Abstract:
An apparatus ( 402 ) and method for measuring a surface energy of a test surface ( 12 ), which includes a viscoelastic polymer layer ( 20 ), disposed on a moveable component ( 34 ), that is compressed against the test surface ( 12 ) with a compressive force. The moveable component ( 34 ) is then moved relative to the test surface ( 12 ) at a predetermined velocity, and a drive force applied to the moveable component ( 34 ) is measured. The surface energy of the test surface ( 12 ) is then determined based at least in part on the compressive force, the predetermined velocity, and the measured drive force.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to an apparatus and a method for measuring the surface energy of a test surface to assess its ability to be bonded. The invention finds particular use in manufacturing control systems to verify the preparation of surfaces to be bonded. 
       BACKGROUND OF THE INVENTION 
       [0002]    Mechanisms that contribute to adhesion at the interface between two solids include polar and non-polar dispersion, hydrogen bonding, covalent and metallic bonding, charged bilayers, cross-linking, polymer entanglement, and mechanical locking. Many manufacturing bonding processes rely on these mechanisms, including painting, printing, plating, adhering, soldering, and spinning. Measurements of surface energy are often used to assess if a surface has been suitably prepared for bonding. 
         [0003]    The surface free energy of a solid is typically defined as half the energy per unit area required to separate a solid into two half planes separated by vacuum. U.S. Pat. No. 5,477,732 describes bringing a characterized solid atomic force microprobe (AFM) into intimate contact with a surface under test, and then measuring the energy required to separate the probe and test surface. The high curvature of the probe tip makes the technique insensitive to most roughness of the test surface. Small AFM tips are generally formed from relatively hard, high surface energy materials. 
         [0004]    Attempting a similar touch probe between a probe and test surface on a more convenient larger scale encounters the problem that most solid surfaces are somewhat rough and unclean. Most of the above mentioned forces are very short range, so that roughness that separates the two surfaces by a few Angstroms on average will reduce the measured force of attraction by more than an order of magnitude. 
         [0005]    While small scale roughness of the surface to be adhered is generally an impediment to measuring the surface energy of that surface, it often improves bonding of the manufactured article. The standard approach to probe rough surfaces is to use a liquid that wets the rough surface to some degree. The surface tension of a liquid is the analog of the surface free energy of a solid. In particular, measuring the contact angle of a sessile droplet on the test surface has been related to the surface free energy of the test surface using relations like the Young-Dupre equation. Related liquid contact angle measurements include the Wilhelmy plate method, the fiber contact angle method, the pendant drop method, and the Du Nouy ring method. Applying droplets of different composition increase the range of measurable surface energies, and allow some differentiation between component contributions from dispersion, polar, and hydrogen bonding. 
         [0006]    Surface roughness remains a problem for contact angle measurements. This is observed in several ways. Since the slope of the test surface is not constant along wetting line around a sessile drop, the wetting line can be a scalloped circle instead of a smooth circle. Typically the drying contact angle is smaller than the wetting contact angle, in part because surface roughness can generate a hysteresis that tends to pin the wetting line. The microscopic contact angle at the wetting front can be different from the measurable macroscopic contact angle. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The present invention relates to an apparatus and method for measuring a surface energy of a test surface having a plurality of asperities and pits, where a viscoelastic polymer layer, disposed on a moveable component, is positioned in contact with the test surface and compressed with a compressive force. The moveable component is then moved relative to the test surface at a predetermined velocity, and a drive force applied to the moveable component is measured. The surface energy of the test surface is then determined based at least in part on the compressive force, the predetermined velocity, and the measured drive force. 
         [0008]    One object of this invention is to improve the above compliant solid probe to account for variations in the observed force resulting from changes in the test surface roughness. 
         [0009]    Another object of this invention is to provide a convenient, inexpensive, and portable meter for rapidly assessing the adequate preparation of surfaces to be bonded. 
         [0010]    Another object of this invention is to improve the range of surface energies that can be probed compared to those accessible to contact angle measurement techniques. 
         [0011]    Another object of this invention is to provide a self-calibration technique for verifying the veracity of the probe surface. 
         [0012]    Another object of this invention is to provide a method to test the relative adhesion of the test surface to a probe surface composed of a polymeric material selected to be most similar to the intended bonding material. 
