RHEOLOGICAL CHANGES IN COSMETICS 323 A Pro• • P•be load ,,. ,•..7 :,,U' ' 7.•. Fr.i.c• ion .. ß Reciprocation D Figure 1. Measuring device used in this study: measuring device (A), diagram of the device (B), block-type probe (C), and roller-type probe (D). Scale bars under (C) and (D) represent 10 mm. pair of strain gauge sensors and then converted to an electric signal (-10-10 V) with an amplifier. The electric signal was digitized through an A/D converter and sent to a computer for data processing. A block diagram of the device is shown in Figure 2. MEASUREMENT PROCEDURE AND DATA PROCESSING A sample (5-40 pl) was applied to the center of the stage, and the probe was placed on the sample, perpendicular to the surface of the stage. Then, reciprocation was applied immediately and continued for 3-15 min. To avoid uneven spreading of the sample on the stage, the probe was pulled up above the stage each time for approximately 5 mm [• :Amplifier //// Probe Lifting ONIOFF • : RøtarylSølenøid [ F Temperature Setting Prøbe•_. L Then-no Couple / •l •----C • Temper/Signa Speed Setting Servomotor Bal?S•rew • 3, •. I Cø II I Position Signal • / Feedback ( •1 Linear Pøtentiø/I"' Figure 2. Block diagram of the measuring device.

324 JOURNAL OF COSMETIC SCIENCE at the end of the sweep area and placed again at the start of measurement in the reverse direction [probe lifting function, as reported by Date and Inaba (14)]. During the measurements, ambient temperature and relative humidity were kept at 25 øC and 50%, respectively. The temperature of the sample stage was kept at 32øC. For calibration, a certain force (0.01-0.1 N) was applied to the probe horizontally by a weight and a pulley. To compensate for zero-point shifting during measurement, the measured fric- tional force in each reciprocating process was calculated with the following equation: Fc F = •c x (Vf - Va) + 2 where Fc is the force applied at calibration, and Vc, Vf, and Vb are the voltages detected at calibration, at the forward process, and at the backward process, respectively. SAMPLES USED FOR MEASUREMENT Various types of commercial cosmetic emulsions were used for measurement. These include massage creams, moisturizing creams, and essences (O/W-type and W/O-type emulsions that can give various skin sensations). Standard cosmetic cream formulations that contained conventional cosmetic ingredients were also used for measurement. Vis- cosity standard oils were purchased from Toki Sangyo (Tokyo, Japan). SENSORY EVALUATION OF COSMETIC SAMPLES Skilled panels carried out evaluation of cosmetic samples to assess their sensory attrib- utes, including spreading, stickiness, absorption, and richness. The panels graded the test samples on a scale from -3 to + 3 against the selected references for each attribute. Special attention was paid to the changes in spreadability and stickiness during appli- cation of the cosmetics. MEASUREMENT OF SPREADING VALUE Spreading values of oils were measured according to the method developed by Zeidler (15), with slight modifications. Oil samples (5 lnl) were placed on the forearms of human volunteers. The oil that spread spontaneously on the skin was blotted on a piece of wax paper to measure the area of spread. The area at 5 min after application was used as the spreading value. During the experiments, ambient temperature and relative humidity were kept at 25øC and 50%, respectively. RESULTS AND DISCUSSION MEASUREMENT USING THE BLOCK-TYPE PROBE When cosmetic emulsions are applied to skin, they usually spread evenly and form thin films on the skin. To simulate this process by rheological measurement, it is important to spread samples evenly on the sample stage. In preliminary experiments using con- ventional friction analyzers that consist of reciprocating sample stages and load cell sensors, however, the probe squeezed and piled up the applied samples at either end of the sweep area during reciprocation. To avoid this phenomenon, our measuring device