , the normal forces and friction forces caused by the water chains may oscillate. As the gap distance decreases between probe  36  and tray  80 , the force response of the water chains may resemble the force response of a polymer (as opposed to the force response of a spring). 
         [0067]    In some embodiments, the water molecules confined between probe  36  and tray  80  on piezotube  22  may form a bundle of water chains through hydrogen bonding. The length of each chain may be approximated by a model called “freely jointed chain” (FJC), in which the individual segments of each water chain move randomly. The FJC model may be expressed by the following equation: 
         [0000]    
       
         
           
             
               〈 
               
                 z 
                 l 
               
               〉 
             
             = 
             
               l 
               · 
               
                 σ 
                  
                 
                   [ 
                   
                     
                       coth 
                        
                       
                         ( 
                         
                           
                             
                               f 
                               l 
                             
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                     - 
                     
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                             f 
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         [0068]    In the foregoing equation, l may represent the number of water joints, σ may represent the diameter of water, f may represent tip force, n may represent the number of water chains, k B  may represent the Boltzmann constant, and T may represent temperature. Applying the FJC model in the present example, the measurements by COIFM  10  may indicate that, as probe  36  approaches tray  80  on piezotube  22 , the number of water chains between probe  36  and silicon substrate may increase while the number of water joints in each chain may decrease. 
         [0069]    Although the foregoing example describes the use of COIFM  10  to measure interfacial forces associated with water chains, it should be understood that COIFM  10  may be used to measure interfacial forces in any suitable substance. 
         [0070]    The present disclosure encompasses all changes, substitutions, variations, alterations and modifications to the example embodiments described herein that a person having ordinary skill in the art would comprehend. Similarly, where appropriate, the appended claims encompass all changes, substitutions, variations, alterations, and modifications to the example embodiments described herein that a person having ordinary skill in the art would comprehend.