         [0013]    Another object of this invention is to provide information on the spatial variation of the surface energy of the test surface. 
         [0014]    Another object of this invention is to increase the reusability of the compliant solid probe by using non-overlapping sectors of its surface for independent measurements. 
         [0015]    Another object of this invention is to provide protection for the compliant solid probe so that it is not contaminated while not in use. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1A  is a sectional view illustrating a method of applying a viscoelastic probe surface to a test surface. 
           [0017]      FIG. 1B  is a sectional view illustrating a method of applying creep and relaxation to obtain an intimate contact between the probe and test surface. 
           [0018]      FIG. 1C  is a sectional view illustrating a method of removing the viscoelastic probe surface from the test surface. 
           [0019]      FIGS. 2A and 2B  are graphs illustrating viscoelastic creep from a constant applied stress. 
           [0020]      FIGS. 3A and 3B  are graphs illustrating viscoelastic relaxation from a constant applied strain. 
           [0021]      FIG. 4A  is a top view of a portable, battery operated meter. 
           [0022]      FIG. 4B  is a bottom perspective view of the portable, battery operated meter. 
           [0023]      FIG. 4C  is a side view of the portable, battery operated meter. 
           [0024]      FIG. 4D  is a front view of the portable, battery operated meter. 
           [0025]      FIG. 5A  is a top view of the portable, battery operated meter, where the case, touch screen display, and fasteners are omitted. 
           [0026]      FIG. 5B  is a bottom perspective view of the portable, battery operated meter, where the case, touch screen display, and fasteners are omitted. 
           [0027]      FIG. 5C  is a side view of the portable, battery operated meter, where the case, touch screen display, and fasteners are omitted. 
           [0028]      FIG. 5D  is a front view of the portable, battery operated meter, where the case, touch screen display, and fasteners are omitted. 
           [0029]      FIGS. 6A and 6B  illustrate side and front views, respectively, of the torque measurement flexure of a preferred embodiment of the invention. 
           [0030]      FIG. 7A  is a top view of the portable, battery operated meter, showing features associated with a self-calibration mechanism. 
           [0031]      FIG. 7B  is a bottom perspective view of the portable, battery operated meter, showing features associated with the self-calibration mechanism. 
           [0032]      FIG. 7C  is a side view of the portable, battery operated meter, showing features associated with the self-calibration mechanism. 
           [0033]      FIG. 7D  is a front view of the portable, battery operated meter, showing features associated with the self-calibration mechanism. 
           [0034]      FIG. 8  is a block diagram illustrating the control system of a preferred embodiment of the invention. 
           [0035]      FIG. 9  is a process flow chart illustrating the decision flow of a measurement process of a preferred embodiment of the invention. 
           [0036]      FIG. 10  is a graph illustrating data measuring the surface energy of a smooth surface by a preferred embodiment of the invention. 
           [0037]      FIG. 11  is a graph illustrating data measuring the surface energy of a rough surface by a preferred embodiment of the invention. 
           [0038]      FIG. 12  is a graph illustrating characteristics of data measuring the surface energy of smooth and rough surfaces by a preferred embodiment of the invention. 
           [0039]      FIG. 13  is a side view illustrating an alternative preferred embodiment of the invention. 
           [0040]      FIG. 14  is a perspective view illustrating measuring indicia associated with the probe surface of a preferred embodiment of the invention. 
           [0041]      FIGS. 15A and 15B  are perspective views illustrating a preferred embodiment of the invention separated from a storage tray and assembled with a storage tray, respectively. 
           [0042]      FIG. 16A  is a side view of a complete apparatus, which is a preferred embodiment of the invention in a compact configuration. 
           [0043]      FIG. 16B  is a sectional view of section A-A taken in  FIG. 16A . 
           [0044]      FIG. 16C  is a perspective view of the complete apparatus with a standoff separated. 
           [0045]      FIG. 16D  is a side view of a disposable applicator assembly. 
           [0046]      FIG. 16E  is a sectional view of section B-B taken in  FIG. 16D . 
           [0047]      FIG. 16F  is also a sectional view of section B-B taken in  FIG. 16D . 
