## 1 Introduction  
A healthy voice and speech are important for socializing and numerous professions. The social and economic disadvantages associated with severe voice and speech disorders are even more pronounced than for other communication disabilities like hearing malfunctions [1]. To overcome these drawbacks, appropriate treatment of voice and speech disorders is necessary, which requires a comprehensive understanding of voice and speech production. In general, voice and speech production is an interplay of aerodynamics, mechanical dynamics and acoustics. The fluid-structure-acoustic interaction of the airflow from the lungs with the deformable tissue of the vocal folds creates the basic tone of voice in the larynx [2]. Supraglottal structures modulate it to the audible sound of the voice, known as speech [3]. Due to the complex nature of voice production, the reasons for voice disorders are numerous. Functional dysphonia, a preserved but disordered phonation, that is not directly caused by structural or neurologic pathologies, is a frequently observed voice disorder clinically. Aperiodic oscillations of the vocal folds or left-right asymmetries that affect glottal closure express it often. In that context, it was found that vocal fold hydration has an impact [4]. A mucus layer hydrates and lubricates the vocal folds, ensuring an accurate oscillation. In general, mucus covers the inner lining of organs, the epithelium, and serves as a protective barrier against pathogens, maintains the hydration of air-exposed organs, or ensures material transport [5]. Ex-vivo studies confirmed the impact of artificial mucus on the vibrational characteristics of the vocal folds, in relation to different viscoelastic properties [6; 7]. Clinical studies reported mucus of varying thickness for persons with and without voice disorders [8; 9]. Specific diseases like active laryngeal tuberculosis [10] or cystic fibrosis [11] reported voice disorders as effects of changed mucus consistency or mucus accumulation on the vocal folds. Vice versa, dehydration of the vocal folds was also found to affect oscillation characteristics [12]. The viscoelasticity of mucus depends on its composition [13]. For several organs, it was already determined. Human respiratory mucus was found by several studies to reveal gel characteristics [14; 15; 16]. Some studies investigated animal mucus that also revealed gel characteristics [16; 17]. The mucins are the main component of mucus, that determine its viscoelasticity [18]. Mucins are glycoproteins that are able to build cross-linked complex hydrogels [16]. It is known that the polymer network of mucins is caused by hydrogen and disulfide bonds plus physical entanglements [19]. A study reported that low mucin concentrations of 1.5% leads to liquid-like mucus whereas a concentration of 5% results in solid-like mucus [20]. Besides the mucin concentration, the concentration of salts and surfactants and the pH affect the network building properties of mucins and thus mucus viscoelasticity [18; 21] pulmonary diseases like cystic fibrosis, chronic obstructive pulmonary disorder and asthma affect the mucin content [15; 22; 23]. Nicotine was also found to affect the thickness of mucus gels [24]. In general, mucus consists of 1-5% of mucins, 90-95% water, 1% electrolytes, 1-2% lipids, other proteins, DNA, cells and cellular debris [5; 25]. Similar mucin content at different locations in the human body results in similar rheological behavior, although the mucin glycoproteins themselves differ [22]. Detailed information about the viscoelasticity of mucus properties is rare. Limited amounts of available mucus collected from each patient restrict the use of bulk rheology. Particle tracking microrheology (PTM) is nowadays a commonly used technique to overcome the limitation of small sample amounts and determine rheological properties [26]. The viscoelastic properties determined by PTM depend on the diameter of the microspheres used in the experiments and their diffusion.