       
    
    
     DETAILED DESCRIPTION 
       [0048]    As a viscoelastic material is peeled off of a surface, work is expended both in fracturing the interface (the surface energy) and in distorting the viscoelastic material (mechanical dissipation). Typically the work expended in mechanical dissipation is much greater than the work needed to overcome the surface energy. For example, the surface energy of polystyrene is about 0.033 Joules per square meter. Conceptually, if a one-atom-thick film could be peeled off of the surface, so that no energy is expended distorting the film, the force required to pull away a one centimeter wide film would be about 330 micro Newtons for polystyrene (independent of the rate that it is peeled). Peeling a typical removable pressure sensitive adhesive (PSA) strip one centimeter wide from polystyrene will typically require several Newtons, or more than 100 times the energy required to overcome the surface energy, and the required force varies considerably with the rate that the strip is peeled. Viscoelastic dissipation therefore acts as a desirable amplifier of the interfacial surface energy, making a small force easier to measure. 
         [0049]      FIGS. 1A through 1C  are sectional views illustrating the principle of utilizing the adhesion of a viscoelastic material to a rough surface to measure the surface energy of that surface. A solid  10  has a surface-under-test  12  which has some degree of roughness. There are many ways to characterize roughness; without precluding any particular characterization method of surface roughness, we will refer to a rough surface as having asperities  16  where the surface  12  protrudes above the average local surface of the solid  10 , and pits  14  where the surface  12  lies below the average local surface of the solid. As the probe surface  22  approaches the test surface  12 , the asperities  16  will make first contact. 
         [0050]    The viscoelastic material (VEM)  20  is selected for both its chemical and mechanical properties. Preferred embodiments include silicones, rubbers, urethanes, acrylics, styrenes, and polyolifins. A most preferred embodiment is ×4 retention gel from Gel-Pak of Hayward, Calif. An alternative most preferred embodiment is the adhesive component of 1310 removable clean room tape from UltraTape Industries of Oregon. The surface  22  of the VEM  20  can have the same composition as the bulk, or in a less preferred embodiment it can be treated with an adhesion promoter or inhibitor. The VEM  20  should leave minimal or no residue on the test surface  12  when it is removed, so that adhesion of the interface is tested and not cohesion of the VEM  20 . The VEM thickness should be at least as great as the local roughness of the test surface; a useful range of VEM thickness is 0.0005 inches to 0.05 inches, with a most preferred thickness of about 0.005 inches. 
         [0051]    A flexible support film  26  of relatively high modulus is useful in removing the VEM  20  from the test surface  12  without permanent stretching distortion. The most preferred support film is 0.002 inch thick Kapton. Kapton films down to 0.0002 inches can be used; the thinner films can experience stresses above their yield stress for high surface energy test surfaces. Kapton films up to 0.01 inches thick can be used; the thicker films can be sufficiently inflexible that don&#39;t conform to the larger lateral scale height variations of the test surface  12 . Alternative preferred flexible support film materials include cellulose, polyesters, metal films, polyamides, and polyethylenes. One combination of the VEM  20  and the flexible support  26  is subsequently referred to herein as the ribbon. 
         [0052]    A compressible layer  28  between the support film  26  and the mandrel  34  tends to make uniform the compression force pressing the VEM  20  against the test surface  12  particularly for length scales of roughness and curvature that are larger than the thickness of the VEM. The most preferred embodiment uses a 0.15 inch thickness of Poron polyurethane open cell foam with a 25% deflection of 1 to 5 psi. Poron is particularly useful in that it recovers quickly to its original configuration after a strain cycle. A preferred embodiment has a range of compressible layer thicknesses from 0.02 inches to 2 inches. Alternative compressible layer materials include open and closed cell foams, rubber, and elastomers. 
         [0053]    A cylindrical mandrel  34  forms the hub of the probe in the most preferred embodiment. An alternative preferred embodiment is made by coating an o-ring or elastomeric torus with a VEM coating. Alternative embodiments of the mandrel  34  give its downward face a flat or spherical configuration; these will be subsequently described. 
         [0054]    In  FIG. 1A  the mandrel  34  is pressed towards the test surface  12  while rotating clockwise and moving from left to right parallel to the test surface  12 . The VEM probe surface  22  comes into contact with the test surface  12  at a leading edge  24 . The attractive interaction between the VEM surface  22  and the test surface  12  tends to advance the mandrel  34  and the leading edge  24  to the right with respect to the test surface  10 . Just behind the leading edge the asperities will tend to be in intimate contact with the VEM surface  22 , such as asperity  32 , while the pits may not be in intimate contact, such as pit  30 . 
         [0055]    In  FIG. 1B  the mandrel  34  continues to press towards the test surface  12  while rotating clockwise and moving from left to right parallel to the test surface  10 . The normal pressure from the hub reaches its maximum where the mandrel tangent is parallel to the test surface  12 . The combination of dwell time of the mandrel  34  over an area of the test surface and the pressure between the mandrel  34  and the test surface  12  causes the VEM  20  to conform to at least a portion of the rough surface that did not make contact near the leading edge. For example, the pit  30  generally makes intimate contact with VEM surface  22  in  FIG. 1B . 
         [0056]    In  FIG. 1C  the mandrel  34  continues to press towards the test surface  12  while rotating clockwise and moving from left to right parallel to the test surface  12 . At the trailing edge  36  the VEM is peeling off of the test surface  12  by the rolling of the mandrel  34 . The attractive interaction between the VEM surface  22  and the test surface  12  tends to retard the clockwise rotation of the hub as well as the linear motion of the mandrel  34  to the right with respect to the test surface  12 . The mechanical dissipation in the VEM  20  that amplifies the surface energy as previously discussed occurs in the vicinity of the trailing edge  36 . 
         [0057]    The torque required to rotate the mandrel  34  in  FIGS. 1A through 1C  gives an averaged measurement of the difference between the forces retarding the trailing edge  36  and advancing the leading edge  24  combined with geometric effects such as the radius of the hub. In other geometries, such as the spherical ball described subsequently, the line of contact is a ring, and an external measure is the normal force required to remove the ball from the surface. We will subsequently refer to “drive force” to include both torque measurements on a mandrel and normal force measurements on a sphere. More generally, drive force is meant to be an external measurement of force acting in part on a line of contact between the test surface and the VEM. 
         [0058]      FIGS. 2A ,  2 B,  3 A and  3 B are graphs illustrating well known properties of viscoelastic materials.  FIG. 2A  shows a constant stress σ 0  being applied for a time t 0 .  FIG. 2B  shows the strain ε that results from that applied stress. If the material were purely elastic, the strain would follow the dashed curve, promptly deflecting to the elastic strain limit E, and then fully recovering after t 0 . A typical VEM follows the solid curve in  FIG. 2B . The VEM also deflects initially to an elastic limit E, and then continues to experience additional strain as the VEM creeps. After t 0  the VEM will promptly partially recover, and then more gradually recover further. The time constants associated with creep and recovery, the relative magnitude of the elastic and viscoelastic strains, and the asymptotic recovery are idiosyncratically dependent on details of the material, the stress, and the geometry. 
         [0059]      FIG. 3A  shows a constant strain ε 0  being applied for a time t 0 .  FIG. 3B  shows the stress σ that results from the applied strain. If the material were purely elastic, the stress would follow the dashed curve, promptly deflecting to the elastic stress limit E, and then fully recovering after t 0 . A typical VEM follows the solid curve in  FIG. 3B . The VEM also deflects initially to an elastic limit E, and then relaxes by VE to an intermediate stress level. After t 0  the VEM will experience some residual negative stress, which too will gradually relax. The time constants associated with relaxation, the relative magnitude of the elastic and viscoelastic strains, and the asymptotic recovery are again idiosyncratically dependent on details of the material, the stress, and the geometry. 
         [0060]    Both creep and relaxation are experienced by the VEM  20  in  FIG. 1A through 1C . For clarity, and without implied restrictions, we will subsequent refer to any combination of creep and relaxation as stress relaxation. 
         [0061]      FIGS. 4A through 4D  show four external views of portable, battery operated meter  402 , which is a preferred embodiment of the invention.  FIG. 4A  is a top view of meter  402 ;  FIG. 4B  is bottom perspective view of meter  402 ;  FIG. 4C  is a left side view of meter  402 ; and  FIG. 4D  is a front view of meter  402 . As shown, portable, battery operated meter  402  is roughly a three inch cube. The cuboid case  404  of injection molded plastic rests on top of a test surface  12  on two freely rotating wheels  412  and a portion of the surface  22  of a VEM  20 . A touch screen display  406  provides visual readout of the surface energy of the test surface  12  upon completion of the measurement, as well as diagnostics, operation directions, and prompts. The touch screen and switches  408  and  414  give the user control of the functioning of the device. Since the apparent surface energy measured by the preferred embodiment is sensitive to temperature, the user can contact a temperature probe  410  to the test surface  12  to normalize the result to a standard temperature. A knob  418  aids in loading and removing ribbons from the assembly of an expandable mandrel  34  and the compressible layer  28 . When the mandrel  34  is in its unexpanded state, the user slides a ribbon over the compressible layer  28 , grasps the knob  418 , and activates the switch  414 . The meter  402  rotates the mandrel  34  by a mechanism described later, expanding the mandrel  34 , and engaging the ribbon with the cylindrical surface of the compressible layer  28 . A reflective optical sensor  416  determines if the unit is resting on a test surface or if it is in some other configuration. 
         [0062]      FIGS. 5A through 5D  are, respectively, a top view, bottom perspective view, left side view, and front view of the meter  402  (corresponding to  FIGS. 4A through 4D , respectively), except that the case  404 , the touch screen display  406 , and various fasteners have been removed for clarity. A DC gear motor  502  drives pulleys  506  using belt  504  to controllably rotate the mandrel  34 . The DC gear motor  502  and mandrel  34  are both mounted to a flexure bulkhead  508 , describe later. A battery pack  512  provides power to a controller mounted on the printed circuit board  510 . Three reflective optical sensors  514  are positioned to detect indicia on the ribbon, described later. 
         [0063]      FIGS. 6A and 6B  are front and side views, respectively, of the flexure bulkhead  508 . The flexure bulkhead  508  mounts to the case  402  with the threaded holes  610 . The DC motor  502  mounts through the hole  602 , and the mandrel  34  mounts through the hole  604 . Two slots  608  and four holes  612  are machined into the flexure bulkhead  508  so that the combination of the motor  502  and mandrel  34  are connected to the threaded holes via four thin flexures. Resistive film strain gages are glued to two of the four thin flexures at  606 . Monitoring the differential resistance of the two strain gages provides the controller a measurement of the torque on the mandrel  34  with respect to the test surface  12 . 
         [0064]      FIGS. 7A through 7D  are, respectively, a top view, bottom perspective view, left side view, and front view of the meter  402  (corresponding to  FIGS. 4A through 4D , respectively), except that most of the constituent parts have been removed so that the self calibration mechanism can be more clearly seen. A bracket  702 , held by threaded holds  712  to the case  402 , carries a small stepper motor  704  and a pivot arm  706 . An eccentric wheel  708  displaces the top end of the pivot arm  706  towards and away from the motor shaft, causing a calibration cylinder  710  to be alternately pressed against the VEM surface  22  or lifted off of the VEM surface  22 . A spring  712  preloads the pivot arm  706  against the eccentric wheel  708 . The calibration cylinder serves several functions. Since its surface energy is known, measuring its surface energy by rolling the calibration cylinder  710  in contact with a known good VEM  22  and measuring the resulting torque allows the strain gages to be calibrated. The more common function of the calibration cylinder is to test the VEM surface  22  to see if a previous test surface  12  has contaminated the VEM, which would reduce the apparent surface energy of the calibration cylinder. 
         [0065]      FIG. 8  is a block diagram illustrating the signal connections of a preferred embodiment of the meter  402 . A rechargeable battery  512  supplies power to the system through a power manager  804 . The power manager can be as simple as a switch; in the most preferred embodiment the power manager is a circuit executing the functions of gas gage, charging control, temperature and short circuit protection, and multi-level power conservation. A connector to an external charging device  802  allows the power manager  804  to recharge the battery  512 . A controller  806 , consisting of one or more processors and their associated memory and programming, coordinates the operation of the peripheral devices and performs calculations related to test surface energy. An example of a controller is a dsPIC30F6014A. Variations in the resistance of the strain gages  606  are converted to a signal by a signal conditioner  814 . Since test surfaces with low surface energies may generate only small torques, a DAC  812  is used to null the bias to the signal conditioner  814 . The four wire output from the touch screen  406  is processed by a converter  824 , which in turn sends interrupts and x-y screen coordinates to the controller  806 . Reflective sensors  514  measure both indicia on the ribbon and the presence of a test surface; signals from the reflective sensors  514  pass through a preamplifier  834  to the controller  806 . A rotary encoder  842  monitors the angular position of the mandrel. The motor driver  808  converts servo motor signals from the controller  806  into higher power drive signals to the servo motor  502  that rotates the mandrel. A second motor driver  828  allows the controller  806  to position the stepper pivot motor  704  for auto-calibration or normal operation. The controller  806  writes to the LCD display  406  through an LCD drive controller  818 . In addition to displaying results on the LCD, an external data interface is available through an Ethernet port  840  connected to the controller  806  through a physical layer interface  838 . 
         [0066]      FIG. 9  is a process flow chart for a preferred embodiment of the meter  402 . Starting with a ribbon-free mandrel  34 , the user slides a ribbon over the foam sleeve of the mandrel and operates the knob  418  and switch  412  to expand the mandrel  34  and engage the ribbon  902 . The controller  806  inputs reflective sensor readings while causing the mandrel  34  to rotate, reading indicia on the ribbon. The indicia are tested for alignment errors  904  like a missing ribbon or a ribbon positioned incorrectly along the axis of the mandrel. If the indicia indicate a valid ribbon is located correctly, viscoelastic parameters  906  are extracted from the indicia. The user places the preferred embodiment  402  with the ribbon surface in contact with the test surface  908 , and the user indicates that the measurement should start. 
         [0067]    To measure the surface energy (SE), the controller causes the mandrel  34  to rotate at a predetermine rate. The dwell time is the time the trailing contact edge  36  intersects a point on the ribbon minus the time the leading contact edge  24  intersects that point on the ribbon. The dwell time is related to the ribbon circumference, the preload, the rotation rate, and the mandrel  34  diameter. When at least half of one dwell time has passed, so that the rolling process has approached steady state, the controller reads the torque required to rotate the mandrel using the strain gage signals. The controller can average several such measurements to improve signal to noise, or can accumulate a sequence of such measurements to create a spatial map. The controller then reverses the direction of rotation of the mandrel  34 , again allows steady state to be reached, and acquires torque data while moving in the reverse direction. The difference between the torque readings in the plus and minus directions makes the measurement process less sensitive to offsets in the sensors; it also compensates for the test surface being out of level. Finally the torque readings are combined with the viscoelastic parameters using a predetermined function to produce the measured surface energy (SE). 
         [0068]    The torque is tested to see if it is too high  912 . For example, if the surface adheres sufficiently to the ribbon, the torque required to rotate the mandrel will lift the wheels  412  completely off of the test surface if the mandrel rotation direction is reversed; in this case, the reverse direction reading is generally not required to improve signal to noise, and only the forward direction is used. If the torque is so high that the strain gage measurement leaves its calibrated range  914 , then the peel rate should be decreased by increasing the dwell time, and the measurement  910  repeated. If the torque is so low that there is little detectable difference between the forward and reverse torques  918 , again the dwell time should be increased to test for the case that the VEM has not undergone enough stress relaxation to compensate for the surface roughness, and the measurement  910  repeated. 
         [0069]    If the SE measurement at dwell time A has passed the range tests  912  and  916 , a second SE measurement is then made at dwell time B  920 , different from dwell time A. Again, range tests  922  are applied. The two surface energy measurements SE(A) and SE(B) are used to compute a rate of change of torque with peel velocity  924 ; if this slope is greater than a value computed from the viscoelastic parameters, a well adhered and sufficiently stress relaxed interface is indicated, and the final surface energy is reported  928 . If the slope is less than a value computed from the viscoelastic parameters  926  the dwell times are increased and the measurement cycle is repeated. 
         [0070]    Since quicker measurements have more value than slower measurements, the first measurement dwell time A tried should be somewhat smaller than the dwell time A used in the last successful measurement cycle. This is done so that repeated measurements performed in a manufacturing application cause the measurement cycle to equilibriate near the shortest practical time. 
         [0071]    The process illustrated in  FIG. 9  assumes that measurements are made in steady state, or with the mandrel  34  rotating at a constant speed. An alternative preferred embodiment for relatively rough low surface energy test surfaces uses a stop-and-go approach. The ribbon is compressed against the test surface by a stationary mandrel  34  for a dwell time, then the mandrel  34  is rotated at a relatively high rate to peel off the ribbon adhered during the dwell time. Since the ribbon contacting the test surface  12  during the dwell time experiences a non-uniform compression, the SE should be computed from the torques measured as the ribbon under the center of the stationary mandrel  34  is peeled off the test surface  12 . 
         [0072]    The moving leading and trailing contacts propelled by the rolling mandrel  34  reach steady state values in a fraction of a full rotation of the mandrel  34 . This allows the preferred embodiment to consider a ribbon as several independent sectors. A sector need be no larger than about two static footprints of the ribbon contacting the test surface  12 . The number of sectors per ribbon is between two and thirty two, with six sectors being the most preferred value. When a new ribbon is engaged on the mandrel  34 , each sector can be treated as a fresh probe surface uncontaminated by previous measurements. 
         [0073]      FIG. 10  is a graph showing experimental data taken from a ribbon rolling across a clean native oxide silicon wafer. A linear torque scale is plotted against a logarithmic leading edge velocity scale. Over about two orders of magnitude of velocity, this particular removable pressure sensitive adhesive shows a reasonable fit to a 0.31 power law dependence on velocity, shown by the dashed curve. 
         [0074]      FIG. 11  is a graph showing experimental data for the same pressure sensitive adhesive rolling across a clean flat steel plate to which 40 μm diamonds have been adhered (a diamond lap tool). At low rolling velocities, the required torque appears to follow a similar power law to the smooth surface data of  FIG. 10 . 
         [0075]    At higher rolling velocities, the required torque changes less rapidly with velocity, following approximately a 0.11 power law. We note that there can be an audible distinction between the low and high velocity cases; the low velocity case is relatively silent, while the high velocity case can produce a crackling sound. 
         [0076]      FIG. 12  is a graph illustrating a generic comparison between rolling torques measured on relatively smooth (S) and relatively rough (R) test surfaces of the same chemical composition. At the lowest velocities (v&lt;v 1 ), the VEM is in intimate contact with both S and R; since there is more area in contact and some mechanical interlocking between the test surface and the VEM in the R case, there is more torque required for the rough surface. As the velocity increase (v 1 &lt;v&lt;v 3 ), the stress relaxation reduces in the R case from forming good contact with the asperities and the pits to forming good contact with just the asperities. In this range the torque is somewhat unpredictable; experimentally the required torque can appear oscillatory. At still higher velocities (v 3 &lt;v 5 ) the rough surface is producing less torque and is following a different power law compared to the smooth surface. The smooth surface ceases to follow its low velocity power law above v 5 . 
         [0077]      FIG. 13  is a side view illustrating an alternative preferred embodiment meter, where the leading and trailing edges move in opposite directions instead of the same direction. An additional wheel  1302  is added that is controlled by a motor to move vertically, thereby pressing the probe surface  22  against the test surface or separating the probe surface from the test surface. In this case, the force exerted by the test surface on mandrel normal to the test surface is measured as a function of the height of the additional wheel  1302 , giving a measurement of the surface energy in the contacting area under the mandrel. The rotating mandrel is useful to move the sensing unit laterally, as well as for advancing the probe surface to a clean, un-used position for the next test sample. In alternative preferred embodiments the probe surface that contacts the test surface is toroidal, flat, or cylindrical prior to contacting the test surface. 
         [0078]    The preferred embodiment meter shown in  FIG. 13  can be used to measure surface energy with constant velocity but varying preload. In this application, the wheels  412  and  1302  support a portion of the weight of the measurement tool, and the mandrel supports the remaining weight. The fraction of the weight of the measurement tool supported by the mandrel is varied by adjusting the vertical position of wheel  1302 . Increasing the stress forcing the VEM into a rough test surface will increase the number of pits in intimate contact with the probe surface for a constant dwell time. 
         [0079]      FIG. 14  is a perspective view illustrating a preferred embodiment of the ribbon with optically detectable indicia. As the mandrel rotates the ribbon, a circumferential band of the ribbon passes under a reflective optical sensor  514 . A light source  1406  in the reflective sensor  514  produces a light beam  1404  that reflects off of the probe surface  22  and onto a light sensor  1408 . The signal from the light sensor  1408  can measure the presence or absence of indicia on the surface, including gray scale values. Several sensors at different axial position will measure indicia in different circumferential bands. These indicia can be coded with alignment marks, serial numbers, sector identifiers, and viscoelastic parameters for the VEM used in the ribbon. 
         [0080]      FIGS. 15A and 15B  are perspective views illustrating the use of a tray  1502  with the preferred embodiment meter  402 . In  FIG. 15A  the meter  402  is separate from the tray  1502 ; in  FIG. 15B  the meter  402  is nested into the tray  1502 . The tray can serve to protect a mounted ribbon from contamination when the meter is not in use. The tray can support the meter so that the mandrel is free to rotate; this is useful during auto calibration with the calibration cylinder  710 . 
         [0081]    In an alternative preferred embodiment, the thickness of the VEM  20  is varied on the flexible support film  26 . A readily implemented version of this embodiment has the VEM thickness varying linearly about the circumference of the ribbon. Since the VEM looses its ability to conform to the surface roughness once the VEM thickness is less than the surface thickness, this is a method provides quantitative surface roughness information in addition to surface energy. 
         [0082]      FIGS. 16A through 16F  illustrate aspects of a compact configuration of the invention.  FIG. 16A  is a side view of the complete apparatus.  FIG. 16B  is a sectional view of section A-A taken in  FIG. 16A .  FIG. 16C  is a perspective view of the complete apparatus with the standoff  1608  separated.  FIG. 16D  is a side view of the disposable applicator assembly  1604 .  FIG. 16E  is a sectional view of section B-B taken in  FIG. 16D .  FIG. 16F  is also a sectional view of section B-B taken in  FIG. 16D , which illustrates the distortion of the disposable application  1604  when pressed against a test surface  12 . 
         [0083]      FIG. 16A  is a preferred embodiment in which the probe surface  22  is initially spherical, and the actions of stressing, relax, and delaminating the interface between the probe surface  22  and the test surface  12  involves motions in a line normal to the surface. An advantage to this configuration is compactness; the AA battery  1618  and linear stepper motor  1616  are to scale for a 6 inch long 0.87″ diameter pen-style design. Externally the user would hold the plastic case  1606  while pressing the removable standoff  1608  against the test surface  12 . 
         [0084]      FIG. 16B  shows cross sections of components of  FIG. 16A . Probe surfaces are stored in a cylindrical stack of disposable applicator assemblies  1604  held within a ferromagnetic guide tube  1610 . The guide tube  1610  holds the disposable applicator assemblies  1604  in a snug sliding fit. A magnet  1612  binds the end of the guide tube  1610  to a sensitive force gage  1614  that generates signals proportional to axial compression or tension. The force gage is mounted in turn on the non-rotating shaft of a linear stepper motor  1616  such as a Haydon Linear 2000. A battery  1618  provides power to a controller (not shown). To expose a fresh probe surface, the user first slides the guide tube  1610  axially away from the magnet  1612  and out of the case  1606 . In a manner analogous to U.S. Pat. No. 3,338,215, the user loosens the previously exposed disposable applicator assembly  1604  from the tapered end of the guide tube  1610 , and then inserts that used applicator assembly  1604  in the end of the guide tube  1610  usually contacting the magnet  1612  such that the previously exposed disposable applicator mates with the disposable applicators still in the guide tube, pushing them through the tube till a newly exposed disposable applicator emerges from the tapered end of the guide tube. Then the user slides the guide tube  1610  with its re-ordered disposable applicator assemblies  1604  back into the case  1606  and in contact with the magnet  1612 . 
         [0085]      FIG. 16D  shows a side view of a disposable applicator assembly  1604 , consisting of a molded cartridge  1620  and a viscoelastic sphere  1622 . When a user initiates a measurement cycle, the standoff  1608  is in place on the end of the case  1604 , and the free end of the standoff is pressed against the test surface  12 . The linear stepper motor  1616  provides sufficient force to drive the exposed disposable applicator assembly  1604  towards the test surface until the shoulder of the molded cartridge  1620  contacts the test surface  12 . After a sufficient time for stress relaxation, the linear stepper motor  1616  retracts the combination of the force gage  1614 , the magnet  1612 , the guide tube  1610 , and the cylindrical stack of disposable applicator assemblies  1604  from the test surface which measuring the force or tension required. The surface energy of the test surface is then computed by the controller. 
         [0086]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.